Note: Descriptions are shown in the official language in which they were submitted.
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FLAT-PLATE MONOPOLE ANTENNAE
FIELD OF THE INVENTION
The present invention relates to a flat-plate antennae. In particular, the
present
invention relates to a flat-plate monopole antennae suitable for use with
mobile
wireless communications devices.
BACKGROUND OF THE INVENTION
Conventional wireless communications devices, such as cellular telephones,
typically use a monopole antennae to transmit and receive electronic
information.
The conventional cellular monopole antennae comprises a tubular antennae
element
grounded to the casing of the cellular telephone, and a metal rod disposed
within the
grounded tubular antennae element. To attain the large gain and bandwidth
characteristics required for cellular communication, the monopole antennae
must
generally be considerably longer than the length of the casing of the cellular
telephone. Since a long monopole antennae would inhibit the portability of the
cellular telephone, the monopole antennae is typically fabricated as a series
of
telescoping monopole antennae sections which allow the monopole antennae to be
retracted into the telephone casing when the cellular telephone is not in use.
However, the monopole antennae must still be extended to obtain the required
gain
and bandwidth characteristics for cellular communication. Accordingly,
attempts
have been made to reduce the size of the monopole antennae without
compromising
antennae gain and bandwidth.
For instance, Yokoyama (US 4,791,423) teaches a microstrip antennae
suitable for use with a mobile telephone. The microstrip antennae comprises a
first
grounding rectangular conductive sheet, a radiating rectangular conductive
sheet
parallel to the grounding conductive sheet, a feed pin disposed within the
substrate
between the first grounding conductive sheet and the radiating conductive
sheet, and
second and third grounding conductive sheets disposed at opposite ends of the
first
grounding conductive sheet and being perpendicular to the first grounding
conductive
sheet and the radiating conductive sheet sleeve so as to improve the beam tilt
characteristics of the antennae. The second grounding conductive sheet
functions as a
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connecting conductive sheet connecting the first grounding conductive sheet to
the
radiating conductive sheet. In one variation, shown in Fig.4a of the patent,
the patch
antenna includes a planar passive element disposed above and in close
proximity to
the radiating conductive sheet for facilitating impedance matching of the
antenna.
Nakase (tJS 5,061,939) teaches a flat-plate antenna suitable for use with a
mobile telephone. The flat-plate antenna comprises a rectangular conductive
ground
plate, a radiating oval-shaped conductive top plate parallel to the ground
plate, a
plurality of elongated connecting members disposed between the ground plate
and the
top plate for electrically connecting the top plate to the ground plate, and a
stripline
resonator disposed between the ground plate and the top plate. The stripline
resonator
includes a feeder line, and a capacitor electrode centrally disposed below the
top plate
for adjusting the resonant frequency of the antenna.
Yokoyama (US 5,148,181) teaches a mobile radio communications device
having an antenna for preventing the antenna gain from being reduced by the
user's
head andlor hands during operation of the communications device. The antenna
is
mounted on the upper surface of the casing of the communications device, and
comprises a rectangular first conductive plate, a rectangular second
conductive plate
disposed below and extending parallel to the first conductive plate, and a
rectangular
third conductive plate extending perpendicularly from the first and second
conductive
plates. The first conductivc plate is spaced a distance from the upper surface
of the
casing of the communications device. The antenna also includes a short circuit
plate
extending perpendicularly from the first conductive plate and is connected to
the
upper surface of the casing in proximity othe location of the earpiece and the
mouthpiece.
Boubouleix (US 4,491,843) teaches a dipole antenna comprising a
parallelopiped-shaped metal box, an amplifier disposed within the metal box, a
metal
plate disposed in proximity to the top end surface of the metal box, and an
inductor
extending through the upper end surface. The inductor is connected at one end
to the
metal plate and at the opposite end to the amplifier. .
Delaveaud ("Small-sized Low-profile Antenna to Replace Monopole
Antennas", Electronics Letters, vol. 34, no. 8, GB, IEE Stevenage, April 16,
1998)
teaches a monopole antenna comprising a parallelepiped-shaped metal box, a
metal
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plate and a coaxial feed probe. The metal box has an upper planar grounding
surface,
and the metal plate is disposed in proximity to the grounding surface. The
coaxial
feed probe extends through the grounding surface of the metal box. The centre
conductor of the probe is connected at one end to the centre of the metal
plate. The
outer conductor of the probe is connected at one end to the metal plate, and
at the
opposite end to the grounding surface.
Yokoyama, Nakase, Boubouliex and Delaveaud provide an antenna which is
significantly smaller than the conventional telescoping monopole antenna.
However,
in each case the bandwidth, resonant frequency and input impedance
characteristics of
the antenna are dictated by fixed parameters such as the dimensions of the
conductive
plates, and the distance between the conductive plates. As a consequence,
close
attention must be paid to manufacturing tolerances to attain the proper
operating
characteristics for the antenna. Therefore, there remains a need for a
monopole
antenna which is significantly smaller than the conventional
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monopole telescoping antennae, and whose operating characteristics are not
compromised by variations from manufacturing tolerances.
SUMMARY OF THE INVENTION
According to the invention, there is provided a wireless communications
device and monopole antenna which address deficiencies of the prior art
monopole
antennae.
The monopole antenna according to the present invention includes a
conductive ground plane, a conductive radiating plate, an antennae interface
terminal,
and a resonant network for defining operating characteristics of the antennae.
The
conductive radiating plate is spaced apart from the ground plane and, together
with the
ground plane, defines a cavity therebetween. The antennae interface terminal
is in
communication with the cavity and is electrically isolated from the ground
plane and
the radiating plate. The resonant network includes an inductive element
electrically
coupled between the interface terminal and the radiating plate.
The wireless communications device according to the present invention
includes a conductive casing for receiving wireless communications hardware
therein;
a conductive radiating plate spaced apart from the casing and, together with
the
ground plane, defining an antenna; and a resonant network for defining
operating
characteristics of the antennae. The conductive casing includes an antenna
communication port for interfacing with the communications hardware. The
resonant
network includes an inductive element electrically coupled between the
communication port and the radiating plate.
In a preferred implementation of the invention, the inductive element is
disposed within the cavity and comprises a first air-core coiled wire inductor
electrically coupled between the radiating plate and the interface terminal,
and a
second air-core coiled wire inductor electrically coupled between the
interface
terminal and the ground plane. Each inductor has a number of wire turns, and
the
resonant network provides the antenna with a resonant frequency determined in
accordance with the number of wire turns.
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BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiment of the invention will now be described, by way of
example only, with reference to the drawings, in which:
Fig. 1 is a perspective view of the wireless communications device, according
to the present invention, depicting the conductive casing (antenna ground
plane), the
radiating plate, and the resonant network;
Fig. 2 is a magnified view of the top end of the wireless communications
device shown in Fig. 1, depicting the air-cored coiled wire inductors of the
resonant
network , ;
Fig. 3 is a longitudinal cross-sectional view of one variation of the wireless
communications device, including an arcuately-shaped radiating plate;
Fig. 4 is a side view of another variation of the wireless communications
device, including a radiating plate inclined relative to the conductive
casing;
Fig. 5a is a schematic view of the wireless communications device including
dimensional symbols as used herein; and
Fig. 5b is a magnified schematic view of the top end of the wireless
communications device including additional dimensional symbols as used herein.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning now to Fig. 1, a wireless communications device, denoted generally
as 100, is shown comprising a conductive casing 102, a conductive radiating
plate 104
spaced apart from the casing 102, and a cavity 106 defined between the casing
102
and the radiating plate 104. Preferably, the casing 102 has a generally
rectangular
parallelopiped shape, configured for receiving wireless communications
circuitry
therein, and includes a pair of opposite faces 108a, 108b, a pair of opposite
sides
110a, 1 l Ob, and a pair of opposite ends 112a, 112b. Also, preferably the
casing 102 is
shaped as a wireless telephone handset, and the wireless communications
circuitry
housed within the casing 102 operates as a wireless telephone. However, it
will be
appreciated that the casing 102 may adopt other shapes and the wireless
communications circuitry may function other than as a wireless telephone, as
the
application demands.
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As shown more clearly in Fig. 2, the casing 102 includes an antenna
communication port 114, and an antenna interface terminal 116 extending
through the
communication port 114.. The interface terminal 116 extends into the cavity
106, and
is electrically isolated from the casing 102 and the radiating plate 104. In
addition,
the interface terminal 116 interfaces with the communications hardware
disposed
within the casing 102, and allows the communications hardware to transmit and
receive data as electromagnetic waves.
The radiating plate104 cooperates with the casing 102 to define a monopole
antenna, with the casing 102 acting as the conductive ground plane for the
antenna.
Preferably, the radiating plate 104 comprises a rectangular planar conductive
plate
fabricated from copper or aluminum. However, the radiating plate 104 may adopt
any
other shape suitable for proper antenna operation. For instance, in one
variation,
shown in Fig. 3, the radiating plate is fabricated with an arcuate shape.
Further, the
radiation plate 104 may have the same or different dimensions as the casing
102.
As shown in Fig. 1, preferably the radiating plate 104 is oriented
substantially
parallel to the casing ends 112 and at right angles to the casing faces 108.
However,
the radiating plate 104 may also be disposed at other angles relative to the
casing
faces 108 so as to alter the antenna pattern of the monopole antenna. For
instance, in
Fig. 4, there is shown a wireless communications device 100' in which the end
112a'
and the radiating plate 104 are inclined relative to the casing faces 108.
Alternately,
the radiating plate 104 may be inclined slightly with respect to the casing
end 112a.
As shown in Figs. 1 and 2, the wireless communication device 100 also
includes a resonant network 118 which, in conjunction with the dimensions of
the
radiating plate 104 and the cavity 106, defines the operating characteristics
of the
monopole antennae. The resonant network 118 is disposed within the cavity 106,
and
includes an inductive antenna element electrically coupled to the interface
terminal
116, the radiating plate 104 and the casing 102. However, it should be
understood
that the resonant network 118 is not limited to including only a single
inductive
antenna element. For instance, in one variation (not shown), in order to
increase the
bandwidth of the antenna, the resonant network 118 includes a plurality of
distinct
inductive antenna elements, each being electrically connected to the interface
terminal
116 and respective locations on the radiating plate 104 and the casing 102
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The inductive antenna element comprises a first inductor 120 electrically
connected between the radiating plate 104 and the interface terminal 116, and
a
second inductor 122 electrically connected between the interface terminal 116
and the
casing 102. Preferably, the first inductor 120 and the second inductor 122 are
coupled
together on a common core.
The resonant network 118 also includes a capacitive antenna element which is
defined by the dimensions of the casing 102, the radiating plate 104 and the
cavity
106. The inductive antenna element is disposed in parallel with the capacitive
antenna element, with the capacitive antenna element and the inductive antenna
element together establishing a parallel resonant circuit. As will be
apparent, the
resonant frequency of the monopole antenna is determined in accordance with
the
capacitance of the capacitive antenna element and the inductance of the
inductive
antenna element.
Preferably, the cavity 106 comprises an air cavity, and the inductors 120, 122
comprise air-cored coiled wire inductors, each having a respective number of
wire
turns so that the resonant frequency of the monopole antenna is defined by the
number
of wire turns of each inductor 120, 122. Alternately, the cavity 106 may
include a
dielectric or a support structure which supports the radiating plate 104.
Also,
preferably the wire used in the wire inductors 120, 122 comprises flexible
wire so as
to allow the distance between the radiating plate 104 and the end 112a (and
therefore
the capacitance of the capacitive antenna element) to be adjusted. In this
manner, the
resonant frequency of the monopole antenna can be adjusted to account for
variations
in manufacturing tolerances.
As will be appreciated, the input impedance of the monopole antenna is
dictated by the number of wire turns (and hence the inductances) of the wire
inductors
120, 122. On the other hand, the capacitance of the capacitive antenna element
is
dictated by the size and shape of the radiation plate 104 and the casing 102.
Accordingly, the present invention offers the significant advantage of
allowing the
input impedance of the monopole antennae to be matched to that of the wireless
communications circuitry by adjusting the number of wire turns, while also
allowing
the resonant frequency and bandwidth of the antenna to be adjusted as desired
by
altering the dimensions of the radiation plate 104 and/or the casing 102. In
each
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case, however, preferably the length and width of the radiation plate 104 and
the end
112a remains less than 10% of the operating wavelength of the antenna at the
resonant
frequency.
The following table describe sample resonant frequencies obtained using
several different dimensional variations of the present invention. For
reference, the
symbols W, L, H, D, N1, and N2, as used below, are depicted in Figs. 5a, Sb.
Given
that the antenna length of the conventional cellular telephone is
approximately 77 mm
(operating in the GSM band), the following table demonstrates the substantial
height
reductions also attainable with the present invention.
FREQ. W* I L* I H* D* ~ N1 N2 TYPE
I I
850 47 23 9.5 4.2 0.6 1.4 Cellular
Mhz turns turns
1800 23 8 4.5 2.3 0.6 1.4 PCS
Mhz turns turns
850 37 20 10 5.2 0.5 1.5 Cellular
Mhz turns turns
1800 23 8 4.5 2.3 0.6 1.4 PCS
Mhz turns turns
* = all dimensions are in millimetres (mm)
The present invention is defined by the claims appended hereto, with the
foregoing description being merely illustrative of the preferred embodiment of
the
invention. Those of ordinary skill may envisage certain additions, deletions
and/or
modifications to the described embodiment, which although not explicitly
described
herein, do not depart from the spirit or scope of the invention, as defined by
the
appended claims.
