The principle and design of an L-band omni-directional mobile communication antenna are discussed. The antenna is a circular wave guide aperture with hybrid circuits attached to higher order mode excitation. It produces polarized and symmetric two split beams in elevation. The circular waveguide is fed by eight probes with a 90 degree phase shift between their inputs. Radiation pattern characteristics are controlled by adjusting the aperture diameter and mode excitation. This antenna satisfies gain requirements as well as withstanding the harsh environment

Kijesky, J.

Kim, C. S.

Moldovan, N.

English

NASA Technical Reports Server

N88-25717
OMNI-DIRECTIONAL L-BAND ANTENNA FOR MOBILE COMMUNICATIONS
DR. C. S° KIM, N. MOLDOVAN, and J° KIJESKY
PRODELIN CORPORATION
1700 NORTHEAST CABLE DRIVE
P. 0. BOX 368
CONOVER, NORTH CAROLINA 28613
UNITED STATES
ABSTRACT
This paper discusses the principle and design of
our L-band omni-directional mobile communication antenna.
The antenna is a circular waveguide aperture with hybrid
circuits attached for higher order mode excitation. It
produces circular polarized and symmetric two split beams
in elevation. The circular waveguide is fed by eight
probes with a 90 degree phase shift between their inputs.
Radiation pattern characteristics are controlled by
adjusting the aperture diameter and mode excitation. This
antenna satisfies gain requirements as well as
withstanding the harsh environment.
INTRODUCTION
This paper describes the development of a circular
polarized, high gain, L-band transmit antenna for mobile
communications. The antenna is a circular waveguide with
an integrated stripline feed circuit to produce circular
polarized conical split-beam patterns into half space.
Its application is for the new radiodetermination
satellite service (RDSS). just approved by the FCC.
During the past few years, several antennas such as
microstrip patches [i] and spiral horns [2] have been
introduced for mobile communications. Both types of
antennas have used hybrid circuits to produce two split
broad conical beams. The ideal RF performance is to
maximize the peak gain (>5 dBi) while maintaining a broad
halfpower beamwidth (>40°). The antenna presented here
has radiation fields that can be controlled by adjusting
aperture radius and modes of excitation (TE21 or TE31).
Radiation patterns are calculated by aperture integration.
Measured radiation patterns show that good symmetry exists
and that the maximum beam axis occur at plus and minus 40%
Halfpower beamwidths of approximately 40 ° have been
achieved.
255
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THEORETICAL BACKGROUND
The basic electromagnetic principle applied to the
analysis of this antenna is Huygen's secondary source to
solve integral equations for vector potentials. Radiation
patterns are calculated by aperture integration over a
cylindrical cavity excited by higher order modes. The
radiation fields in spherical coordinates for linear
polarization (E l & E 2) can be obtained from Silver [3].
These fields can be circularly polarized by introducing a
90 ° phase difference. Physical_y, this corresponds to
phasing adjacent probes with a 90 phase difference as
shown in Figure i.
For circular polarization and desired mode
excitation, probes with proper spacing are needed. These
probes are located perpendicular to the side wall so that
their positions are parallel with the maximum E-field
vector. The fields generated from two adjacent probes are
orthogonal to each other inside the circular waveguide.
With this angular spacing (_) chosen, as shown in Figure
I, one probe is alwavs situated in the null field region
of the other probe, thus causing very little mutual
coupling between probes [1] . In order to keep beam
symmetry (Omni-Directional in azimuth) and cross
polarization low, many probes should be used to suppress
unwanted modes. To determine the radiation pattern for
circular polarization the following equation is used:
WHERE :
N =
m =
number of probes
360/N degrees
N/4 (mode number)
(i + (n-l)_)
RADIATION PATTERN AND GAIN CALCULATION
Radiation patterns are calculated by the equation
depending on the mode excitation. Gain calculation is on
the basis of G=4_Pk/Pt, where Pk is the peak power at the
beam peak axis and Pt is the total power radiated •
Radiation patterns are shown in Figure 2 for different
modes. Gain computations are also illustrated in Table 1
for both different modes and diameters.
256
On the basis of Table 1 and Figure 2, radiation
patterns and gains are in very close relationship to each
other. Usually as aperture diameter increases, higher
order modes are used. Since gains of modes 2 and 3 are
approximately above 5dB and beam axis is controlled
between 30 ° and 50 o radiation pattern and gain can be
optimized, depending on requirements.
TABLE 1 CALCULATED CAIN AND RADIATION PATTERN
MODE
(TE)
4
DIAMETER
(WAVELENGTH)
.6
.8
1.0
1.0
I.I
1.2
I .34
1.4
I .46
1.7
1.8
1.9
PEAK DIRECTION
FROM BROADSIDE
0o
0°
0 o
m u
41 °
38 °
36 °
51 °
50 °
48 °
i
56 °
54 °
52 °
CAIN
(dBi)
J
8.3
10.9
11.8
5.73
6.75
7.52
3.66
5.32
6.0
3.25
5.13
5.99
*BEAMWlDTH
J_
90 °
74 °
60 °
54 °
50 °
46 °
49 °
48 °
47 °
46 °
44 °
42 °
*BEAMWlDTH IS ONE SIDE FOR TE21 , TE31 , AND TE41
EXPERIMENTAL RESULTS
Two prototype antennas (TE21 , 8 probes and TE31 , 8
probes) were fabricated to verify their predicted
performances. Each prototype consisted of a circular
waveguide cavity, feed probes and off-the-shelf power
dividers. In order to provide the proper phase difference
between probes, coaxial delay lines were connected between
the probes and power dividers.
Comparisons between calculated and measured data
are shown in Figure 3 and 4. Radiation patterns with TE21
and TE31 have shown that a good symmetric, conical pattern
exists and that both beam axis and beam widths satisfy
calculated results.
257
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FIGURE i
CIRCULAR WAVEGUIDE CAVITY SHOWING PROBES WITH ANGULAR SPACING c_.
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--I10. O--
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FIGURE 2
CALCULATED RADIATION PAIWERNS FOR DIFFERENT MODE EXCITATIONS.
258
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OS _00_ _t _-a-_,'_'W'_
FIGURE 3
COMPARISON BETWEEN COMPUTED AND
MEASURED PATTERNS, TE21, DIA.
10.2"
F = 1.27 GHz
FIGURE 4
COMPARISON BETWEEN COMPUTED AND
MEASURED PATTERNS, TE31, DIA.
11.5"
F = 1.62GHz
FIGURE 5
AXIAL RATIO PATTERN, TE21, DIA.
10.2"
F = 1.62 GHz
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4'
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259
The axial ratio in Figure 5 is within 2 - 3 dB near
the main beam axis• The phase feeding circuitry is very
sensitive to the antenna performance; small errors in
phase-amplitude of the combiner and phase lines cause
major changes in radiation patterns. Since edge
diffraction terms are not included, errors exist between
computed and measured data around 90 ° . Overall measured
radiation patterns are in agreement with calculated
values.
REFERENCES
• J. Huang, "Circularly Polarized Conical Patterns
from Circular Microstrip Antennas". IEEE Trans. on
Antennas and Propagt., Vol. AP-32, No. 9, Sept.
1984, pp. 991-994.
Q R. Johnson and H. Jasik, "Antenna Engineering
Handbook", McGraw-Hill Book Co., 1984 Edition, pp.
14.3 - 14.24.
• Silver, "Microwave Antenna Theory and Design", New
York, Dover 1965, pp. 334-341.
260


Omni-directional L-band antenna for mobile communications

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