Note: Descriptions are shown in the official language in which they were submitted.
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WIDEBAND CIRCULARLY POLARIZED HYBRID DIELECTRIC RESONATOR
ANTENNA
FIELD OF THE INVENTION
The present invention relates to wideband circularly polarized
antennas.
BACKGROUND OF THE INVENTION
Most satellite communication and navigation systems transmit
signals using circularly polarized (CE) waves to benefit from
the advantages that CP waves offer. Circularly polarized
antennas having good axial ratio (AR) over the operating
frequency band and over a wide half-power beamwidth (HPBW) are
then required to establish and maintain satellite links from
any location on Earth. In particular, the navigation
applications using any satellite navigation systems (SNS) need
antennas exhibiting an excellent AR over a wide frequency band
(or multiple bands) and over a wide beamwidth to overcome low
horizon signal reception.
Some of the prior art antennas that meet some of these
requirements are: (1) the printed stacked patch antenna, (2)
the cross printed dipole, and (3) the Folded Printed
Quadrifilar Helical Antenna (FPQHA).
Dielectric Resonator Antennas (DRAs) offer high-radiation
efficiency, a high degree of flexibility, and have inherently a
wide operating bandwidth. In addition, compact antennas based
on dielectric resonators are achievable by optimizing the width
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to height ratio or using high permittivity material. However, in
the prior art, little attention has been given to multi-band and
wideband circularly polarized DRA designs.
A more recent approach to improve the bandwidth of DRA antennas
consists of combining two radiating bands, one using the
dielectric resonator and one using the feed network. In this
case, the feed network is performing a dual function: providing
feeding to the DRA and also radiating on its own, but at a
predefined band. Such an antenna is referred to as a hybrid
dielectric resonator antenna. This type of antenna can have a
very wide bandwidth while maintaining its radiation
characteristics over the operating frequency band.
Several techniques have been proposed to generate CF when using
DRAs. The different techniques can be classified into two
categories: (1) single probe feed, and (2) multiple probe feed.
Single probe feed schemes generally do not achieve AR bandwidth
as wide as multiple probe feed. Their frequency bandwidth is
usually limited to a few percent. By contrast, multiple probe
configurations allow broad AR bandwidth, in the range of 20%.
In the prior art, Leung et al. disclose that DRA designs fed by
conformal lines are interesting solutions to generate CF over a
wide bandwidth [K. W. Leung, W. C. Wong, K. M. Luk, and E. K. N.
Yung, "Circular-polarised dielectric resonator antenna excited
by dual conformal strips," Electron. Lett., vol. 36, no. 6, pp.
484-486, March 2000]. However, the bandwidth obtained here is
not sufficient to cover the 32.2% bandwidth including all the
SNS, from 1.16 to 1.61 GHz. Buerkle et al. also presented a
dual-band DRA achieving a bandwidth over 25% [A. Buerkle, K.
Sarabandi, H. Mosallaei, "Compact Slot and
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Dielectric Resonator Antenna With Dual-Resonance, Broadband
Characteristics," IEEE Trans. Antennas and Propag., vol. 53, no.
3, pp. 1020-1027, March 2005].
Based on the aforementioned shortcomings of the prior art, the
present invention seeks to provide an improved hybrid DRA
design.
SUMMARY OF INVENTION
The present invention provides a hybrid antenna comprised of a
DRA and four sequentially rotated feed slots to enhance the AR
bandwidth in order to cover the entire SNS frequency bandwidth
with one antenna.
The hybrid DRA design of the present invention offers a greater
bandwidth and a better axial ratio compared to other CP DRA
presented in the prior art. Among the advantages of this
antenna are its compact geometry and its relatively low profile.
In one aspect, this document discloses a dielectric resonator
antenna comprising: a dielectric resonator; a ground plane,
operatively coupled with the dielectric resonator, the ground
plane having four independent slots with each slot being arc in
shape and forming a ring configuration; and a substrate,
operatively coupled to the ground plane, having a feeding
network consisting of four microstrip lines, with each
microstrip line feeding independently into each slot, wherein
the four slots are constructed and geometrically arranged to
ensure proper circular polarization and coupling to the
dielectric resonator; and wherein the antenna feeding network
combines the four microstrip lines with a 90 degree phase
difference to generate circular polarization over a wide
frequency band and wherein the feeding network includes compact
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wideband rat-race coupler combined with two surface mount (SMT)
branch-line hybrid couplers.
BRIEF DESCRIPTION OF THE DRAWINGS
The embodiments of the present invention will now be described
by reference to the following figures, in which identical
reference numerals in different figures indicate identical
elements and in which:
FIGURE 1 shows an exploded view of a hybrid DRA in
accordance with an embodiment of the present invention;
FIGURE 2 shows an exploded view of a hybrid DRA in
accordance with another embodiment of the present
invention;
FIGURE 3 shows an exploded view of a hybrid DRA in
accordance with another embodiment of the present
invention;
FIGURE 4 shows a cross-sectional and side sectional view
of the hybrid DRA in accordance with another embodiment
of the present invention;
FIGURE 5 shows a graphical representation of a simulated
reflection coefficient and boresight gain of the hybrid
DRA in accordance with another embodiment of the present
invention;
FIGURE 6 shows a graphical representation of simulated
coherent polarization radiation patterns of the hybrid
DRA in accordance with another embodiment of the present
invention;
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FIGURE 7 shows a circuitry layout of the hybrid DRA
feeding network in accordance with another embodiment
of the present invention;
FIGURE 8a shows a top view and FIGURE 8b shows a
bottom view of a hybrid DRA with the antenna feeding
network fabricated in accordance with another
embodiment of the present invention;
FIGURE 9 shows a graphical representation of an
experimental reflection coefficient of the hybrid DRA
in accordance with another embodiment of the present
invention;
FIGURE 10 shows a graphical representation of
experimental maximum realized gain as a function of the
frequency in accordance with another embodiment of the
present invention;
FIGURE 11 shows a graphical representation of
experimental radiation patterns as a function of the
elevation angle for the cut p=00in accordance with
another embodiment of the present invention;
FIGURE 12 shows a graphical representation of an
experimental axial ratio at boresight as a function of
the frequency in accordance with another embodiment of
the present invention;
FIGURE 13 shows cross-sectional and side views of the
hybrid DRA showing arc-shaped slots in accordance with
another embodiment of the present invention; and
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FIGURE 14 shows a graphical representation of a
simulated reflection coefficient and boresight gain of
the hybrid DRA shown in FIGURE 13.
The Figures are not to scale and some features may be
exaggerated or minimized to show details of particular elements
while related elements may have been eliminated to prevent
obscuring novel aspects. Therefore, specific structural and
functional details disclosed herein are not to be interpreted
as limiting but merely as a basis for the claims and as a
representative basis for teaching one skilled in the art to
variously employ the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention includes a cylindrical DRA fed by four
slots that are constructed and geometrically arranged to ensure
proper circular polarization and coupling to the dielectric
resonator. FIGURE 1 shows an exploded view of the hybrid DRA
configuration according to an embodiment of the present
invention.
As shown in FIGURE 1, the hybrid DRA consists of a dielectric
resonator 10, a ground plane 20 that includes four (4) slots
30A, 30B, 30C, 30D, a substrate 40 that includes four (4)
feeding lines 50A, 50B, 50C, 50D, and a black plate housing 60.
The dielectric resonator 10 is operatively coupled to the
ground plane 20. The ground plane 20 is in turn operatively
coupled to the substrate 40. Finally, the substrate 40 may be
operatively coupled to a back plate housing 60 in accordance
with an alternative embodiment of the present invention.
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In FIGURE 1, the four (4) slots 30A, 30B, 30C, 30D are arc-
shaped. However, the present invention contemplates other
shapes, such as rectangular. FIGURE 2 is an exploded view of a
hybrid DRA in accordance with another embodiment of the present
invention, in which the four slots are rectangular in shape.
Therefore, the present invention is not limited to a specific
shape for each of the slots.
While the dielectric resonator 10 shown in FIGURE 1 is
cylindrical in shape, other shapes are contemplated by the
present invention. For example, FIGURE 3 is an exploded view of
a hybrid DRA in accordance with another embodiment of the
present invention, in which the dielectric resonator is
rectangular in shape.
In one embodiment of the present Invention, the dielectric
resonator 10 was glued to the ground plane 20 for operatively
coupling.
Also, according to another embodiment, plated through holes were
inserted into the substrate 40 to connect the ground plane 20 of
the antenna to the ground plane of components of the feeding
network for operative coupling (FIGURES 7 and 8A and 83 show the
holes and the feeding network).
In accordance with another embodiment of the present invention,
FIGURE 4 shows a cross-sectional view in the upper portion of
the drawing and a side sectional view of the hybrid DRA
according to another embodiment of the present invention. Here,
the slots shown are rectangular, rather than arc-shaped. In this
embodiment, the hybrid DRA also has a dielectric resonator that
is cylindrical, as shown in FIGURE 1. For exemplary
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purposes, the cylindrical radius is a = 31.75 mm and the
cylindrical height is h = 22 mm, wherein the dielectric
resonators has permittivity equal to 10. The dielectric
resonator shown in FIGURE 4 has been designed to resonate at
around 1.5 GHz.
According to the present invention and with further reference
to FIGURE 4, four degenerate HE116 modes are excited using the
four slots and are fed by the four microstrip feeding lines
with a 900 phase difference to generate CP.
It should be mentioned here that the hybrid mode, referred to
as HE if the electrical component is dominant or EH if the
magnetic component is dominant, is commonly used to excite
cylindrical DRAs. The HE115 mode radiates like a short
magnetic dipole, which is desirable for wide coverage. The mode
subscripts refer to field variations in the azimuth, radial,
and axial directions, respectively, in cylindrical coordinates.
In accordance with the present invention, the substrate 40
shown in FIGURES 1 through 4 and 13 may be made of FR-4 (the
National Electrical Manufacturers Associations - NEMA) grade
designation for glass reinforced epoxy laminate sheets)
material (cr=4.4) to accommodate the feeding circuit of the
DRA. Alternatively and as a further example, the substrate may
be made of CER-10 material, which is manufactured by TaconicTm.
The CER-10 substrate is an organic-ceramic laminate based on
woven glass reinforcement. This material provides excellent
dimensional stability and enhanced flexural strength.
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As shown in FIGURE 1, the slots 30A, 30B, 30C, 30D are etched
in the ground plane. In the exemplary embodiment of FIGURE 4,
the Wgnd dimension of the ground plane is approximately 160 mm.
Also in the exemplary embodiment of FIGURE 4, the length of the
rectangular slots is close to Ag/2 at approximately 1.25 GHz,
and thus the length dimension Ls is approximately 36 mm and the
width Ws is approximately 8.8 mm. The feeding line stub length
Lm is approximately 12.9 mm. The slots coordinates relative to
the dielectric center are Ssx is approximately 4 mm along the x
direction, and Ssy is approximately 19.4 mm in the y direction.
The position of the feeding lines Smx relative to the vertical
centerline of the substrate is approximately 11 mm.
In addition, the following hybrid dielectric resonator antennas
have been designed using different dielectric permittivity,
dielectric and slot shapes. Configurations [1], [2], and [5]
have been fabricated and tested. The different configurations
are summarized below in Table 1:
Config. Dk a h Dielectric Slot shape
Substrate
[mm] [mm] shape
material
[1] 10 50 24 Square
Rectangular FR-4
[2] 10 31.75 22
Cylindrical Rectangular FR-4
[3] 10 31.75 22 Cylindrical
Arc FR-4
[4] 16 25.4 18 Cylindrical
Arc CER-10
[5] 30 19.05 15 Cylindrical
Arc CER-10
Table 1 - Various hybrid DRA configurations
The last column in Table 1 specifies the type of substrate
material used. In configurations [1] through [3], the substrate
material used was FR-4, which has an approximate permittivity
of 4.4. In configurations [4] and [5], the substrate material
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used was CER-10. The permittivity of this CER-10 material is 10
and is very stable over a range of frequencies.
The simulation and/or real testing of the various
configurations demonstrated that both square and cylindrical
shapes are suitable shapes for the dielectric resonator. It was
found that both dielectric resonator shapes lead to similar
performance. The arc-shaped slots also yielded very similar
performance to the rectangular slots. A general consistency was
observed between the simulations and the real measurements.
In configuration [5], the permittivity of this dielectric
resonator was increased to significantly reduce its physical
size. To determine the size of the resonator, equation [1] was
used to calculate the required length of the slot, so as to
ensure that the four slots could operatively fit underneath the
dielectric resonator.
(1) Ls = A0 / (2*sqrt(Dk)) where X 0 = 3e8/f
__
wherein: f=1.25 GHz and Dk is the dielectric
permittivity
For example, the required length for the slots, where the
dielectric resonator has a permittivity of 16, is Ls = 30 mm.
The available perimeter is the area delimited by the dielectric
resonator perimeter and is estimated at 122 mm (based on an
equation of 2*pi*(a-Ws/2-1 mm) with a=50.8 mm and Ws=10 mm),
which is below 4*Ls. Based on these preceding calculations,
further optimizations and adjustments may be required for
adequate matching and coupling. The matching is tuned using a
serial microstrip line stub of length Lm, starting at the
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center of the slot, and the coupling is adjusted using the slot
location and width.
For the hybrid DRA shown in FIGURE 4, a graphical
representation of a simulated reflection coefficient and
boresight gain is shown in FIGURE 5. The simulations using the
commercial software HFSS ["High Frequency Structure Simulator
v. 11.0," Ansoft Corp., 2008, online: www.ansoft.com.] show
very good matching from 1.07 GHz to 1.65 GHz, corresponding to
an impedance bandwidth of 44%. The gain at boresight is above 0
dBic from 1.11 to 1.68 GHz.
For the hybrid DRA shown in FIGURE 4, FIGURE 6 shows a
graphical representation of simulated coherent polarization
radiation patterns of this hybrid DRA. The antenna feeding
network was not part of the simulated model, and a 90 phase
difference was applied between each of the four microstrip
lines. The simulated half-power beamwidth (HPBW) is 900 at the
lower and central frequencies, and increases to 1100 towards
the high end of the bandwidth. The obtained AR at boresight is
under 0.1 dB over the entire band. The antenna presents an AR
beamwidth (AR < 3 dB) of 850 at 1.15 GHz, 1000 at 1.4 GHz and
1100 at 1.6 GHz.
It should be noted that the use of a rectangular dielectric
resonator leads to a very similar configuration when exciting
degenerate TE511 and TE151 (Transverse Electric) modes. The
transverse electric mode, referred to as TE, is commonly used
to excite rectangular DRAs. The TE511 and TE151 radiates like a
short magnetic dipole. The subscripts represent the field
variation in the X-, y-, and z-directions, respectively, in
Cartesian coordinates. A square-shaped dielectric resonator is
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also contemplated. Therefore, the present invention is not
limited to the shape of the dielectric resonator. However, the
cylindrical shape may be more suitable in commercial
applications because it has a more compact surface area.
FIGURE 7 shows a circuitry layout of the hybrid DRA feeding
network in accordance with another embodiment of the present
invention. The antenna feeding network has to provide 900 phase
difference between the four slots over a wideband. To achieve
this, a compact wideband rat-race coupler as detailed in the
prior art [M. Caillet, M. Clenet, A. Sharaiha, and Y.M.M. Antar,
"A Compact Wide-Band Rat-Race Hybrid Using Microstrip Lines,"
IEEE Microw. Wireless Compon. Lett., vol. 19, no. 4, pp. 191-
193, Apr. 2009] has been combined with two surface mount (SMT)
branch-line hybrid couplers [3-dB / 900 hybrid coupler, "Model
XC1400P-03S" Anarera, online: www.anaren.com].
The antenna shown in FIGURE 4 was fabricated using Emerson &
Cuming Eccostock HIK10 dielectric of an approximate permittivity
of 10 for the dielectric resonator, and an FR4 substrate of
approximately 30 mil (0.76 mm) thickness for the feeding
network.
FIGURE 8a shows a top view and FIGURE 8b shows a bottom view of
a hybrid DRA fabricated in accordance with another embodiment of
the present invention. Plated thru holes were inserted into the
substrate to operatively connect the ground plane of the antenna
to the ground of the SMT branch-line hybrid couplers of the
feeding network shown in FIGURE 7.
FIGURE 9 shows a graphical representation of an experimental
reflection coefficient of the hybrid DRA shown in FIGURE 4. It
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can be seen that the DRA covers the 1.08 to 1.82 GHz frequency
band, corresponding to an impedance bandwidth of 51%.
Concerning the radiation characteristics, they were measured
from 1.125 to 1.625 GHz in an anechoic chamber. FIGURE 10 shows
a graphical representation of experimental maximum realized
gain as a function of the frequency of the hybrid DRA shown in
FIGURE 4. The experimental maximum realized gain remains above
1.5 dBic over the entire band, with a peak around 3.75 dBic at
1.475 GHz.
FIGURE 11 shows a graphical representation of an experimental
radiation patterns as a function of the elevation angle for the
cut 9=00 of the hybrid DRA shown in FIGURE 4. The measured HPBW
is 750 at 1.175 GHz, 800 at 1.375 GHz and 850 at 1.575 GHz.
FIGURE 12 shows a graphical representation of an experimental
axial ratio at boresight as a function of the frequency for the
hybrid DRA shown in FIGURE 4. The AR at boresight remains under
1.5 dB over the entire band. The AR beamwidth is 1400 at 1.175
GHz, 2000 at 1.375 GHz and 1950 at 1.575 GHz for the planes
9=00 and 9=900. Regarding the cut at 9=450, a narrower AR
beamwidth of 1000 has been noticed at all investigated
frequencies.
The antenna efficiency of the hybrid DRA shown in FIGURE 4 was
evaluated by comparing the directivity and the measured gain,
and found to be over 70%. The overall performance of the
fabricated antenna is very similar to the simulated results.
Due to the presence of the slots, back-radiation does occur.
The front to back radiation ratio varies from 5 dB at 1.15 GHz
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to 10 dB at 1.6 GHz. In accordance with an embodiment of the
present invention, the back-radiation level can be reduced
using a metallic back plate housing appropriately positioned at
the back of the antenna. For instance, a front to back
radiation ratio of 10 dB was achieved at 1.15 GHz using an
approximately 150 x 150 mm2 metallic sheet located 15 mm behind
the slots. No significant effect has been observed regarding
the antenna characteristics (impedance, gain, radiation
patterns and AR).
It should be clearly understood by the skilled artisan that the
back plate housing is an optional element of the present
invention.
To make the antenna more compact in size, the present invention
contemplates reducing the surface area it occupies.
Permittivities of approximately 16 and 30 have been
successfully used for the dielectric resonator. Also, as
previously mentioned with reference to FIGURE 3, the shape of
the slots may be modified to an arc, and this provides more
efficient coupling than using rectangular-shaped slots as the
slots are completely confined within the circle corresponding
to the DRA circumference. The resultant geometry is shown in
Figure 13. The surface of the compact dielectric resonator
design using a permittivity of 30 is approximately 28% the
surface of the cylindrical-shaped design having a permittivity
of 10. In FIGURE 13, each of the four arc slots has a radius
of approximately 19 mm, an approximate angle as of 890, and Ws
is approximately 12 mm wide. Also, the height h of this
dielectric resonator is approximately 15 mm. The angle at is
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approximately 100 and the length Lm is approximately 8mm. The
width of the ground plane Wgnd is approximately 100 mm.
FIGURE 14 shows a graphical representation of a simulated
reflection coefficient and boresight gain of the hybrid DRA
shown in FIGURE 13. The simulated reflection coefficient and
gain bandwidth are slightly reduced compared to the DRA using a
dielectric resonator having a permittivity of approximately 16,
but it still provides enough bandwidth to cover all the SNS
applications. Radiation patterns and axial ratio are almost
identical to the rectangular-shaped geometry.
It should also be mentioned that the present invention includes
a conventional unilayer substrate material, where basic shapes
such as square or cylinder can be used for the DRA, and no
drilling into the dielectric resonator is required.
By using a higher permittivity dielectric, the DRA surface
width and height may be significantly reduced over the prior
art designs. Yet, performance of the hybrid DRA is very
similar to the original antenna. This new wideband CP hybrid
DRA has shown close performance compared to other SNS antennas
of the prior art.
The compact geometry of the hybrid DRA of the present
invention, whose smallest simulated radius is approximately 19
mm and whose smallest corresponding height is approximately 15
mm, is among the smallest SNS antennas present in the
literature. For example, the stack patch antenna of the prior
art is 61 mm wide, the cross printed dipole of the prior art is
70 mm wide and 50 mm height, or the FPQHA (folded planar
quadrifilar helical antenna) of the prior art has a radius of
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36 mm, and a height of 130 mm. In accordance with the present
invention, hybrid DRAs of smaller size can be fabricated with
higher dielectric constant material.
The embodiments of the invention described above are intended
to be only exemplary, and not a complete description of every
aspect the invention. The scope of the invention is therefore
intended to be limited solely by the scope of the appended
claims.
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