Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02245882 2003-12-12
AN ANTENNA
This invention relates to an antenna for operation at frequencies in excess of
200 MHz.
and to a radio communication unit includins the antenna.
The antenna requirements of a cellular or cordless telephone handset are
primarily that it
should be compact and omnidirectional. For a handset operating within the
frequency
range of 800 MHz to 2 GHz the antenna is typically an extendable rod having a
length
approximately equivalent to the a quarter wavelength when extended. or a
helical wire
having several turns. The antenna is usually mounted partially within the
handset unit and
partly projecting from the end of the unit adjacent the earphone. One
difficulty with radio
telephone handsets is the perceived health hazard associated with prolonged
irradiation
of the user's head by the intense electric and magnetic fields generated close
to the
antenna. Typically, 90 per cent of the radiated power is absorbed by the head,
particularly
by the blood-rich parts such as the ears and lips. Absorption of radiation by
the head can
also lead to radiation inefficiency and consequent reduction of the operating
range of the
handset. depending on the orientation of the handset and user with respect to
the nearest
base station.
Other antennas for operation within the frequency range (800 MHz to 2 GHz)
employed
by cellular telephones include the so-called Inverted-F antenna. This has two
resonant
patches. one spaced above the other. However, the antenna is mechanically
bulky.
In U.S. Patent 5,854,608 there is disclosed a miniature satellite navigation
antenna
having elements formed by four helical conductive tracks on the outer surface
of a
ceramic rod made of a material with a relative dielectric constant of 36.
The helical elements are arranged primarily for receiving circularly polarised
signals.
One of the objects of the present invention to provide an improved radio
telephone
handset antenna which results in reduced radiation into the user's head.
CA 02245882 1998-07-24
WO 97127642 PCT/GB97/00085
2
According to a first aspect of this invention, an antenna for operation at
frequencies in
excess of 200 MHz comprises an electrically insulative core of a material
having a relative
dielectric constant {E~) greater than 5, and an antenna element structure
disposed on or
adjacent the outer surface of the core, the material of the core occupying the
major part
of the volume defined by the core outer surface, wherein the antenna element
structure '
comprises a single pair of elongate antenna elements disposed in an opposing
configuration on or adjacent the core outer surface and interconnected at
respective ends
so as to form together a path of conductive material around the core, the
other ends of the
antenna elements constituting a feed connection. In a preferred antenna in
accordance
with the invention, the core is cylindrical, having a central axis, and the
antenna elements
are co-extensive, each element extending between axially spaced-apart
positions on the
outer cylindrical surface of the core. The elements are preferably metallised
tracks
deposited or bonded onto the core and arranged such that at each of the spaced-
apart
positions the respective spaced-apart portions of the elements are
substantially
diametrically opposed. The spaced-apart portions all lie substantially in a
single plane
containing the central axis of the core, and the portions at one of the spaced-
apart
positions are connected together by a link conductor to form the Loop, the
portions at the
other of the spaced-apart positions being coupled to feed connections for the
loop by cross
elements extending generally radially on an end face of the core. The feed
connections
rnay be connected to a coaxial feeder structure. The radiation pattern of the
antenna has
a null directed perpendicularly on each side of the plane. With the exception
of the two
nulls, the radiation pattern is omnidirectional.
By mounting the antenna in a telephone handset, the intensity of the radiation
coupled into
the user's head is substantially reduced. At the frequencies of interest {in
the region of
800 to 900 MHz, and 1800 to 2000 MHz), the antenna can be constructed so as to
be
particularly compact. For example, a DECT (Digital European Cordless
Telephone) ,
antenna operating in the frequency region 1880 - 1900 MHz can typically have a
length
of 20.2mm and a diameter of Smm, using a dielectric material having Er = 36.
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3
Thus, according to a second aspect of the invention there is provided a
handheld radio
communication unit having a radio transceiver, an integral earphone for
directing sound
~ energy from an inner face of the unit which, in use, is placed against the
user's ear, and
an antenna coupled to the transceiver and located in the region of the
earphone, wherein
the antenna comprises: an electrically insulative core having a relative
dielectric constant
(Er) greater than 5, an antenna element structure including a pair of antenna
elements
disposed co-extensively in an opposing configuration on or adjacent the core
outer surface
and connected together to form a loop, the antenna element structure thereby
having a
radiation pattern which has a null in a direction transverse to the antenna
element, and
IO wherein the antenna is so mounted in the unit that the null is directed
generally
perpendicularly to the inner face of the unit to reduce the level of radiation
of the
transceiver in the direction of the user's head. In the case of the antenna
core being in the
form of a cylinder, which may be drum- or rod-shaped, and with a pair of co-
extensive
antenna elements the ends of which lie in the plane containing the central
axis of the core,
I S the plane is preferably parallel to the inner face of the unit. Providing
the antenna with
a trap or balun in the form of a metallised sleeve not only allows the antenna
loop to be
fed in a substantially balanced condition, but also reduces the effect of the
comparatively
small ground mass represented by the unit and provides a useful surface area
for secure
mounting of the antenna, e.g. by soldering or clamping.
For reasons of physical and electrical stability, the material of the core may
be ceramic,
e.g. a microwave ceramic material such as a zirconium-titanate-based material,
magnesium calcium titanate, barium zirconium tantalate, and barium neodymium
titanate,
or a combination of these. The preferred relative dielectric constant (er) is
upwards of 10
or, indeed, 20, with a figure of 36 being attainable using zirconium-titanate-
based
material. Such materials have negligible dielectric loss to the extent that
the Q of the
antenna is governed more by the electrical resistance of the antenna elements
than core
loss.
A particularly preferred embodiment of the invention has a cylindrical core of
solid
material with an axial extent at least as great as its outer diameter, and
with the diametrical
_4_
extent of the solid material being at least 50 per cent of the outer diameter.
Thus,
the core may be in the form of a tube having a comparatively narrow axial
passage of a diameter at most half the overall diameter of the core.
While it is preferred that the antenna elements are helical, with each element
executing a half-turn around the core, it is also possible to form the
elements such
that they are parallel to the central axis and still achieve a radiation
pattern having
a null which is directed transversely to the axis, as in the case of the above-
described antenna with helical elements.
l0
In the preferred antenna, the antenna elements are fed from a distal end, the
core
having a central passage housing a coaxial feeder structure extending from a
proximal or mounting end of the core and opening out at the distal end where
radial elements couple the antenna elements on the cylindrical outer surface
of
the core respectively to the inner and outer conductors of the feeder
structure.
This link conductor may then be annular, and advantageously is constituted by
a
cylindrical sleeve on the outer surface of the proximal part of the core.
The choice of antenna element configuration affects the bandwidth of the
antenna, insofar as the use of helical elements tends to increase bandwidth
compared with antenna elements parallel to the central axis of the core.
According to a first aspect of the invention, there is provided an antenna for
operation at frequencies in excess of 200 NMZ, comprising an electrically
insulative pore of a solid material having a relative dielectric constant
greater than
5, and an antenna element structure disposed on or adjacent the outer surface
of
the core, the material of the core occupying the major part of the volume
defined
by the core outer surface, wherein the antenna element structure comprises a
single pair of elongate antenna elements which are disposed in an opposing
configuration on or adjacent the core outer surface and which as substantially
co-extensive, with each element extending between positions spaced apart in
the
direction of a central axis of the antenna, the antenna elements being
CA 02245882 2002-O1-03
~4a_
interconnected at respective ends so as to form, together a path of canductive
material around the com, the other ends of the antenna elements constituting a
feed connection, the antenna having a radiation pattern which is
omnidirectional
with the exception of a null centered on a null axis passing through the core
transversely with respect to the axis.
According to a second aspect of the invention, there is provided an antenna
for
operation at frequencies in excess of 200 MHz, comprising an electrically
insulative core having a central axis and being formed of a solid material
having a
relative dielectric constant greater than 5, and an antenna element structure
disposed on or adjacent the outer surface of the com the material of the core
occupying the major part of the volume defined by the core outer surface"
wherein
the antenna element structure comprises a loop extending around the core and
terminated at a feed connection, the combination of the antenna element
structure
and the core having a radiation pattern which is omnidirectional with the
exception
of a null centered on a null axis passing through the core transversely with
respect to the central axis.
According to a third aspect of the invention, there is provided a handheld
radio
communication unit having a radio transceiver, an integral earphone for
directing
sound energy from an inner face of the unit which, in use, is placed against
the
user's ear, and an antenna coupled to the transceiver and located in the
region of
the earphone, wherein the antenna comprises: an elastically insulative core
having a relative dielectric constant greater than 5, an antenna element
structure
including a pair of antenna elements disposed co-extensively in an opposing
configuration on or adjacent the core outer surface and connected together to
form a loop, the antenna element structure thereby having a radiation pattern
which has a null in a direction transverse to the antenna elements, and
wherein
the antenna is so mounted in the unit that the null is directed generally
perpendicularly to the said inner face of the unit to reduce the level of
radiation
from the unit in the direction of the user's head.
CA 02245882 2002-O1-03
_4~_
The invention will now be described by Way of example with reference to the
drawings, in which:
Figure 1 is a perspective view of an antenna in accordance With the invention;
Figure 2 is a diagram illustrating the radiation pattern of the antenna of
Figure 1;
Figure 3 is a perspective view of a telephone handset, incorporating an
antenna in
accordance with the invention; and
CA 02245882 2002-O1-03
CA 02245882 1998-07-24
WO 97/27642 PCTlGB9?laQQ8S
Figure 4 is a perspective view of a second antenna in accordance with the
invention.
Referring to Figure 1, an antenna 10 in accordance with the invention has an
antenna
element structure with two longitudinally extending antenna elements LOA, 10B
formed
5 as metallic conductor tracks on the cylindrical outer surface of a ceramic
core 12. The
core 12 has an axial passage 14 with an inner metallic Lining 16, and the
passage houses
an axial inner feeder conductor 18. The inner conductor 18 and the lining I6
in this case
form a feeder structure for coupling a feed line to the antenna elements 1 OA,
1 OB at a
feed position on the distal end face 12D of the core. The antenna element
structure also
includes corresponding radial antenna elements 1 OAR, IOBR formed as metallic
tracks
on the distal end face 12D connecting diametrically opposed ends lOAE, lOBE of
the
respective longitudinally extending elements 10A, 1 OD to the feeder
structure. The other
ends 10AF, IOBF of the antenna elements 1 OA, 1 OB are also diametrically
opposed and
are linked by an annular common virtual ground conductor 20 in the form of a
plated
sleeve surrounding a proximal end portion of the core I2. This sleeve 20 is in
turn
connected to the lining 16 of the axial passage 14 by plating 22 on the
proximal end face
12P of the core 12.
In this preferred embodiment, the conductive sleeve 20 covers a proximal
portion of the
antenna core 12, thereby surrounding the feeder structure i 6, 18, the
material of the core
12 filling the whole of the space between the sleeve 20 and the metallic
lining 16 of the
axial passage I4. The sleeve 20 forms a cylinder connected to the lining 16 by
the plating
22 of the proximal end face 12P of the core I2, the combination of the sleeve
20 and
plating 22 forming a balun so that signals in the transmission Line formed by
the feeder
structure 16, 18 are converted between an unbalanced state at the proximal end
of the
antenna and a balanced state at an axial position approximately in the plane
of the upper
edge 20U of the sleeve 20. To achieve this effect, the axial length of the
sleeve 20 is such
that in the presence of an underlying core material of relatively high
dielectric constant,
the balun has an electrical length of about ~./4 at the operating frequency of
the antenna.
Since the core material of the antenna has a foreshortening effect, the
annular space
surrounding the inner conductor 18 is filled with an insulating dielectric
material I7
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WO 97/27642 PCT/GB97I00085
6
having a relatively small dielectric constant, and the feeder structure
distally of the sleeve
20 has a short electric length. As a result, signals at the distal end of the
feeder structure
16, 18 are at least approximately balanced.
A fits ther effect of the sleeve 20 is that for signals in the region of the
operating frequency
of the antenna, the rim 20U of the sleeve 20 is effectively isolated from the
ground
represented by the outer conductor 16 of the feeder structure. This means that
currents
circulating between the antenna elements I OA, l OB, are confined to the rim
20U and the
loop formed by the antenna element structure is isolated. The sleeve 20 thus
acts as an
isolating trap.
In this embodiment, the longitudinally extending elements 10A, l0B are of
equal length,
each being in the form of a simple helix executing a half turn around the axis
12A of the
core 12.
The antenna elements 10A, l OB are connected respectively to the inner
conductor 18 and
outer lining 16 of the feeder structure by their respective radial elements
lOAR, l OBR.
It will be seen, then, that the helical elements i OA, I OB, the radial
elements lOAR, I OBR,
and the sleeve 20 together form a conductive loop on the outer surface of the
core 12, the
loop being fed at the distal end of the core by a feeder structure which
extends through the
core from the proximal end and lies between the antenna elements 10A, 1 OB.
The antenna
consequently has an end-fed bifilar helical structure.
It will be noted that the four ends lOAE, lOAF, 1 OBE, 1 OBF of the antenna
elements 10A,
l OB all lie in a common plane containing the axis 12A of the core 12. This
common plane
is indicated by the chain lines 24 in Figure 1. The feed connection to the
antenna element
structure also lies in the common plane 24. The antenna element structure is
so
configured that the integral of currents induced in elemental segments of this
structure by
a wave incident on the antenna from a direction 28 normal to the plane 24 and
having a
planar wavefront sums to zero at the feed position, i.e. where the feeder
structure 16, 18
is connected to the antenna element structure. In practice, the two elements 1
OA, 1 OB are
CA 02245882 1998-07-24
wo 97~276a2 PCTlGB9'7100085
7
equally disposed and equally weighted on either side of the plane 24, yielding
vectorial
symmetry about the plane. Each element 10A, l OB may be regarded as being made
up of
' a plurality of increments, each one of which Iies diametrically opposite a
corresponding
complementary increment of the other of the elements 1 OA, 1 OB at an equal
distance from
the central axis 12A.
The antenna element structure with half turn helical elements 10A, l OB
performs in a
manner similar to a simple planar loop, having a null in its radiation pattern
in a direction
transverse to the axis 12A and perpendicular to the plane 24. The radiation
pattern is,
therefore, approximately of a figure-of eight form in both the vertical and
horizontal
planes transverse to the axis 12A, as shown by Figure 2. Orientation of the
radiation
pattern with respect to the perspective view of Figure 1 is shown by the axis
system
comprising axes X, Y, Z shown in both Figure l and Figure 2. The radiation
pattern has
two nulls or notches, one on each side of the antenna, and each centred on the
line 28
shown in Figure 1.
The antenna has particular application at frequencies between 200 MHz and 5
GHz. The
radiation pattern is such that the antenna lends itself especially to use in a
handheld
communication unit such as a cellular or cordless telephone handset, as shown
in Figure
3. To orient one of the nulls of the radiation pattern in the direction of the
user's head, the
antenna is mounted such that its central axis 12A (see Figure 3) and the plane
24 (see
Figure 1 ) are parallel to the inner face 30I of the handset 30, and
specifically the inner
face 30I in the region of the earphone 32. The axis 12A also runs
longitudinally in the
handset 30, as shown. Again, the relative orientations of the antenna, its
radiation pattern,
and the handset 30 are evident by comparing the axis system X, Y, Z as it is
shown in
Figure 3 with the representations of the axis system in Figures l and 2.
The preferred material for the core 12 of the antenna is a zirconium-titanate-
based
material. This material has a relative dielectric constant of 36 and is noted
also for its
dimensional and electrical stability with varying temperature. Dielectric loss
is negligible.
The core may be produced by extrusion or pressing.
CA 02245882 2003-12-12
The antenna elements 10A, l OB, lOAR, l OBR are metallic conductor tracks
bonded to the
outer cylindrical and distal end surfaces of the core 12, each track being of
a width of at
least four times its thickness over its operative length. The tracks may be
formed by
initially plating the surfaces of the core 12 with a metallic layer and then
selectively
etching away the layer to expose the core according to a pattern applied in a
photosensitive
layer similar to that used for etching printed circuit boards. Alternatively,
the metallic
material may be applied by selective deposition or by printing techniques. In
all cases, the
formation of the tracks as an integral layer on the outside of a dimensionally
stable core
leads to an antenna having dimensionally stable antenna elements.
With a core material having a substantially higher relative dielectric
constant than that of
air, e.g. Er = 36, an antenna as described above for the DECT band in the
region of 1880
MHz to 1900 MHz typically has a core diameter of about Smm and the
longitudinally
extending elements 10A, l OB have a longitudinal extent (i.e. parallel to the
central axis
12A) of about 12.7mm. The width of the elements 10A, lOB is about 0.3mm. At
1890
MHz the length of the balun sleeve 20 is typically in the region of 7.Smm or
less.
Expressed in terms of the operating wavelength ~, in air, these dimensions
are, for the
longitudinal (axial) extent of the elements 10A, IOB: 0.08., for the core
diameter:
0.0315., for the balun sleeve: 0.047.1 or less, and for the track width:
0.00189,. Precise
dimensions of the antenna elements 10A, l OB can be determined in the design
stage on
a trial and error basis by undertaking eigenvalue delay measurements.
Adjustments in the dimensions of the plated elements during manufacture of the
antenna
May be performed in the manner described in U.S. patent 5,854,608 with
reference to
Figures 3 to 6 thereof. The whole of the subject matter of the co-pending
application
Is incorporated in the present application by reference.
The small size of the antenna renders it particularly suitable in handheld
devices such as
a mobile telephone handset and other personal communication devices. The
plated balun
sleeve 20 and/or the plated layer 22 on the proximal end face 12P of the core
12 allow the
antenna to be directly mounted on a printed circuit board or other ground
structure in a
CA 02245882 1998-07-24
WO 97127642 PCTlGB97100085
9
particularly secure manner. Typically, if the antenna is to be end-mounted,
the proximal
end face 12P can be soldered to a ground plane on the upper face of a printed
circuit board
' with the inner feed conductor 18 passing directly through a plated hole in
the board for
soldering to a conductor track on the lower surface. Alternatively, sleeve 20
may be
clamped or soldered to a printed circuit board ground plane extending parallel
to the axis
12A, with the distal part of the antenna, bearing antenna elements I OA, l OB,
extending
beyond an edge of the ground plane. It is possible to mount the antenna. 10
either wholly
within the handset unit, or partially projecting as shown in Figure 3.
An alternative embodiment within the scope of the invention is shown in Figure
4.
Referring to Figure 4, the antenna elements 10A, l OB plated on the
cylindrical surface of
core I2 are, in this case, parallel to the central axis 12A on opposite sides
of the latter. As
in the embodiment of Figure 1, the antenna elements 10A, l OB are connected
respectively
to the inner and outer conductors 18, 16 of the feeder structure via radial
elements 1 OAR,
l OBR on the distal end face 12D of the core 12. Again sleeve 20 forms an
isolating trap
so that its upper rim forms part of a Ioop extending around the core from one
feeder
conductor 16 to the other 18. In other respects, the antenna of Figure 4 is
similar to that
of Figure 1. It has a similar radiation pattern, with nulls directed
transversely of the
central axis and perpendicular to the plane containing elements 10A,, l OB,
and the feeder
structure i 6, 18.
