An L-band circularly polarized microstrip array antenna with relatively wide bandwidth has been developed. The array has seven subarrays which form a single cluster as part of a large overlapping cluster reflector feed array. Each of the seven subarrays consists of four uniquely arranged linearly polarized microstrip elements. A 7.5 percent impedance (VSWR less than 1.5) as well as axial ratio (less than 1 dB) bandwidths have been achieved by employing a relatively thick honeycomb substrate with special impedance matching feed probes

Huang, John

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N88-25747
A MICROSTRIP ARRAY FEED FOR MSAT SPACECRAFT REFLECTOR ANTENNA
DR. JOHN HUANG, Mobile Satellite Program, Jet Propulsion Laboratory,
United States.
JET PROPULSION LABORATORY
4800 Oak Grove Drive, 161-213
Pasadena, California 91109
United States
ABSTRACT
An L-band circularly polarized microstrip array antenna with
relatively wide bandwidth has been developed. The array has seven
subarrays which form a single cluster as part of a large overlapping
cluster reflector feed array. Each of the seven subarrays consists of
four uniquely arranged linearly polarized microstrip elements. A 7.5%
impedance (VSWR <1.5) as well as axial ratio (<I dB) bandwidths have
been achieved by employing a relatively thick honeycomb substrate with
special impedance matching feed probes.
INTRODUCTION
Very large multiple-beam satellite reflector antennas in the 20 to
55 meter range have been planned for the future U.S. Land Mobile
Satellite Systems (LMSS)[I]. From 40 to 90 contiguous beams covering
Continental United States (CONUS) are to be generated from overlapping
cluster feed arrays [2] with diameters of up to 6 meters. A structure
of this size should have the capability of being folded and stowed in
the satellite launching vehicle. Consequently, the feed array should
be low in profile and light in weight. An earlier experimental feed
array to demonstrate the feasibility of generating contiguous beams by
using the overlapping cluster feed concept was successfully developed
and tested[3]. In that feed array, relatively narrowband microstrip
patches were used in order to reduce thecomplexity of the experiment.
This paper describes the development of a new array which has a
relatively broadband performance sufficient to cover the bandwidth
requirements of the Land Mobile Satellite System. To meet the light
weight requirement of the system and to cover both downlink frequencies
(1545 to 1559 MHz) and uplink frequencies (1646 to 1660 MHz), the
microstrip antenna with a half-inch thick honeycomb-supported substrate
has been selected. Since a half-inch (0.07 wavelength) substrate is
relatively thick for microstrip radiators, four feed probes are
required [A] per single element to suppress the undesired higher order
modes and thus to generate acceptable circular polarization (c.p.)
across the total bandwidth. For a large array, such a single-patch
four-probe feed system would increase the complexity of an already
complex feed beam-forming network, and will make it heavier and more
prone to RF losses.
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https://ntrs.nasa.gov/search.jsp?R=19880016363 2020-03-20T06:52:06+00:00Z
In a previous paper [5] the theoretical background was presented
for obtaining a circularly polarized array from single-probe fed
linearly polarized patch elements. The c.p. is achieved by having a
basic 2 x 2 subarray with specific orientation and phase arrangement
for the feed elements. As shown in Figure i, the feed elements'
angular orientation as well as phase are arranged in the 0 °, 90 ° , 180 ° ,
270 ° fashion. With such a system, not only is the feed complexity
reduced, but also the bandwidth performance is improved. To
demonstrate the concept, a single cluster array composed of seven
subarrays with 28 single-feed linearly polarized patches has been
constructed and tested.
HARDWARE DESCRIPTION AND RESULTS
The overlapping cluster array concept can be demonstrated by 2
clusters of arrays as shown in Figure 2. The two large circles
represent the arrays to generate two contiguous beams, and each small
circle is a sub-array consisting of four microstrip patches. The array
that has been constructed and tested is one of the large circles and
has a total of 28 microstrip patches. Dimensions of this array are
illustrated in Figure 3. The relatively sparse element spacing is a
result of the overlapping cluster arrangement for optimum reflector
illumination with a minimum number of elements in the array. The array
has an amplitude taper so that the reflector is illuminated with proper
edge taper required for achieving low sidelobe levels. Each of the 7
subarrays are fed by a stripline 4-way hybrid power divider so that 0 °,
90 ° , 180 °, 270 ° feeding phases for the elements can be realized.
The fabricated array is illustrated in Figure 4 where the
placement of the feed probes in the 4 patches of each subarray is
similar to that shown in Figure I. Since the honey-comb supported
microstrip substrate is relatively thick, it generates a large amount
of surface waves which cause significant mutual coupling between
subarrays. These surface waves and the consequent mutual coupling
effect cause asymmetry in the field components and degrade the c.p.
performance. Introduction of metallic baffles between the subarrays
can block out most of the surface waves and thus reduce the mutual
coupling between subarrays. It is found that one-inch tall metallic
baffles are required between all the subarrays to bring the on-axis
axial- ratio from 3 dB to less than I dB. Within each subarray,
however, due to the 0 °, 90 ° , 180 °, 270 ° element orientation and phase
arrangement, most of the unwanted field components caused by surface
waves are cancelled and field symmetry is preserved.
In feeding a microstrip antenna with a thick substrate, generally
a large inductance occurs around the feed probe. Normally, an
impedance matching circuit can be etched prior to each feed probe for
tuning out the probe inductance. This, however, introduces complexity
and additional loss to the hybrid circuit. Furthermore, there may not
be enough real estate available on the hybrid circuit board layer to
accommodate the matching circuits and, consequently, an additional
layer of circuit board may be needed. To reduce this complexity, a
tear-drop shaped probe has been used to effectively tune out the
inductance and provide 7.5% bandwidth. The tear-drop shape introduces
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an appropriate amount of capacitance to cancel the undesired inductance
and provide a gradual change of the field from the coaxial line into
the thick microstrip antenna substrate.
The measured radiation patterns of the array shown in Figures 3
and 4 are illustrated in Figure 5. These patterns are produced by a
spinning-linear-dipole technique to demonstrate the c.p. quality of the
array. Figure 5(a) is the pattern measured at 1545 MHz for _ - 45 °
plane cut, while Figure 5(b) is measured at 1660 MHz. Patterns
measured at other plane cuts will be presented in the conference. The
worst axial ratio on the main beam peak is less than 0.8 dB for both
frequencies. The main beam of the array is rather symmetrical, and its
10 AR h==mw_h= fnr _ = OO, _5 0 =n_ qo 0 nlnnp= n_ n_nrlv equal _
main beam below i0 dB and the sidelobes, however, are different for
various cuts. The design goal was to achieve circular symmetry of the
main beam down to 15 dB from the peak. A reflector with edge taper of
this level will produce better than 30 dB sidelobe levels. Because of the
inherent circular asymmetry of the 7-subarray feed, perfect symmetry
for the feed pattern in all the cuts is hardly feasible.
The relatively high feed array sidelobe levels are a consequence
of the relatively large separation of the subarrays (1.5 and 1.3
wavelengths in 0° and 90 ° plane) imposed by the overlapping cluster
requirement of the contiguous multibeam system[2]. These high
sidelobes will result in large spill-over past the reflector edge and
hence lower reflector efficiency when compared with a gain optimized
reflector. A complete three dimensional pattern integration shows that
the relatively high sidelobes encountered here will only contribute to
less than 0.3 dB of gain loss.
CONCLUSION
A circularly polarized feed array for a spacecraft reflector
antenna has been constructed by using linearly polarized microstrip
elements. The array achieved an axial ratio better than 0.8 dB at the
pattern peak and better than 3 dB down to 20 degrees from the peak,
across a 7.5% frequency bandwidth. A tear-drop shaped feed probe is
used to achieve wideband input impedance matching for the relatively
thick microstrip substrate. Due to the success of this experimental
study, a circularly polarized microstrip array can have a simplified
feed mechanism, reduced RF loss in the feed circuit, and improved
radiation performance over a wide frequency bandwidth. It is expected
that I0 to 15 percent bandwidth can be achieved by using the same
technique if the substrate thickness is further increased.
REFERENCES
[1] F. Naderi, G. H. Knouse, and W. J. Weber, "NASA's Mobile Satellite
Communications Program; Ground and Space Segment Technologies,"
35th International Aeronautics Federation Congress, Lausanne,
Switzerland, October i0, 1984.
473
[2] V. Jamnejad, "Multibeam Feed System Design Considerations for
Single Aperture Large Space Antennas," NASA Conference on Large
Space Antenna Technology, December 1984.
[3] K. Woo, "Multiple Beam Antenna Feed Development," 1986 AP-S
International Symposium Digest, pp 409-412, June 1986.
[4] J. Huang, "Circularly Polarized Conical Patterns from Circular
Microstrip Antennas," IEEE Transactions on Antennas and
Propagation, Vol. AP-32, No. 9, pp 991-994, September 1984.
[5] J. Huang, "A Technique for an Array to Generate Circular
Polarization with Linearly Polarized Elements," IEEE Transactions
on Antennas and Propagation, Vol. AP-34, No. 9, pp 1113-1124,
September 1986.
= 270°_
_=0 °
= 180 °
I t I
= 90 °
Figure I. Square grid 2 x 2 subarray of 4 identical lineary
polarized microstrip elements that together generate
C.P. The heavy dot on each patch indicates the feed
location and @ indicates the feed phase. Arrows are
E-field polarization directions.
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Figure 2. Two overlapping 7-subarray cluster feeds, df is
separation distance between any two adjacent subarrays
and between two cluster feeds.
Y
m a _ n _ a l
I -13 dB I _ 0 dB"----T'_-'_-13 dB / = x
_---_ f-_'- ---__//_--_..x_ 0.65X
- 3dB I 13 d 0.65X
'E3 E_
"_ _ ..-_ / I_ _'_
xl_ _1F °'75 I-0.75 x-I
Figure 3. 7-subarray single cluster feed with 28 mlcrostrip
patches.
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Figure 4. Constructed 7tsubarray single cluster feed with 28 
microstrip patches. Each subarray is enclosed by 1.0- 
inch tall aluminum baffles. 
Figure 5. Measured array patterns at 6 - 45O plane cut for: 
(a) frequency - 1.54 GHz and (b) frequency - 1.66 GHz. 
476 


A microstrip array feed for MSAT spacecraft reflector antenna

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