Note: Descriptions are shown in the official language in which they were submitted.
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
AN ANTENNA AND A FEED NETWORK FOR AN ANTENNA
BACKGROUND OF THE INVENTION
I. Field of the Invention
' The present invention relates to an antenna and to a feed network for
an antenna. More specifically, the present invention relates to a helical
antenna with a feed network wherein a portion of the feed network is
provided in an area coincident with radiators of the antenna.
II. Description of the Related Art
Contemporary personal communication devices are enjoying
widespread use in numerous mobile and portable applications. With
traditional mobile applications, the desire to minimize the size of the
communication device, such as a mobile telephone for example, has led to a
moderate level of downsizing. However, as the portable, hand-held
applications increase in popularity, the demand for smaller and smaller
devices has increased dramatically. Recent developments in processor
technology, battery technology and communications technology have
enabled the size and weight of the portable device to be reduced drastically
over the past several years.
One area in which reductions in size are desired is the device's
antenna. The size and weight of the antenna play an important role in
downsizing the communication device. The overall size of the antenna can
impact the size of the device's body. Smaller diameter and shorter length
antennas can allow smaller overall device sizes as well as smaller body sizes.
Size of the device is not the only factor that needs to be considered in
designing antennas for portable applications. Another factor to be
considered in designing antennas is attenuation and/or blockage effects
resulting from the proximity of the user's head to the antenna during
normal operations. Yet another factor is the characteristics of the
communication link, such as, for example, desired radiation patterns and
operating frequencies.
' An antenna that finds widespread usage in satellite communication
systems is the helical antenna. One reason for the helical antenna's
popularity in satellite communication systems is its ability to produce and
receive circularly-polarized radiation employed in such systems.
Additionally, because the helical antenna is capable of producing a radiation
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
2
pattern that is nearly hemispherical, the helical antenna is particularly well
suited to applications in mobile satellite communication systems and i n
satellite navigational systems. -
Conventional helical antennas are made by twisting the radiators of
the antenna into a helical structure. A common helical antenna is the
quadrifilar helical antenna which utilizes four radiators spaced equally
around a core and excited in phase quadrature (i.e., the radiators are excited
by signals that differ in phase by one-quarter of a period or 90°). The
length
of the radiators is typically an integer multiple of the quarter wavelength of
the operating frequency of the communication device. The radiation
patterns are typically adjusted by varying the pitch of the radiator, the
length
of the radiator (in integer multiples of a quarter-wavelength), and the
diameter of the core.
Conventional helical antennas can be made using wire or strip
technology. With strip technology, the radiators of the antenna are etched or
deposited onto a thin, flexible substrate. The radiators are positioned such
that they are parallel to each other, but at an obtuse angle to the sides (or
edges) of the substrate. The substrate is then formed, or rolled, into a
cylindrical, conical, or other appropriate shape causing the strip radiators
to
form a helix.
This conventional helical antenna, however, also has the
characteristic that the radiator lengths are an integer multiple of one
quarter
wavelength of the desired resonant frequency, resulting in an overall
antenna length that is longer than desired for some portable or mobile
applications.
Additionally, in applications where transmit and receive
communications occur at different frequencies, dual-band antennas are
desirable. However, dual-band antennas are often available only in less than
desirable configurations. For example, one way in which a dual band
antenna can be made is to stack two single-band quadrifilar helix antennas
end-to-end, so that they form a single cylinder. A disadvantage of this
solution, however, is that such an antenna is longer than would otherwise
be desired for portable, or hand-held applications.
Another technique for providing dual-band performance has been to
utilize two separate single band antennas. However, for hand-held units,
the two antennas would have to be located in close proximity to one
another. Two single band antennas, placed in close proximity on a portable,
or hand-held unit would cause coupling between the two antennas, leading
to degraded performance as well as unwanted interference.
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
3
SUMMARY OF THE INVENTION
In one aspect the invention provides a helical antenna, comprising: a
substrate; a radiator portion comprising a plurality of radiators disposed o n
said substrate, wherein said substrate is shaped such that said radiators are
configured in a helical fashion; a feed portion, adjacent to said radiator
portion and comprising a substrate; a feed network comprising a first set of
one or more traces disposed on said substrate of said feed portion and a
second set of one or more traces disposed on said substrate of said radiator
portion.
In another aspect the invention provides a feed network, comprising:
a first set of one or more traces disposed on a feed portion of an antenna;
and
a second set of one or more traces disposed on a radiator portion of said
antenna.
In a further aspect the invention provides a dual band helical
antenna, comprising: a first antenna section comprising a first feed network
disposed on a first side of a substrate on a first feed portion of the first
antenna, a first ground plane disposed on a second side of said substrate and
opposite said feed network, and a first set of one or more radiators disposed
on said substrate and extending from said feed network; a second antenna
section comprising a second feed network disposed on said substrate on a
second feed portion, a second ground plane disposed on said substrate
opposite said feed network; a second set of one or more radiators disposed on
said substrate and extending from said feed network; and means for
providing a path for current to flow from said radiators of said second
antenna along the axis of said second antenna to thereby increase the energy
radiated in the directions perpendicular to the axis; wherein said first feed
network comprises a first set of one or more traces disposed on said first
feed
portion of the antenna and a second set of one or more traces disposed on a
radiator portion of said first antenna section, and said second feed network
comprises a third set of one or more traces disposed on said second feed
. portion and a fourth set of one or more traces disposed on a radiator
portion
of said second antenna section.
In a further aspect the invention provides an antenna wherein two
sets of interdigitated tracks are provided on a common substrate which is
formed into a curved surface so that the tracks follow respective
substantially helical paths, and a feed network is coincident with a portion
of
one set of tracks.
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
4
~'he present invention is embodied in a novel and improved feed
network for an antenna which includes a radiator portion and a feed
portion. The feed network is configured such that a section of the feed
network is disposed on the radiator portion of the antenna and the
remainder of the feed network is disposed on the feed portion. Because part
of the feed network is disposed on the radiator portion, the remainder of the
feed network requires less area on the feed portion. As a result, the feed
portion of the antenna can be smaller as compared to antennas having
conventional feed networks. Because this configuration requires less area
on the feed portion, the feed network is said to be area-efficient.
In a preferred embodiment, the traces of the feed network that are
disposed on the radiator portion are disposed opposite the ground portion of
the radiators. As such, the ground portion of the radiators serves as a
ground plane for this part of the feed network.
The feed network can be implemented with numerous different types
of antennas of varying configurations, including single-band and multi-band
helical antennas.
One advantage of the invention is that the overall size of the antenna
and the amount of loss in the feed are reduced as compared to antennas
having conventional feed networks.
In one embodiment, the feed network is implemented with a dual-
band helical antenna having two sets of one or more helically wound
radiators. The radiators are wound, or wrapped, such that the antenna is in
a cylindrical, conical, or other appropriate shape to optimize or otherwise
obtain desired radiation patterns. According to this implementation, one set
of radiators is provided for operation at a first frequency and the second set
is
provided for operation at a second frequency which preferably is different
from the first frequency. Each set of radiators has an associated feed network
to provide the signals to drive the radiators. Thus, the dual-band antenna
can be described as being comprised of two single-band antennas, each single-
band antenna having a radiator portion and a feed portion.
A tab can be provided to feed the signal to the first single-band
antenna. The tab extends from the feed portion of the first single-band
antenna. When the antenna is formed into a cylinder or other appropriate
shape, the tab is aligned with the axis of the antenna. More specifically, in
a
preferred embodiment, the tab extends radially inward to provide a centrally
located feed structure. Thus, the tab and the feed line do not interfere with
the signal patterns of the second single-band antenna.
CA 02284673 1999-09-24
WO 98/44590 PCT/US98105873
BRIEF DESCRIPTION OF THE DRAWINGS
The features, objects, and advantages of the _ present invention will
become more apparent from the detailed description set forth below of an
5 embodiment of the invention when taken in conjunction with the drawings
in which like reference characters identify correspondingly throughout.
Additionally, the left-most digits) of a reference number identifies the
drawing in which the reference first appears.
FIG.1A is a diagram illustrating a conventional wire quadrifilar
helical antenna.
FIG.1B is a diagram illustrating a conventional strip quadrifilar
helical antenna.
FIG. 2A is a diagram illustrating a planar representation of an open-
circuited, or open terminated, quadrifilar helical antenna.
FIG. 2B is a diagram illustrating a planar representation of a short-
circuited quadrifilar helical antenna.
FIG. 3 is a diagram illustrating current distribution on a radiator of a
short-circuited quadrifilar helical antenna.
FIG. 4 is a diagram illustrating a far surface of an etched substrate of a
strip helical antenna.
FIG. 5 is a diagram illustrating a near surface of an etched substrate of
a strip helical antenna.
FIG.6 is a diagram illustrating a perspective view of an etched
substrate of a strip helical antenna.
FIG.7A is a diagram illustrating an open-circuit coupled multi-
segment radiator having five coupled segments according to one
embodiment of the invention.
FIG.7B is a diagram illustrating a pair of short-circuited coupled
multi-segment radiators according to one embodiment of the invention.
FIG. 8A is a diagram illustrating a planar representation of a short-
circuited coupled multi-segment quadrifilar helical antenna according to one
embodiment of the invention.
' FIG 8B is a diagram illustrating a coupled multi-segment quadrifilar
helical antenna formed into a cylindrical shape according to one
embodiment of the invention.
FIG. 9A is a diagram illustrating overlap s and spacing s of radiator
segments according to one embodiment of the invention.
FIG.9B is a diagram illustrating example current distributions on
radiator segments of the coupled multi-segment helical antenna.
CA 02284673 1999-09-24
WO 98144590 PCT/US98/05873
6
FIG.10A is a diagram illustrating two point sources radiating signals
differing in phase by 90°.
FIG.10B is a diagram illustrating field patterns for the point sources
illustrated in FIG.10A.
FIG.10C is a diagram illustrating circular polarization field patterns
for a conventional helical antenna and circular polarization field patterns
for a helical antenna having a feed tab aligned with the axis of the antenna.
FIG.11 is a diagram illustrating the embodiment in which each
segment is placed equidistant from the segments on either side.
FIG.12 is a diagram illustrating an example implementation of a
coupled multi-segment antenna according to one embodiment of the
invention.
FIG.13 is a diagram illustrating planar representations of the surfaces
of a stacked dual-band helical antenna according to one embodiment of the
invention.
FIG.14 is a diagram illustrating planar representations of the surfaces
of a stacked dual-band helical antenna according to one embodiment of the
invention in which the feed points for the radiators are positioned at a
distance from the feed network.
FIG.15 is a diagram illustrating a planar representation of a tab used to
feed one antenna of the stacked dual-band helical antenna according to one
embodiment of the invention.
FIG.16 is a diagram illustrating example dimensions for a stacked
dual-band helical antenna according to one embodiment of the invention.
FIG.17 is a diagram illustrating an example of a conventional
quadrature phase feed network.
FIG.18 is a diagram illustrating a feed network having portions that
extend into the radiators of the antenna according to one embodiment of the
invention.
FIG.19 is a diagram illustrating feed networks along with the signal
traces, including the feed paths, for antennas according to one embodiment
of the invention.
FIG. 20 is a diagram illustrating an outline for the ground plane of
antennas according to one embodiment of the invention.
FIG. 21 is a diagram illustrating both the ground planes and the signal
traces of a dual band antenna superimposed according to one embodiment of
the invention.
CA 02284673 1999-09-24
WO 98144590 PCTIUS98/05873
7
FIG.22A is a diagram illustrating a structure for maintaining an
antenna in a cylindrical or other appropriate shape according to one
embodiment. -
FIGS. 22B-22E are diagrams illustrating the formation of an antenna
in a cylindrical or other appropriate shape according to the embodiment
illustrated in FIG. 22A.
FIG. 23A is a diagram illustrating a form suitable for use in supporting
an antenna in a cylindrical or other appropriate shape according to one
embodiment.
FIGS.23B and 23C are diagrams illustrating the formation of an
antenna in a cylindrical or other appropriate shape according to the
embodiment illustrated in FIG. 23A.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
I. Overview and Discussion of the Invention
The present invention is directed toward an area-efficient feed
network for an antenna. A portion of the feed network is provided on a
radiator portion of the antenna. This decreases the area required for the feed
portion of the antenna.
II. Example Environment
In a broad sense, the invention can be implemented in any system for
which helical antenna technology can be utilized. One example of such an
environment is a communication system in which users having fixed,
mobile and/or portable telephones communicate with other parties through
a satellite communication link. In this example environment, the
telephone is required to have an antenna tuned to the frequency satellite
communication link.
The present invention is described in terms of this example
environment. Description in these terms is provided for convenience only.
It is not intended that the invention be limited to application in this
example environment. In fact, after reading the following description, it
will become apparent to a person skilled in the relevant art how to
implement the invention in alternative environments.
CA 02284673 1999-09-24
WO 98144590 PCT/US98105873
8
III. Conventional Helical Antennas
Before describing the embodiments of the invention in detail, it is
useful to describe the radiator portions of some conventional helical
antennas. Specifically, this section of the document describes radiator
portions of some conventional quadrifilar helical antennas. FIGS.1A and
1B are diagrams illustrating a radiator portion 100 of a conventional
quadrifilar helical antenna in wire form and in strip form, respectively. The
radiator portion 100 illustrated in FIGS. 1A and 1B is that of a quadrifilar
20 helical antenna, meaning it has four radiators 104 operating in phase
quadrature. As illustrated in FIGS.1A and 1B, radiators 104 are wound to
provide circular polarization.
FIGS. 2A and 2B are diagrams illustrating planar representations of a
radiator portion of conventional quadrifilar helical antennas. In other
words, FIGS. 2A and 2B illustrate the radiators as they would appear if the
antenna cylinder were "unrolled" on a flat surface. FIG. 2A is a diagram
illustrating a quadrifilar helical antenna which is open-circuited, or open
terminated, at the far end. For such a configuration, the resonant length .~
of
the radiators 208 is an odd integer multiple of a quarter-wavelength of the
desired resonant frequency.
FIG. 2B is a diagram illustrating a quadrifilar helical antenna which is
short-circuited, or electrically connected, at the far end. In this case the
resonant length .~ of radiators 208 is an even integer multiple of a quarter
wavelength of the desired resonant frequency. Note that in both cases, the
stated resonant length .2 is approximate, because a small adjustment is
usually needed to compensate for non-ideal short and open terminations.
FIG. 3 is a diagram illustrating a planar representation of a radiator
portion of a quadrifilar helical antenna 300, which includes radiators 208
having a length .~ _ ~,~2, where ~, is the wavelength of the desired resonant
frequency of the antenna. Curve 304 represents the relative magnitude of
current for a signal on a radiator 208 that resonates at a frequency of f =
v/a,
where v is the velocity of the signal in the medium.
Example implementations of a quadrifilar helical antenna
implemented using printed circuit board techniques (a strip antenna) are
described in more detail with reference to FIGS. 4 - 6. The strip quadrifilar
helical antenna is comprised of strip radiators 104A-104D etched onto a
dielectric substrate 406. The substrate is a thin flexible material that is
rolled
into a cylindrical, conical or other appropriate shape such that radiators
104A-104D are helically wound about a central axis of the cylinder.
CA 02284673 1999-09-24
WO 98/44590 PCT/US98105873
9
FIGS. 4 - 6 illustrate the components used to fabricate a quadrifilar
helical antenna 100. FIGS. 4 and 5 present a view of a far surface 400 and
near surface 500 of substrate 406, respectively. The antenna 100 includes a
radiator portion 404, and a feed portion 408.
In the embodiments described and illustrated herein, the antennas are
described as being made by forming the substrate into a cylindrical shape
with the near surface being on the outer surface of the formed cylinder. In
alternative embodiments, the substrate is formed into the cylindrical shape
with the far surface being on the outer surface of the cylinder.
In one embodiment, dielectric substrate I00 is a thin, flexible layer of
polytetraflouroethalene (PTFE), a PTFE/glass composite, or other dielectric
material. In one embodiment, substrate 406 is on the order of 0.005 in., or
0.13 mm thick, although other thicknesses can be chosen. Signal traces and
ground traces are provided using copper. In alternative embodiments, other
conducting materials can be chosen in place of copper depending on cost,
environmental considerations and other factors.
In the embodiment illustrated in FIG. 5, feed network 508 is etched
onto feed portion 408 to provide the quadrature phase signals (i.e., the
0°,
90°, 180° and 270° signals) that are provided to
radiators 104A-104D. Feed
portion 408 of far surface 400 provides a ground plane 412 for feed circuit
508.
Signal traces for feed circuit 508 are etched onto near surface 500 of feed
portion 408.
For purposes of discussion, radiator portion 404 has a first end 432
adjacent to feed portion 408 and a second end 434 (on the opposite end of
radiator portion 404). Depending on the antenna embodiment
implemented, radiators 104A-104D can be etched into far surface 400 of
radiator portion 404. The length at which radiators 104A-104D extend from
first end 432 toward second end 434 is approximately an integer multiple of a
quarter wavelength of the desired resonant frequency.
In such an embodiment where radiators I04A-104D are an integer
multiple of ~,~2 radiators 104A-104D are electrically connected to each other
(i.e., shorted, or short circuited) at second end 434. This connection can be
made by a conductor across second end 434 which forms a ring 604 around
the circumference of the antenna when the substrate is formed into a
cylinder. FIG. 6 is a diagram illustrating a perspective view of an etched
substrate of a strip helical antenna having a shorting ring 604 at second end
434.
One conventional quadrifilar helical antenna is described in U.S.
Patent No. 5,198,831 to Burrell et. al. (referred to as the '831 patent),
which is
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
incorporated herein by reference. The antenna described in the '83I patent is
a printed circuit board antenna having the antenna radiators etched or
otherwise deposited on a dielectric substrate. The substrate is formed into a
cylinder resulting in a helical configuration of the radiators.
5 Another conventional quadrifilar helical antenna is disclosed in U.S.
Patent No. 5,255,005 to Terret et al (referred to as the '005 patent) which is
incorporated herein by reference. The antenna described in the '005 patent is
a quadrifilar helical antenna formed by two bifilar helices positioned
orthogonally and excited in phase quadrature. The disclosed antenna also
10 has a second quadrifilar helix that is coaxial and electromagnetically
coupled
with the first helix to improve the passband of the antenna.
Yet another conventional quadrifilar helical antenna is disclosed in
U.S. Patent No. 5,349,365, to Ow et al (referred to as the '365 patent) which
is
incorporated herein by reference. The antenna described in the '365 patent is
a quadrifilar helical antenna designed in wireform as described above with
reference to FIG. lA.
IV. Coupled Multi-Segment Helical Antenna
In order to reduce the length of radiator portion 100 of the antenna,
one form of helical antenna utilizes coupled multi-segment radiators that
allow for resonance at a given frequency at shorter lengths than would
otherwise be needed for a helical antenna with an equivalent resonant
length.
FIGS. 7A and 7B are diagrams illustrating planar representations of
example embodiments of coupled-segment helical antennas. FIG.7A
illustrates a coupled mufti-segment radiator 706 terminated in an open
circuit according to one single-filar embodiment. An antenna terminated in
an open-circuit such as this may be used in a single-filar, bifilar,
quadrifilar,
or other x-filar implementation.
The embodiment illustrated in FIG.7A is comprised of a single
radiator 706. Radiator 706 is comprised of a set of radiator segments. This
set
is comprised of two end segments 708, 710 and p intermediate segments 712,
where p = 0, 1, 2, 3 . . . (the case where p = 3 is illustrated). Intermediate
segments are optional (i.e., p can equal zero). End segments 708, 710 are
physically separate from but electromagnetically coupled to one another.
Intermediate segments 712 are positioned between end segments 708, 710
and provide electromagnetic coupling between end segments 708, 7I0.
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
11
In the open-terminated embodiment, the length .BSI of segment 708 is
an odd-integer multiple of one-quarter wavelength of the desired resonant
frequency. The length .~52 of segment 710 is an integer multiple of one-half
the wavelength of the desired resonant frequency. The length ~Sp of each of
the p intermediate segments 7I2 is an integer multiple of one-half the
' wavelength of the desired resonant frequency. In the illustrated
embodiment, there are three intermediate segments 712 (i.e., p = 3).
FIG.7B illustrates radiators 706 of the helical antenna when
terminated in a short circuit 722. This short-circuited implementation is not
suitable for a single-filar antenna, but can be used for bifilar, quadrifilar
or
other x-filar antennas. As with the open-circuited embodiment, radiators
706 are comprised of a set of radiator segments. This set is comprised of two
end segments 708, 710 and p intermediate segments 712, where p = 0,1, 2, 3 . .
. (the case where p = 3 is illustrated). Intermediate segments are optional
{i.e., p can equal zero). End segments 708, 710 are physically separate from
but electromagnetically coupled to one another. Intermediate segments 7I2
are positioned between end segments 708, 710 and provide electromagnetic
coupling between end segments 708, 710.
In the short-circuited embodiment, the length .ESZ of segment 708 is an
odd-integer multiple of one-quarter wavelength of the desired resonant
frequency. The length ~SZ of segment 710 is an odd-integer multiple of one
quarter wavelength of the desired resonant frequency. The length ~Sp of each
of the p intermediate segments 712 is an integer multiple of one-half the
wavelength of the desired resonant frequency. In the illustrated
embodiment, there are three intermediate segments 712 {i.e., p = 3).
FIGS. 8A and SB are diagrams illustrating a coupled multi-segment
quadrifilar helical antenna radiator portion 800 according to one
embodiment of the invention. FIGS. 8A and 8B illustrate one example
implementation of the antenna illustrated in FIG. 7B, where p = zero (i.e.,
there are no intermediate segments 712) and the lengths of segments 708, 710
are one-quarter wavelength.
. The radiator portion 800 illustrated in FIG. BA is a planar
representation of a quadrifilar helical antenna, having four coupled
radiators 804. Each coupled radiator 804 in the coupled antenna is actually
comprised of two radiator segments 708, 710 positioned in close proximity
with one another such that the energy in radiator segment 708 is coupled to
the other radiator segment 710.
CA 02284673 1999-09-24
WO 98/44590 PCTILIS98/05873
12
More specifically, according to one embodiment, radiator portion 800
can be described in terms of having two sections 820, 824. Section 820 is
comprised of a plurality of radiator segments 708 extending from a first end
832 of the radiator portion 800 toward the second end 834 of radiator portion
800. Section 824 is comprised of a second plurality of radiator segments 710
extending from second end 834 of the radiator portion 800 toward first end
832. Toward the center area of radiator portion 800, a part of each segment
708 is in close proximity to an adjacent segment 710 such that energy from
one segment is coupled into the adjacent segment in the area of proximity.
This is referred to in this document as overlap.
In a preferred embodiment, each segment 708, 710 is of a length of
approximately .Pl = .~2 = ~,/4. The overall length of a single radiator
comprising two segments 708, 710 is defined as tot' The amount one
segment 708 overlaps another segment 710 is defined as 8 = .~1 + ~2 - ~tot~
For a resonant frequency f = v/~, the overall length of a radiator tot is
less than the half-wavelength length of ~,/2. In other words, as a result of
coupling, a radiator, comprising a pair of coupled segments 708, 710,
resonates at frequency f = v/~, even though the overall length of that
radiator is less than a length of 7~/2. Therefore, the radiator portion 800 of
a
1/2 wavelength coupled mufti-segment quadrifilar helical antenna is shorter
than the radiator portion of conventional half-wavelength quadrifilar
helical antenna 800 for a given frequency f .
For a clearer illustration of the reduction in size gained by using the
coupled configuration, compare the radiator portions 800 illustrated in FIG 8
with those illustrated in FIG. 3. For a given frequency f = v/~,, the length
.~ of
radiator portion 300 of the conventional antenna is ~,/2, while the length tot
of radiator portion 800 of the coupled radiator segment antenna is less than
~/2~
As stated above, in one embodiment, segments 708, 710 are of a length
~1 = .EZ = ~,/4. The length of each segment can be varied such that .~1 is not
necessarily equal to .~2, and such that they are not equal to ~,/4. The actual
resonant frequency of each radiator is a function of the length of radiator
segments 708, 710 the separation distance s between radiator segments 708,
710 and the amount by which segments 708, 710 overlap each other.
Note that changing the length of one segment 708 with respect to the
other segment 710 can be used to adjust the bandwidth of the antenna. For
example, lengthening .~1 such that it is slightly greater than ~,/4 and
CA 02284673 1999-09-24
WO 98!44590 PCT/US98/05873
13
shortening .~2 such that it is slightly shorter than ~,~4 can increase the
bandwidth of the antenna.
FIG. 8B illustrates the actual helical configuration of a coupled multi-
segment quadrifilar helical antenna according to one embodiment of the
. 5 invention. This illustrates how each radiator is comprised of two segments
708, 710 in one embodiment. Segment 708 extends in a helical fashion from
first end 832 of the radiator portion toward second end 834 of the radiator
portion. Segment 710 extends in a helical fashion from second end 834 of
the radiator portion toward first end 832 of the radiator portion. FIG. 8B
further illustrates that a portion of segments 708, 710 overlap such that they
are electromagnetically coupled to one another.
FIG.9A is a diagram illustrating the separation s and overlap b
between radiator segments 708, 710. Separation s is chosen such that a
sufficient amount of energy is coupled between the radiator segments 708,
710 to allow them to function as a single radiator of an effective electrical
length of approximately ~,~2 and integer multiples thereof.
Spacing of radiator segments 708, 710 closer than this optimum
spacing results in greater coupling between segments 708, 710. As a result,
for a given frequency f the length of segments 708, 710 must increase to
enable resonance at the same frequency f . This can be illustrated by the
extreme case of segments 708, 710 being physically connected (i.e., s = 0). In
this extreme case, the total length of segments 708, 710 must equal ~,~2 for
the
antenna to resonate. Note that in this extreme case, the antenna is no longer
really 'coupled' according to the usage of the term in this specification, and
the resulting configuration is actually that of a conventional helical antenna
such as that illustrated in FIG. 3.
Similarly, increasing the amount of overlap 8 of segments 708, 710
increases the coupling. Thus as overlap S increases, the length of segments
708, 710 increases as well.
To qualitatively understand the optimum overlap and spacing for
segments 708, 710, refer to FIG. 9B. FIG. 9B represents a magnitude of the
current on each segment 708, 710. Current strength indicators 911, 928
illustrate that each segment ideally resonates at ~,~4, with the maximum
signal strength at the outer ends and the minimum at the inner ends.
To optimize antenna configurations for the coupled radiator segment
antenna, the inventors utilized modeling software to determine correct
segment length .~1, .~2, overlap 8, and spacing s among other parameters. One
such software package is the Antenna Optimizer (AO) software package. AO
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
14
is based on a method of moments electromagnetic antenna-modeling
algorithm. AO Antenna Optimizer version 6.35, copyright 1994, was written
by and is available from Brian Beezley, of San Diego, California.
Note that there are certain advantages obtained by using a coupled
configuration as described above with reference to FIGS. SA and SB. With
both the conventional antenna and the coupled radiator segment antenna,
current is concentrated at the ends of the radiators. Pursuant to array factor
theory, this can be used to an advantage with the coupled radiator segment
antenna in certain applications.
To explain, FIG.10A is a diagram illustrating two point sources, A, B,
where source A is radiating a signal having a magnitude equal to that of the
signal of source B but lagging in phase by 90° (the e1 wt convention is
assumed). Where sources A and B are separated by a distance of ~,~4, the
signals add in phase in the direction traveling from A to B and add out of
phase in the direction from B to A. As a result, very little radiation is
emitted in the direction from B to A. A typical representative field pattern
shown in FIG. 10B illustrates this point.
Thus, when the sources A and B are oriented such that the direction
from A to B points upward, away from the ground, and the direction from B
to A points toward the ground, the antenna is optimized for most
applications. This is because it is rare that a user desires an antenna that
directs signal strength toward the ground. This configuration is especially
useful for satellite communications where it is desired that the majority of
the signal strength be directed upward, away from the ground.
The point source antenna modeled in FIG.10A is not readily
achievable using conventional half wavelength helical antennas. Consider
the antenna radiator portion illustrated in FIG.3. The concentration of
current strength at the ends of radiators 208 roughly approximates a point
source. When radiators are twisted into a helical configuration, one end of
the 90° radiator is positioned in line with the other end of the
0° radiator.
Thus, this approximates two point sources in a line. However, these
approximate point sources are separated by approximately ~,~2 as opposed to
the desired ~,~4 configuration illustrated in FIG.10A.
Note, however that the coupled radiator segment antenna embodying
the invention provides an implementation where the approximated point
sources are spaced at a distance closer to ~.~4. Therefore, the coupled
radiator
segment antenna allows users to capitalize on the directional characteristics
of the antenna illustrated in FIG.10A.
CA 02284673 1999-09-24
WO 98/44590 PCTIUS98/05873
The radiator segments 708, 710 illustrated in FIG. 8 show that segment
708 is very near its associated segment 710, yet each pair of segments 708,
710
are relatively far from the adjacent pair of segments. In one alternative
embodiment, each segment 710 is placed equidistant from the segments 708
5 on either side. This embodiment is illustrated in FIG.11.
Referring now to FIG.11, each segment is substantially equidistant
from each pair of adjacent segments. For example, segment 708B is
equidistant from segments 710A, 710B. That is, s~ = s2. Similarly, segment
710A is equidistant from segments 708A, 708B.
10 This embodiment is counterintuitive in that it appears as if unwanted
coupling would exist. In other words, a segment corresponding to one phase
would couple not only to the appropriate segment of the same phase, but
also to the adjacent segment of the shifted phase. For example, segment
708B, the 90° segment would couple to segment 710A (the 0°
segment) and to
15 segment 710B (the 90° segment). Such coupling is not a problem
because the
radiation from the top segments 710 can be thought of as two separate
modes. One mode resulting from coupling to adjacent segments to the left
and the other mode from coupling to adjacent segments to the right.
However, both of these modes are phased to provide radiation in the same
direction. Therefore, this double-coupling is not detrimental to the
operation of the coupled multi-segment antenna.
FIG.12 is a diagram illustrating an example implementation of a
coupled radiator segment antenna. Referring now to FIG.12, the antenna
comprises a radiator portion 1202 and a feed portion 1206. Radiator portion
includes segments 708, 710. Dimensions provided in FIG.12 illustrate the
contribution of segments 708, 710 and the amount of overlap b to the overall
length of radiator portion 1202.
The length of segments in a direction parallel to the axis of the
cylinder is illustrated as .~lsina for segments 708 and .~2sina for segments
710,
where a is the inside angle of segments 708, 710.
Segment overlap as illustrated above in FIGS. SA and 9A, is illustrated
by the reference character S. The amount of overlap in a direction parallel to
the axis of the antenna is given by &sina , as illustrated in FIG. 12.
Segments 708, 710 are separated by a spacing s, which can vary as
described above. The distance between the end of a segment 708, 710 and the
end of radiator portion 1202 is defined as the gap and illustrated by the
reference characters ~yl, y2, respectively. The gaps ~yl, 'y2 can, but do not
have
CA 02284673 1999-09-24
WO 98144590 PCT/US98/05873
16
to be, equal to each other. Again, as described above, the length of segments
708 can be varied with respect to that of segments 710.
The amount of offset of a segment 710 from ~ one end to the next is
illustrated by the reference character c~a. The separation between adjacent
segments 710 is illustrated by the reference character ceps, and is determined
by
the helix diameter.
Feed portion 1206 includes an appropriate feed network to provide the
quadrature phase signals to the radiator segments 708. Feed networks are
well known to those of ordinary skill in the art and are thus not described i
n
detail herein.
In the example illustrated in FIG.12, segments 708 are fed at a feed
point that is positioned along each segment 708 a distance from the feed
network that is chosen to optimize impedance matching. In the
embodiment illustrated in FIG.12, this distance is illustrated by the
reference
characters s feed'
Note that continuous line 1224 illustrates the border for a ground
portion on the far surface of the substrate. The ground portion opposite
segments 708 on the far surface extends to the feed point. The thin portion
of segments 708 is on the near surface. At the feed point, the thickness of
segments 708 on the near surface increases.
Dimensions are now provided for an example coupled radiator
segment quadrifilar helical antenna suitable for operation in the L-Band at
approximately 1.6 GHz. Note that this is an example only and other
dimensions are possible for operation in the L-Band. Additionally, other
dimensions are possible for operation in other frequency bands as well.
The overall length of radiator portion 1202 in the example L-Band
embodiment is 2.30 inches (58.4 mm). In this embodiment, the pitch angle a
is 73 degrees. With this angle a, the length of segments 708 .~lsina for this
embodiment is 1.73 inches {43.9 mm). In the embodiment illustrated, the
length of segments 710 is equal to the length of segments 708.
In one example, segment 710 is positioned substantially equidistant
from its adjacent pair of segments 708. In one implementation of the
embodiment where segments 710 are equidistant from adjacent segments
708, the spacing sl = s2 = 0.086 inches. Other spacings are possible
including,
for example, the spacing s of segments 710 at 0.070 inches {1.8 mm) from an
adjacent segment 708.
The width ~ of radiator segments 708, 710 is 0.11 inches (2.8 mm) in
this embodiment. Other widths are possible.
.... .. . ~ . ..
CA 02284673 1999-09-24
WO 98/44590 PCTIUS98/05873
17
The example L-Band embodiment features a symmetric gap yl = y2 -
0.57 inches (14.5 mm). Where the gap y is symmetric for both ends of the
radiator portion 1202 (i.e., where yl = y2), the radiators 708, 710 have an
overlap 8sina of 1.16 inches (29.5 mm) (1.73 inches - 0.57 inches).
The segment offset cep is 0.53 inches and the segment separation ws is
0.393 inches (10.0 mm). The diameter of the antenna is 4c~s/~.
In one embodiment, this is chosen such that the distance S feed from
the feed point to the feed network is s feed - 1.57 inches (39.9 mm). Other
feed points can be chosen to optimize impedance matching.
Note that the example embodiment described above is designed for
use in conjunction with a 0.032 inch thick polycarbonate radome enclosing
the helical antenna and contacting the radiator portion. It will become
apparent to a person skilled in the art how a radome or other structure
affects the wavelength of a desired frequency.
I5 Note that in the example embodiments just described, the overall
length of the L-Band antenna radiator portion is reduced from that of a
conventional half-wavelength L-Band antenna. For a conventional half-
wavelength L-Band antenna, the length of the radiator portion is
approximately 3.2 inches (i.e., ~,~2(sina)), where a is the inside angle of
segments 708, n0 with respect to the horizontal), or (81.3 mm). For the
example embodiments described above, the overall length of the radiator
portion 1202 is 2.3 inches (58.42 mm). This represents a substantial savings
in size over the conventional antenna.
V. Stacked Dual-Band Helical Antenna
Having thus described several embodiments of a single-band helical
antenna, a dual-band helical antenna embodying the present invention is
now described. The present invention is directed toward a dual-band helical
antenna capable of resonating at two different operating frequencies. Two
helical antennas are stacked end to end, with one antenna resonating at a
first frequency and the other antenna resonating at a second frequency. Each
antenna has a radiator portion comprised of one or more helically-wound
radiators. Each antenna also has a feed portion comprised of a feed network
and a ground plane. The two antennas are stacked such that the ground
plane of one antenna is used as a shorting ring across the far end of the
radiators of the other antenna.
CA 02284673 1999-09-24
WO 98144590 PCT/US98/05873
18
FIG.13 is a diagram illustrating planar representations of far surface
400 and near surface 500 of a dual-band helical antenna according to one
embodiment of the invention. The dual-band helical antenna is comprised
of two single-band helical antennas: helical antenna 1304 operating at a first
resonant frequency and helical antenna 1308 operating at a second resonant
frequency.
In the embodiment illustrated in FIG.13, feed network 508, radiators
104A-104D and first antenna 1304 are disposed on near surface 500 of first
antenna 1304. Also disposed on near surface 500 is the ground plane 412 for
the feed network 508 of second antenna 1308. On far surface 400 are feed
network 508 and radiators 104A-104D of second antenna 1308 as well as
ground plane 412 for the feed portion of first antenna 1304.
As discussed above with reference to FIGS.2A and 2B, where the
resonant length .~ of radiators 104A-104D is an even integer multiple of a
quarter-wavelength of the desired resonant frequency, the far end of the
radiators 104A-104D is shorted. . As illustrated in FIG.13, this shorting is
accomplished using ground plane 412 of first antenna 1304. As a result of
this configuration, an additional shorting ring does not need to be added to
the end of radiators 104A-104D.
Note that in the embodiment illustrated in FIG. I3, first antenna 1304
is illustrated as resonating at odd integer multiples of a quarter-wavelength
of the desired resonant frequency because the ends of radiators 104A-104D
are open circuited. In an alternative embodiment, a shorting ring (not
illustrated) could be added to the far end of radiators 104A-104D of first
antenna 1304, while changing the length of these radiators 104A-104D such
that they are an even-integer multiple of a quarter-wavelength of the desired
resonant frequency.
Radiators 104A-104D of the dual-band antenna described with
reference to FIG.13 are illustrated as being fed at a first end near feed
network 508. It is well known that a feed point of radiators 104A-104D of the
helical antenna can be positioned at any point along the length of radiators
104A-104D where such positioning is primarily determined based on
impedance matching considerations. FIG.14 is a diagram illustrating one
embodiment of a dual-band helical antenna in which the feed points of
radiators 104A-104D are positioned at a predetermined distance from feed
network 508. Specifically, in the embodiment illustrated in FIG.14, a feed
point A of first antenna 1304 is positioned at a distance .~~ED1 fr'°m
feed
network 508 and feed point B of second antenna 2308 is positioned at a
distance ~FEED2 from feed network 508.
CA 02284673 1999-09-24
WO 98/44590 PCT/US98105873
19
This embodiment illustrates that radiators 104A-104D are comprised
of a ground trace 1436 on a first surface of the substrate 406, a feed trace
1438
on a second surface of substrate 406 and opposite said ground trace 1436, and
a radiator trace 1440 on the second surface of substrate 406.
As with the embodiment illustrated in FIG.13, in this embodiment,
ground plane 412 of first antenna 1304 serves as a shorting ring for radiators
104A-104D and second antenna 1308 such that the radiators of second
antenna 1308 resonate at an even integer multiple of a quarter-wavelength
of the desired resonant frequency.
In order to decrease the overall length of the stacked antenna, the
edge-coupled technology discussed above can be utilized. In such
embodiments, radiators 104A-104D of first antenna 1304 and/or second
antenna 1308 as illustrated in FIGS.13 and 14 are replaced with edge-coupled
radiators as illustrated, for example, in FIG.12.
One challenge of providing a dual-band antenna such as that
illustrated in FIGS. 13 and 14 is that of feeding first antenna 1304. To this
end, first antenna 1304 is fed by means of a tab extending from the lower area
of the feed portion of first antenna 1304.
FIG.15 is a diagram illustrating such a tab used to feed first antenna
1304. Referring now to FIG.15, a tab 1504 extends from the side of the feed
portion of first antenna 1304 on substrate 406. In the embodiment illustrated
in FIG.15, tab 1504 is approximately "L" shaped such that it extends
horizontally from the feed portion of first antenna 1304 at a given distance
and is then angled axially through the center in the direction of the feed
portion of second antenna 1308. Although 1504 is illustrated as being shaped
with a right angle, other angles could be used as could curves of various
radii.
Ideally, when substrate 406 is rolled into a cylinder or other
appropriate shape to form the helical antenna, axial component 1524 of tab
1504 is substantially along the axis of the dual-band helical antenna. Having
axial component 1524 of tab 1504 coincident with the axis of the helical
antenna minimizes the impact of this member on the radiation patterns of
the antenna. As illustrated in FIG.15, in a preferred embodiment, tab 2504
extends from feed portion of first antenna 1304 at a vertical position that is
as
far as possible from first antenna 1304. This is done to minimize the effect
of
tab 1504 on the radiation patterns of first antenna 1304. Because second
antenna 1308 is a coupled-segment one-half wavelength antenna and the
ends of radiators 104A-104D of second antenna 1308 are shorted by ground
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
plane 412 of first antenna 1304, tab 1504 has a minimal effect on the
radiation
patterns of second antenna 1308.
Preferably, the length .~~ of feed portion 1206 of first antenna 1304 can
be determined by considering two factors at the appropriate operating
5 frequency. First, it is desirable to minimize the amount of current flowing
from the radiators of first antenna 1304 to second antenna 1308, and vice
versa. In other words, it is desirable to achieve isolation between the two
antennas. This can be accomplished by ensuring that the length is great
enough such that the currents do not extend form one set of radiators to the
10 other at the frequency of interest.
Another challenge is the goal of not allowing current from radiators
104A-D of first antenna 1304 from reaching tab 1504. Currents from first
antenna 1304 are attenuated as they travel across the feed portion of first
antenna 1304 toward tab 1504. Tab 1504 creates an asymmetrical
15 discontinuity in these currents. Therefore, it is desired to minimize the
magnitude of the currents reaching tab 1504 to the extent practical.
After reading this description, it will become apparent to a person
skilled in the art how to implement feed portion 1206 of appropriate length
.~~ based on the materials used, the frequencies of interest, the expected
20 power levels in the antenna, and other known factors. This decision may
also entail a tradeoff between size and performance.
Note that the effects of tab 1504 are not non-existent in this
embodiment. Because tab 1504 is close to the radiators of second antenna
1308, some current from second antenna 1308 is coupled into tab 1504, and,
therefore, along the axis of the antenna. This current affects the radiation
of
second antenna 1308, resulting in increased radiation to the sides of the
antenna. For applications where the antenna is mounted vertically, this
results in increased radiation in the direction of the horizon and decreased
radiation in the vertical direction. As a result, this application is well-
suited
for satellite communication systems where low-earth-orbiting satellites are
used to relay communications from or to the communication device.
This effect is illustrated in FIG.10C, where circular polarization
radiation pattern 1010 is a representation of a typical radiation pattern for
a
conventional helical antenna, and radiation pattern 1020 is a representation
of a radiation pattern for second antenna 1308. As FIG.10C illustrates,
pattern 1020 is "flatter" and "wider" than conventional pattern 1010.
To enable coupling of a signal to first antenna 1304, tab 1504 includes a
connector such as a crimp or solder connector or other connector suitable for
making a connection between a feed cable and the signal trace on tab 1504.
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
21
Various types of cable or wire can be used to connect transceiver RF circuitry
to the antenna at tab /504. Preferably a low loss flexible or semi-rigid cable
is
utilized. Of course, as is well known in the antenna art, it is desired to
match the impedance of the feed input with that of the interface cable to
maximize power transfer to the antenna. However, if the input transition is
poor, the radiation patterns will still be symmetric, only their gains will be
lowered by the corresponding amount of reflection loss. In addition to a low
insertion loss, it is also important that the connector provide a sturdy
mechanical connection between the cable and tab 1504.
Also illustrated in FIG.15 is the outline for an example substrate
shape. After reading this description, it will become apparent to a person
skilled in the art how to implement the antenna with a tab 1504 utilizing
substrates having other shapes.
FIG. 16 is a diagram illustrating one embodiment of a stacked antenna
with example dimensions. In this embodiment, first antenna 1304 is an L
band antenna and second antenna 1308 is an S-band antenna. In this
embodiment, S-band antenna 1308 is an edge-coupled antenna wherein each
radiator 104 is comprised of two segments. Note that this embodiment is
provided for example only. Alternative frequency bands can be chosen for
operation. Also note that either first antenna 1304 or second antenna 1308 or
both could utilize the edge-coupled technology.
Example dimensions are now described for the L-band and S-band
antenna illustrated in FIG.16. The radiating aperture of the L-band antenna
is a total axial height of 1.253 inches, while the S-band aperture is a total
height of 1.400 inches. In this embodiment, the height of feed portion 412 of
first antenna 1304 is 0.400 inches. This yields a total radiating aperture of
3.093 inches. The inclination angle of radiators 104A-104D is 65°.
The above dimensions are provided by way of example only. As
discussed above with reference to conventional helical antennas, the overall
length of radiators I04A-I04D determines the precise resonating frequency of
the antenna. The resonating frequency is important because the highest.
average gains and the most symmetric patterns occur at the resonant
frequency. If the antenna is made longer, the resonating frequency shifts
down. Conversely, if the antenna is made shorter, the resonating frequency
shifts up. The percentage of the frequency shift is approximately
proportional to the percentage that the radiators 104A-104D are lengthened
or shortened. At L-band operating frequencies, roughly 1 mm of length in
the direction of the antenna axis corresponds to 1 MHz.
CA 02284673 1999-09-24
WO 98/44590 PCT/US98I05873
22
In the illustrated embodiment, both first antenna 1304 and second
antenna 1308 have four excited filar arms, or radiators 104A-204D. Each of
these radiators 104A-104D are fed in phase quadrature. The quadrature
phase excitation of four radiators 104A-104D for each antenna 1304, 1308 is
implemented using a feed network. While conventional feed networks
capable of providing quadrature phase excitation can be implemented, a
preferred feed network is discussed in detail below.
Another important dimension is the feedpoint axial length. The
feedpoint axial length defines the distance of the feedpoint from the feed
network for embodiments where the feedpoint is positioned along radiators
104A-104D as illustrated in FIG.13. The feedpoint axial length dimension
indicates the position at which the microstrip flares out to continue the
radiator and is actually the feedpoint position for the entire radiator 104.
In
the example illustrated in FIG.16, the feedpoint length for first antenna 1304
is 1.133 inches. The feedpoint length for second antenna 1308 is 0.638 inches.
These dimensions yield 50 ohm impedances at 1618 and 2492 MHz,
respectively. If the feedpoint position is shifted lower, the impedance is
lower. Conversely, if the feedpoint position is shifted higher, the impedance
is higher. It is important to note that when the overall radiator length is
being adjusted to tune the frequency, the feedpoint position should also be
shifted by a proportional amount in the direction along the axis of the
antenna to maintain the correct impedance match.
Preferably, the antenna having dimensions as illustrated in FIG.16 is
rolled into a cylinder having a diameter of 0.500 inches.
VI. Feed Network
The helical antennas described in this document can be implemented
using a mono-filar, quadrifilar, octafilar or other x-filar configuration. A
feed network is utilized to provide the signals to the filars at the necessary
phase angle. The feed network splits the signal and shifts the phase
provided to each filar. The configuration of the feed network is dependent
on the number of filars. For example, for a quadrifilar helical antenna, the
feed network provides four equal-power signals in a quadrature phase
relationship (i.e., 0, 90, 180, and 270 degrees).
To conserve space on the feed portion of the antenna a unique feed
network layout may be utilized. The traces of the feed network extend into
one or more radiators 104A-104D of the antenna. For convenience, the feed
network is described in terms of a feed network designed to provide four
r._ .4.~..... , , .
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
23
equal-power signals in a quadrature phase relationship. After reading this
description, it will become apparent to a person skilled in the relevant art
how to implement the feed network for other x-filar configurations.
FIG.17 illustrates the electrical equivalent of a conventional
quadrature phase feed network. For conventional quadrature phase feed
networks, the network provides four equal-power signals, each separated i n
phase by 90 degrees. The signal is provided to the feed network via a first
signal path 1704. At a first signal point A (referred to as a secondary feed
point), the 0-degree phase signal is provided to a first radiator 104. At
signal
point B, the 90-degree phase signal is provided to a second radiator 104. At
signal points C and D, the 180- and 270-degree phase signals are provided to
third and fourth radiators 104.
Signals A and B are combined at a point P2 to yield a 25-ohm
impedance. Likewise, signals C and D are combined at a point P3 to yield a
25-ohm impedance. These signals are combined at P1 to yield a 12.5-ohm
impedance. Therefore, a 25-ohm, 90-degree transformer is placed at the
input to convert this impedance to 50-ohms. Note that in the network
illustrated in FIG.17, part of the transformer is placed before the P1 split
to
shorten the feed and also to decrease losses. However, because it is before
the split, it must be twice the impedance after the split.
The conventional feed network is modified such that the traces of the
feed network are disposed on portions of the substrate defined for radiators
104A-104D. Specifically, in a preferred embodiment, these traces are disposed
on the substrate in an area which is opposite from the ground traces of the
one or more of the radiators 104A-104D.
FIG.18 is a diagram illustrating an example embodiment of the feed
network in a quadrifilar helical antenna environment. Specifically, in the
example illustrated in FIG.18, two feed networks are illustrated: a first feed
network 1804 for implementation with first antenna 1304; and a second feed
network 1808 for implementation with second antenna 1308. Feed networks
1804, 1808 have points A, B, C, and D, for providing the 0, 90, 180, and 270-
degree signals to radiators 104A-104D. The dashed lines provided on FIG.18
approximately illustrate an outline for the ground plane of radiators 104A-
104D on a surface of the substrate opposite the surface on which feed
networks 1804,1808 are disposed. Thus, FIG.18 illustrates those portions of
feed networks 1804, 1808 which are disposed on, or extend into, radiators
104A-104D.
Note that according to conventional wisdom, the feed network is
provided on an area that is designated for the feed network and that is
CA 02284673 1999-09-24
WO 98/44590 PCTIUS98/05873
24
separate from the radiators. In contrast, the feed network described herein is
laid out such that a portion of the feed network is deposited on the radiator
portion of the antenna. As such, the feed portion of the antenna can be
reduced in size in comparison to the feed portion for a conventional feed
networks.
FIG.19 is a diagram illustrating feed networks 1804, 1808 along with
the signal traces, including the feed paths, for antennas 1304, 1308. FIG. 20
illustrates an outline for the ground plane of antennas 1304, 1308. FIG. 21 is
a diagram illustrating both the ground planes and the signal traces
superimposed.
An advantage of these feed networks is that the area required for the
feed portion of the antenna to implement a feed network is reduced over
conventional feeding techniques. This is because portions of the feed
network which would otherwise be disposed on the feed portion of the
antenna are now disposed on the radiator portion of the antenna. As a
result of this, the overall length of the antenna can be reduced.
An additional advantage of such a feed network is that because the
secondary feed point is moved closer to the feed point of the antenna,
transmission line loss is decreased. Additionally, a transformer can be
integrated into the routing line of the feed network for impedance
matching.
Thus, an area-efficient network is configured such that a section of the
feed network is disposed on a radiator portion of an antenna and the
remainder of the feed network is disposed on a feed portion. Because part of
the feed network is disposed on the radiator portion, the remainder of the
feed network requires less area on the feed portion. As a result, the feed
portion of the antenna can be smaller as compared to antennas having
conventional feed networks. Preferably, the traces of the feed network that
are disposed on the radiator portion are disposed opposite the ground
portion of the radiators. As such, the ground portion of the radiators serves
as a ground plane for this part of the feed network. The area-efficient feed
network can be implemented with numerous different types of antennas of
varying configurations, including single-band and multi-band helical
antennas. As a result of this configuration, the overall size of the antenna
and the amount of loss in the feed are reduced as compared to antennas
having conventional feed networks.
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
VII. Antenna Assembly
As described above, one technique for manufacturing helical antennas
5 is to dispose radiators, feed networks and ground traces on a substrate and
to
wrap the substrate in an appropriate shape. Although the above-described
antenna configurations can be implemented using conventional techniques
for wrapping the substrate in the appropriate shape, an improved structure
and technique for wrapping the substrate is now described.
10 FIG. 22A is a diagram illustrating one embodiment of a structure used
to maintain the substrate in an appropriate (e.g., cylindrical) shape. More
specifically, FIG. 22A illustrates an example structure added to an antenna
having an area efficient feed network. After reading this description, it will
become apparent to a person skilled in the relevant art how to implement
15 the invention with helical antennas of other configurations.
FIGS.22B through 22F depict cross-sectional views of an example
structure used to hold the antenna in a cylindrical or other appropriate
shape. Referring now to FIGS. 22A through 22F, the example includes a
metallic strip 2218 on, or as an extension of, ground plane 412, solder
20 material 2216 opposite metallic strip 2218, and one or more vias 2210.
Metallic strip 2218 can be comprised of a portion of ground plane 412,
or a metallic strip added to ground plane 412. Preferably, in one
embodiment, metallic strip 2218 is provided by merely extending the width
of ground plane 412 by a predetermined amount. In the embodiment
25 illustrated in FIG. 22A, this width is shown by wst~p.
A series of vias 2210 are provided in ground plane 412 in the area of
metallic strip 2218. Preferably, for a solid connection, the vias 2210 are
added
to radiator portions of both first antenna 1304 and second antenna 1308. The
pattern chosen for vias 2210 is based on known mechanical and electrical
properties of the materials used. While the invention can be implemented
with only one or two vias 2210 on each ground plane 412, to obtain a desired
level of mechanical strength and electrical connection several vias 2210 may
be employed. While not necessary, the portion of each ground plane 412
used can extend laterally, or circumferentially, beyond the antenna radiators.
As seen in FIG. 22B, vias 2210 extend completely through the material
of ground plane 412 and through support substrate 406 (100) from one
surface to the next. The vias are manufactured as metallized or metal coated
vias using well known techniques in the art. A relatively small portion or
CA 02284673 1999-09-24
WO 98144590 PCT/US98/05873
26
region of an opposite edge 2214 of ground plane 412 is coated with solder
material 2216.
The embodiments illustrated in FIGS. 22B and 22D, include a small
metallic strip 2218 formed on substrate 406 on the opposite side from ground
plane 412, but adjacent to first edge 2212. In this embodiment, the vias
extend through the substrate to metallic strip 2218. While metallic strip 2228
is not necessary in all applications, it will be readily apparent to those
skilled
in the art that metallic strip 2218 facilitates solder flow and improved
mechanical bonding. A specific material for manufacturing metallic strip
2218 is chosen according to known principles based on the ground plane
material being used, the solder chosen, and so forth.
When the antenna support substrate is rolled into the generally
cylindrical shape to form desired helical antenna structures, edges 2212 and
2214 are brought into close proximity with one another as illustrated in
FIG. 22D. Vias 2210 and metallic strip 2218 (if provided) are positioned to
overlap solder material 2216 on opposite ground plane edge 2214. Heat is
applied using well known soldering techniques and equipment while strip
2218 is held in contact with solder material 2216.
As solder material 2216 is melted, it flows into vias 2210 and onto
metallic strip 2218. The heat is then reduced or removed, and the solder
forms a permanent, but removable or serviceable, joint or bond between the
two outer edges or ends of ground plane 412. In this manner, the antenna
support substrate 406 and the antenna components deposited thereon are
now mechanically held in the desired cylindrical form without requiring
other materials such as dielectric tape, adhesives, or the like. This reduces
the time, cost, and labor previously required to assemble a helical antenna of
this type. This may also allow increased automation of this operation and
provide more; readily reproducible antenna dimensions. In addition, one
edge of ground plane 412 is now electrically connected to the other edge,
providing a continuous conductive ring from the ground plane, as desired.
This electrical connection is accomplished without complicated soldering or
connecting wires.
This technique can also be extended to provide support or
engagement along other portions of the antenna. For example, a series of
one or more metallic pads or strips 2220 can be deposited at spaced apart
locations along the length of one or both sets of antenna radiators. As seen
in FIG. 22E, the metallic pads or strips 2220 are positioned adjacent one or
more radiators 104A-D but on the opposite side of support substrate 406 (100).
These pads or strips are positioned so that when the antenna substrate is
_, , .
CA 02284673 1999-09-24
WO 98/44590 PCTIUS98/05873
27
rolled pr curved to produce the desired antenna, as seen in FIG. 22F, metallic
pads or strips 2220 are positioned over a portion of radiators 104A-D on the
opposite edge of the support substrate. Specifically, in one embodiment,
metallic pads or strips 220 are positioned over a ground trace 1436 of
radiators 104A-D. Metallized vias may be formed in pads 2220 where desired
for the application or to improve transfer of heat to melt the solder.
If a small amount of solder 2226 is previously applied to a mating
portion on the surface of ground trace 1436, it can be used to join these
radiators to the strips. This provides additional joints or bonding points
which efficiently hold the antenna structure together in the desired form.
Where electrical connection is desired, metallized vias can be formed in the
pads or strips which extend through to the opposite side. These pads can be
used in conjunction with or without the strips previously discussed for the
ground planes. Such a structure is especially useful where very long
radiators, or multiple stacks of antenna radiators are contemplated which
result in tall antenna structures.
FIGS. 23A - 23C illustrate a series of views of an example embodiment
of a form 2310 used for rolling substrate 406 into the desired shape. The
example illustrated in FIG.23 is a form 2310 of cylindrical shape used in
rolling the antenna and to provide continued support and rigidity for the
antenna structure. In one embodiment, form 2310 can be provided with a
series of prongs or teeth 2312 extending radially outward from an outer
surface of form 2310. To interface with form 2310 and teeth 2312, a series of
"tooling" or assembly guide" holes or passages 2230 are provided in substrate
406 for mating with teeth 2312.
In FIG.22A, tooling holes 2230 are illustrated as being positioned
within ground planes 412. The metallic material of ground plane 412 acts to
reinforce the holes and prevent deformation and movement when a
relatively soft support substrate material is used. This assists with
alignment
accuracy for the antenna structure. However, there is no requirement for
holes 2230 to be placed within a metallic layer.
Referring again to FIGS.23A - 23C, and commencing with the
perspective view of FIG. 23A, substrate 406 is shown positioned to engage a
support form 2310 by mating teeth 2312 with holes 2230. As seen in the side
views of FIG. 23B and 23C, as support form 2310 is rotated about its axis, or
substrate 406 is otherwise wrapped around support form 2310, holes 2230
engage teeth 2312 which help position substrate 406 in place against or o n
support from 2310. Eventually, the entire substrate 406 is engaged against
support form 2310. In FIG. 23C, substrate 406 is illustrated as having been
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
28
wrapped around support form 2310 until it overlaps itself so that strips 2218,
2220 engage solder 2216, 2226 as described above.
Of course, where strips 2218, 2220 and solder 2216, 2226 are not used to
join the substrate sections, substrate 406 does not need to overlap on support
form 2310. Additionally, there is no requirement that support form 2310
extend the entire length of the antenna(s), radiators 104A-D or substrate 406.
In some applications, some or all of the portions of the antenna may be self
supporting, without the need for a form 2310. This feature can be
advantageous, for example, to minimize the impact of the form 2310 on
radiation patterns at certain frequencies.
For purposes of clarity and ease of illustration, in FIGS. 23A - 23C, only
substrate 406 is shown, without material layers for ground planes, radiators,
feeds, feed networks, and so forth. It will also be readily apparent to those
skilled in the relevant art how to size holes 2230 to match the dimensions of
teeth 2312.
Form 2310, as illustrated in FIG. 23, can be constructed using a solid or
hollow structure formed in a cylindrical or other desired shape, with teeth or
prongs 2312 protruding therefrom. In this embodiment, form 2310 can be
thought of, for example, as a variation of the toothed drum found in many
music boxes. As would be apparent to one of ordinary skill in the art after
reading this disclosure, alternative structures can be implemented to
provide form 2310 including an axle/spoke arrangement, an axle/sprocket
arrangement, or other appropriate configuration.
Note that it is contemplated that the spacing of the prongs 2312 or
spokes may not be symmetrical about the support element. That is, the
spacing may be larger in some portions in order to impart a greater amount
of consistent tension in rolling, and smaller in some areas to better control
substrate positioning where the substrate edges overlap. Preferably tooth
spacing is chosen such that teeth 2312 apply a certain amount of tension to
hold substrate 406 in place and to make the entire assembly a more rigid
structure.
The use of holes 2230 and teeth 2312 provide improved
manufacturing capabilities through position and assembly automation, and
in precision placement or positioning of the substrate on a form that can be
mounted within an antenna radome. This allows more precise structural
definition and positioning of the antenna assembly, resulting in more
precise control and compensation for the impact of the radome on radiation
patterns.
.....,.. ~rt...... T , ,
CA 02284673 1999-09-24
WO 98/44590 PCT/US98/05873
29
The above description of the placement of metallic strips 2218, solder
material 221b, and vias 2210 is provided by way of example. After reading
this description, it would be apparent to a person skilled in the art how
these
components could be placed in alternative locations depending on the
configuration desired. For example, these components can be positioned
such that the antenna can be rolled to have right-hand or left-hand circular
polarization and to have the radiators 104A-D an either the inside or the
outside of the shape.
VIII. Conclusion
While various embodiments of the present invention have been
described above, it should be understood that they have been presented by
way of example only, and not limitation. Thus, the breadth and scope of the
I5 present invention should not be limited by any of the above-described
exemplary embodiments, but should be defined only in accordance with the
following claims and their equivalents.
The previous description of the preferred embodiments is provided to
enable any person skilled in the art to make or use the present invention.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood by those
skilled in the art that various changes in form and details may be made
therein without departing from the spirit and scope of the invention.
