Note: Descriptions are shown in the official language in which they were submitted.
156.1L4
In general, microstrip radiators are specially shaped
and dimensioned conductive surfaces formed on one surface of
a planar dielectric substrate, the other surface of such sub-
strate having formed thereon a further conductive surface
S commonly termed the "ground plane". Microstrip radiators are
typically formed, either singly or in an array, by convention-
al photoetching processes from a dielectric sheet laminated
between two conductive sheets. The planar dimensions of
the radiating element are chosen such that one dimension is on
the order of a predetermined portion of the wavelength of a
predetermined frequency signal within the dielectric sub-
strate, and the thickness of the dielectric substrate chosen
to be a small fraction of the wavelength. A resonant cavity
is thus formed between the radiating element and ground plane,
;;,1 15 with the edges of the radiating element in the non-resonant
j dimension defining radiating slot apertures between the radi-
~, .
ating element edge and the underlying ground plane surface.
: ~ 1
A dilemma arises in the prior art with respect to
constraints on the minimum size of antenna elements. By
definition, the effective resonant dimension of the resonant
cavity, defined by the radiating element (commonly called
the "E-plane dimension") must be approximately a predeterm~ned
portion of a wavelength of the operating fre~uency siynal in
the dielectric. The prior art has generally attempted to
reduce the size of the antenna elements by utilizing suh-
strates with high dielectric constants to, in effectj reduce
the wavelength of the resonant frequency within the dielectric
substrate and thereby allow for a smaller resonant dimension.
Such an approach, however, is disadvantageous in that the use
of a high dielectric substrate increase the loss conductance
-2- ~
1~561~1
of the cavity and results in a larger non-resonant dimension,
as will be explained, or significantly lower efficiency of
lthe antenna or both.
The non-resonant dimension, commonly termed the "H-
plane dimension", is determined in major part by the beam
width and efficiency of the antenna. ~he efficiency of
the antenna is typically expressed as a ratio of the power
actually radiated to the power input, where the power input
is (neglecting any reflected components) substantially equal
to the sum of the power radiated and the power loss throush
heat dissipation in the dielectric. The equivalent circuit
of the antenna element, with respect to power dissipation,
may be expressed as a parallel combination of a radiation
resistance and a dielectric loss resistance where the radia-
tion and dielectric loss resistances are respectively definedas the resistances which, when placed in series with the
antenna element, would dissipate the same ~nount of power as
actually radiated by the element and as dissipated by the
dielectric, respectively. The radiation power and dielectric
loss are thus inversely proportional to the respective values
; of the radiation and loss resistances. The radiation resis-
tance, however, is inversely proportional to the non-resonant
dimension of the element. For a given dielectric, a required
efficiency therefore prescribes the minimum non-resonant
dimension of the element. Thus, conflicting criteria for
reducing the respective dimensions of an antenna element
existed in the prior art, in that the required effective
resonant dimension of the element is determined by the wave-
length of the resonant fre~uency signal in the dielectric
and substrates having high dielectric constant to reduce such
3--
wayelength typically present ~ lo~ loss ~esistance~ requi~ing~ therefore,
a wider non-resonant dimension.
It should be appreciated that minimum size constraints can cause
significant problems in applications where a large multiplicity of radiating
elements are required, but limited space is available for antenna area9
for example, a communication system antenna for use on an astronaut's back-
pack.
The problem of maintaining minimum size constraints is e~acerbated
when the generation of orthogonally polarized signals is attempted.
It is a broad object of the present invention to reduce the planar
size of an antenna structure for radiating two orthogonally polarized
signals without significantly clecreasing the eEficiency of the structure.
This broad object is achieved by providing according to the present
invention, an antenna structure for radiating two orthogonally polarized
signals comprising: a first radiating element including a first resonant
cavity and at least a first radiating aperture; a second radiating element
including a second resonant cavity and at least a second aperture; and
means for applying a first signal to said first radiating element and apply-
ing a second signal, 90 out of phase with respect to said first signal, to
second radiating element; said first and second radiating elements being
rela~ively disposed such that said first and second resonant cavities over-
lay each other and the radiating apertures thereof are relatively disposed
at 90.
The present invention further provides an antenna structure for
radiating two orthogonally polarized signals comprising: a first interdigit-
ated structure, including first and second sets of interdigitated conductive
sheets defining a first resonant cavity therebetween having at least a first
radiating aperture; a second interdigitated structure, including third and
fourth sets of interdigitated conductive sheets defining a second resonant
cavity therebetween, having at least a second radiating aperture; said first
and second interdigitated structures being relatively disposed in a stacked
manner, said first and second apertures being
~ _
orthogonally disposed with respect to each o~her; and means
for applying a first signal to said first interdigitated
structure and a second signal to said second interdigitated
structure, said second signal being 90 out of phase with
said first signal.
A description of the preferred embodiment follows with
reference to the accompanying drawing, wherein like numerals
denote like elements, and: ~
Figure l is a perspective view of a microstrip radiat- ;
ing element with narrowed non-resonant dimension;
Figures 2 and ~, respectively, are sectional and
perspective views of a folded microstrip radia-ting element;
Figure 4 is a sectional view of an interdigitated
antenna structure utilizing standoffs; and
Figure 5 shows a microstrip radiator in accordance
with one aspect of the present invention adapted to radiate
circularly polarized signals.
With reference to Figure 1, a planar conductive radi-
ating element lO is insulated from a conductive ground plane
12, disposed parallel thereto, by a dielectric substrate 14.
Signals of a predetermined operating frequency are applied
to radiating element lO and ground plane 12, for example, by
a coaxial cable 16. Coaxial cable 16 is preferably coupled
to radiating element 10 at a point 18 where the impedance of
element 10 matches the impedance (typically 50 ohms) of the
cable. Radiating element lO is generally rectangular, having
planar dimensions such that one set of edges 20 and 22 defines
a resonant dimension approximately equal to one-half of the
wa~elength of the predetermined frequency signal in dielectric
substrate 14, for example, 0.45 of the free space ~avelength
.. .
~ ~5~
of the signal. Dielectric substrate 14 is a fraction oE a
wavelength, :Eor example, 0.002 times the free space wavelength
of the resonant frequency. A resonant cavity if formed be-
tween radiating element 10 and ground plane 12 with radiation
emanating from radiating aperture slots 28 and 30 formed be-
tween edges 24 and 26 and ground plane 12.
Dielectric substrate 14 is preferably a low density,
low loss expanded dielectric substance such as a honeycombed
or foamed structure. Briefly, such expanded dielectric com-
- lO prises, in substantial portion, voids to provide a rigid, low
weight, low density, low loss structure. Expanded dielectrics,
however, typically present a lower dielectric constant than
non-expanded dielectric substrates~ such as teflon-fi~erglass
typically used i.n the prior art. Thus, use of an expandecl
dielectric generally requires an elongation of the effective
resonant dimension. However, the present inventors have dis-
covered that the loss resistance of such expanded dielectric
substrate is far greater than the loss resistance oE non-
expanded dielectric substrate, providing for a reduction in
the minimum non-resonant dimension, substantially exceeding
the increase in the resonant dimension required due to de-
creased dielectric COTIStant. For example, the non-resonant
dimension can be chosen to be 0.1 times the free space wave-
length of the applied signal, as compared with 0.3-0.9 times
the free space wavelength typical for the prior art. Thus,
in accordance with one aspect of the present invention, a
radiating element of reduced planar area can be constructed
by utilizing an expanded dielectric substrate, and narrowing
the non-resonant dimension. For example, a radiator o:E given
efficiency utili~ing a *Teflon-*Fiberglass substrate is 0.15
*Trademarks - 6 -
times the square of the free space wavelength, while a typical
radiating element of such efficiency utilizing an exp~nded
dielectric substrate and narrowed non-resonant dimension in
accordance with the present invention i5 0 . 0 5 times the -~
square of the free space wavelength, a rPduction in area by
a actor on the order of 3.
The planar area of a radiating element can be further
reduced in accordance with the present invention by, in effect
folding the resonant cavity.. For example, the cavity can be
folded along one or more axes perpendicular to ~he resonant
dimension to create a tiered or layered structure. Alter-
nately, a reduction in the planar size of the resonant cavity
can be effected by folding or bending the microstrip into,
for example, a "V" ~r "U" shape. Figures 2 and 3 depict an
antenna wherein an interdigitated structure is utilized to
effect a folded resonant cavity. Referring to Figures 2 and
3, generally ground plane 12 includes a plurality of longi-
tudinally disposed planar conductive sheet sections 31-35
electrically co~nected by vertical side members 36 and 38.
Radiating element 10 comprises a plurality of generally
planar, longitudinally disposed conductive sheets 40-42 dis-
posed in an interdigitated manner with respect to ground plane
sections 31-35 separated therefrom by dielectric 14, and
electrically connected by a ~ertical member 44, disposed
parallel to side members 36 and 38. Apertures 28 and 30 are
defined by the vertical most edges of radiating element 10.
The cumulative distance from aperture 28 to aperture 30,
through dielectric ~4, is approximately equal to one-half
wavelength of the operative fre~uency within the dielectric.
Thus, radiating element 10 and ground plane 12 de:Ei.ne a
resonant cavity having radiating slot apertures 28 and 30
defined by edges 24 and 26 of radiating element 10 on oppo-
site longitudinal sides of the antenna structure.
Such an interdigitated structure is, in effect, a
planar microstrip element, for example such as shown in
Figure 1, folded from each end toward the middle, then folded
again back toward the end along axes perpendicular to the
resonant dimension and parallel to radiating apertures 28 and
30 r such folding se~uence repeated four times to provide a
five tiered structure. It should be appreciated that inter-
digitated structures may be utilized to provide resonant
cavities folded along a greater or lesser number of axes,
with axes not necessa-rily parallel to the radiating aperture
not perp~ndicular to the resonant dimension. ~hile is it
not necessary, i-t is preferred that an odd number of tiers
be effected such that the apertures are on opposite longi-
tudinal sides of the antenna structure.
An input signal is applied to the radiating element
via coaxial cable 16, with the center conductor connected to
radiating element 10 at a point 18 of appropriate impedance.
While cable 16 is shown coupled through the side of the
antenna element in Figures 2 and 3, it should be appreciated
that connection can be made in any appropriate manner such as,
for example, through the bottom of ground plane 12 or from
the resonant dimension sideO
The planar length (L) of a five-tiered interdigitated
structure, such as shown in Figures 2 and 3 having a non-
resonant dimension on the order of 0.1 times the free space
wavelength of the operating fre~uency, is also on the order
of 0.1 times the free space wavelength, as opposed to a ~ 45
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11~561~L
times the free space wavelength typical in a non-folded
structure such as shown in Figure 1. The height or thickness
~H) of the interdigitated structuxe is on the order of 0.01
times the free space wavelength, as opposed to 0.002 times
the free space wavelength in the unfolded element.
It should be appreciated that, while Figures 2 and 3
show an interdigitated structure wherein both of the side
members are formed by ground plane 12~ folded resonant cavi-
ties can be effected by interdigitated structures wherein one
~r both of the side members are formed by radiating element
10, and by interdigitated structures wherein a plurality of
vertically disposed conductive elements are connected by
longitudinally disposed members. Further, the conductive
sheets need not be planar, but can be curved, nor need all
the conductive sheets be of the same planar sizeO Moreover,
the spacing between sheets need not be uniform or constant,
It should be appreciated that dielectric 14 can com-
prise a void with radiating element 10 being isolated ~rom
ground plane 12 by standoffs. Such a structure is shown in
Figure 4. Non-conductive s-tandoffs 46 and 48 are disposed be~
tween ground plane element 35 and radiator element 40, to
effect spatial separation between ground plane 12 and radiat-
ing element 10. The conductive sheets of radiator 10 and
ground plane 12, in an embodiment utilizing standoffs, must
be rigid enough to maintain the interdigitated separation.
Where a solid orhoneycombed or o-therwise expanded dielectric
is used, the conductive sheets can be extremely thin, with
the dielectric providing structural support.
The interdigitated structure depicted in Figures 2
3~ and 3 is particularly advantageous in the generatlon of
g_
6.~
circular or elliptica~ly polarized signals. Circular or
elliptical polarization is generated utilizing a flat radiat
ing element by applying equal amplitude signals, 90 out of
phase, to ad]acent (intersecting), perpendicular edges of the
element. Such a technique is not feasible for use with folded
or interdigitated elements. To provide circular or elliptical
polarization, two interdigitated or folded elements are~ in
effect, stacked and rotated with respect to each other by 90
as shown in Figure 5~ Quadrature signals, as generated ~y,
for example~ a ~uadrature hybrid 50, are applied to respective
stacked elements 52 and 54 via coaxial cables 56 and 58. Due
to a masking effect by the upper element, it was found desir-
able to utilize cavities a approximately a half wavelength,
and that the cavities maintain two radiating apertures on
opposite sides of the element. It should be appreciated
that, where the coaxial cables are coupled through vert~cal
sides in the non-resonant dimension of the respective ele-
ments, the coaxial cables can be routed straight downward
without interfering with the operation of radiating apertures
60-63. The thickness (T) of such stacked elements are
typically on the order of 0.02 times the free space wavelength
of the operating frequency.
Radiating elements utilizing folded resonant cavities
in accordance with the present invention have been built for
operational fre~uency of between 259.7 MHz to 296.8 MHz. The
elements constructed were interdigitated scructures similar
to that shown in Figures 2 and 3, and were stacked as shown
in Figure 5 to provide circular polariæation. ~ radiation
pattern of -10 db gain was achieved over approximately 80%
spherical coveraye. The physical package was ~" x 18" x 3"
--10--
and weighed less than 0.45 Kg. The conductive sheets were
formed of aluminum 0.005-.020 inch thick. The sheets were
set in an interdigitated arrangement, furnace brazed and ~h~n
sealed with tin. The structure was then set in a mold and
the space between the conductive sheets filled with liquid
expanding insulating resin. The resin hardened to provide
rigidity.
An interdigitated antenna structure has also been
constructed on a layer-by- layer approach, sandwiching a
- 10 layer ofhoneycomb material between conductive sheets.
A seven tiered interdigitated antenna structure
utilizing a dielectric comprising standofs and a void has
also been constructed. The conductive sheets were formed of
brass on the order of 0~020 inch thick, and spacing between
the interdigitated elements was maintained at 0.1 inch ~y
transverse nylon screws running through the interdigitated
elements.
It should be appreciated that folded cavities in
accordance with the present invention can also be o lengths
other than one-half wavelength. For example, quarter-wave
cavities have been cvnstructed.
