A BROADBAND PLANAR ANTENNA EMPLOYING

WAVEGUIDE PARALLEL FEED CIRCUIT

Kazuhisa YOSHIKI', Yasuhiro FUJII, and Shigeru EGASHIRA'

Matsushrta Electric Works, Ltd., Osaka, 57 I Japan
' Department of Electronics, Saga University, Saga, 840 Japan

1. INTRODUCTION
Recently, planar antennas with a relatively high gain of about 30 dBi have
drawn increasing attention due to their ease of installation and aesthetical
appearance. And it has therefore been under development by many researchers.
Ejch of the planar antennas comprises of multiple radiators arrayed on a plane.
They are roughly classified by feed method into a series feed t y v and a parallel
feed type.
The series feed planar antennas".*,3'are simple in the construction of feed
circuit, especially using waveguides to feed power, those have extremely small
feed loss. In theory, however, those with a large-scale array structure have a
disadvantage of narrow bandwidth.
On the other hand, parallel feed planar antennas mostly use strip lines to
constitute a parallel feed circuit. This type of construction has made it possible
to realize a very thin antenna for X - b a ~ ~ d ' However,
~~'. the strip lines, use a
dielectric, which provides large dielectric and radiating losses, so they ax not
suitable to large-scale array structures required at the higher frequency hands.
Similarly, they are not suitable for high-power applications. such as
transmission antennas, either.
With this background, we have been investigating a highly efficient,
broadband planar antenna that uses low loss rectangular waveguides with a
parallel feed circuit. The antenna is thus compatible with a large array structure
while having a high manufacturabiiity and wide band impedance
characteristics.

2. ANTENNA STRUCTURE
The basic structure of the antenna is shown in Fig. I . The antenna operates
at X-band, and consist of a 16element array antenna with an array spacing of
0.9 A ( A : free space wavelength). The antenna section is formed of subarray
units each comprising of 2 X 2 aperture elements. The four aperture elements
are excited through a cavity by a single waveguide aperture. Fig. 2 shows the
detail of a subarray unit. The subarray unit, connected to the aperture at one of
the L-comers of a feed circuit, has a step-shaped impedance matching section
and a cavity. A lattice frame is provided over the radiators. The frame separates
the apertures from one another and thus improves the matching characteristic
with free space.
The subarray unit design enables construction of a rectangular-waveguide
parallel feed circuit. For a 16-element array antenna, four subarray units are
arranged in a square. Therefore, the feed circuit for the antenna is composed of
a combination of four L-corners and three T-junctions, all formed of

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rectangular waveguides. For each T-junction connected to L-corners, the
branching portion is offset from the center by A g/4 ( A g: guide wavelength)
to excite the four subarray units in phase. The T-junctions do not use posts but
have such a construction that does not require adjustment for impedance
matching. An H-shaped waveguide parallel feed circuit has been realized by
combining these L-comers and T-junctions. To meet the demand for a larger-
scale array structure., two or moE H-shaped circuits may be combined.
Both the antenna section and the H-shaped feed circuit can be molded into an
integral body by casting or the otherwise. The antenna thus has a very simple
construction and can be assembled with a small number of parts.

3. EXPERIMENTAL RESULTS
The transmission characteristics of the developed H-shaped parallel feed
circuit were. measured. The frequency bandwidth of the circuit for -20 dB or
lower return loss (R.L.) and -6.3 dB or higher transmission loss (T.L.) was
4.8% (12.2-12.8 GHz).
Fig. 3 shows the frequency characteristic of R.L. of a parallel feed antenna
with a 16element array. As shown, this antenna provides a broad-band R.L.-
frequency characteristic, with the frequency bandwidth for -20 dB or lower
R.L. being 9% ( I 1.9-13.0 GHz), and that for -30 dB or lower R.L. being
4% (12.25- 12.75 GHz). The frequency characteristic of the gain is shown in
Fig. 4. As shown, the aperture efficiency of this antenna is 90% or higher in
the frequency band from 11.7 to 12.4 GHz, indicating that the antenna has a
broad bandwidth and a high efficiency. Satisfactory operation was also
confirmed with a Wlement array antenna.

4. CONCLUSIONS
The basic structure of a waveguide parallel feed antenna has been described.
An H-shaped parallel feed circuit for a 16element array antenna was developed
by combining four L-comers and three T-junctions. The feed circuit is a basic
unit for constructing a larger-scale array. Subarray units each having four
apertures excited by a single waveguide aperture were combined with the
above-mentioned H-shaped circuit to constitute a 16-element array antenna.
Thus, a broad-band, highly efficient waveguide panllel feed antenna has been
obtained as X-band. The antenna is extremely simple in structure, and since it
is entirely made of metal it is also heat-resistant. Furthermore,the low loss
structure provides several advantages for applications at higher frequencies
such as millimeter wave.

REFERENCES
[I] A G Demeryd, S R Rengarajan, S-0 Brattstrom: "A Slotted Waveguide
Antenna Employing Resonant compound Coupling Slots for beam Shaping";
1992 International IEEEAP-S Symposium Digest, pp.57-60
[2] P. Kale, S. B. Sharma, R. K. Malavia, K. Vellimalai: "Slotted Waveguide
Planar Array at C-Band"; The 3rd Asia-Pacific Microwave Conference Proc.
Tokyo, 87-90, 1990
131 J. Takada, M. Ando, N. Goto: "A Reflection Canceling Slot Set in a
Linearly Polarized Radial Line Slot Antenna"; IEEE Trans. on Antennas
Propagation, W.40, No. 4, Apl. 1992

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[4] K. Tsukamoto, K. Okuno. T. Abko: "A Aperture Array Antenna for DBS
Reception"; Tech. Rep. IEICE (JAPAN), AP90-104, Feb. 1991
[ 5 ] S . Nakahara. M. Matsunaga, S . Mano: "Planar Antenna with Notch and
Projection Degeneracy-Removing Elements"; The 3rd Asia-Pacific Mlcrowave
Conference Proc. Tokyo, 173-176, 1990

aped waveguide
le1 feed circuit

Fig. I A basic design of the antenna.

- - 38.5

f'
A
-+A
A-A' cross section
Top view
Fig.2 A subarray unit with 4 aperture elements.

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Fig.3 Measured return loss characteristicsof the antenna.

Fig.4 Measured gain of the antenna

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