A low profile electronically steered phased array was successfully developed for the Mobile Satellite Experiment Program (MSAT-X). The newly invented cavity-backed printed crossed-slot was used as the radiating element. The choice of this element was based on its low elevation angle gain coverage and low profile. A nineteen-way radial type unequal power divider and eighteen three-bit diode phase shifters constitute the beamformer module which is used to scan the beams electronically. A complete hybrid mode pointing system was also developed. The major features of the antenna system are broad coverage, low profile, and fast acquisition and tracking performance, even under fading conditions. Excellent intersatellite isolation (better than 26 dB) was realized, which will provide good quality mobile satellite communication in the future

Chung, H. H.

Foy, W.

Nelson, J.

Pagels, W.

Peng, S. Y.

Schaffner, G.

Vayner, M.

English

NASA Technical Reports Server

N88-25712
MSAT-X ELECTRONICALLY STEERED PHASED ARRAY ANTENNA SYSTEM
H.H. CHUNG, W. FOY, G. SCHAFFNER, W. PAGELS, M. VAYNER, J. NELSON and S.Y. PENG
TELEDYNE RYAN ELECTRONICS
8650 Balboa Avenue
San Diego, California 92123
ABSTRACT
A low profde electronically steered phased array has been
successfully developed for the Mobile Satellite Experiment Program (MSAT-
X), under the contract with Jet Propulsion Laboratory (JPL). The newly
invented cavity-backed printed crossed-slot was used as the radiating
element. The choice of this element was based on its low elevation angle
gain coverage and low profile. A nineteen-way radial type Unequal power
divider and eighteen three-bit diode phase shifters constitute the beamformer
module which is used to scan the beams electronically. A complete hybrid
mode pointing system, which uses an external rate sensor and internal error
signals to optimize the angle tracking accuracy, has also been developed. The
major features of the developed antenna system are broad space coverage,
low profde (0.75 inch exclusive of the input RF connector and beam control
DC connector), and its fast acquisition and tracking performace, even under
fading conditions. Excellent intersatellite isolation (better than 26 dB) was
realized which will provide good quality mobile satellite communication in
the future.
INTRODUCTION
The development of low cost vehicle phased array for the MSAT-X program has been a
challenging task for the antenna designer. Broad spectrum of considerations have been taken into
account in the design phase so that the best antenna system was successfully developed during the
contrat with JPL. The design philosophy of the antenna array, beamformer and pointing system as well
as the measured array performance will be discussed in the following sections.
ANTENNA ARRAY DESIGN
Among the low profde antenna candidates, the crossed-slot and patch element were
considered. The cavity-backed printed stripline fed crossed-slot was selected, however, because it has
(1) wider beamwidth, (2) better gain and axial ratio at low elevation angle, and (3) thinner for the same
operating frequency band than that of patch antenna.
The stripline fed crossed-slot element is basically made of two separate boards. As shown in
Figure 1, crossed-slot was etched on a single sided copper clad substrate and the stripline feeders was
etched on another board of the same thickness. The two boards were bonded together. A set of plated
through holes were required to form a cavity wail. Circular polarization was obtained by feeding the
crossed-slot in phase quadrature via a integrated feed network of a 90 degrees branch line hybrid and
two 180 degrees corporate circuit (Figure 2). The four point feed with symmetrical arrangement and
phase rotations (0-90-180-270 degrees) is required to eliminate the higher order mode inside the cavity
so that the crossed-slot can radiate efficiently.
223
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Fig. I .  Photo of the crossed-slot Fig. 2. Photo or the hybrid feed network 
The  array aperture design was performed c o  i1mt the required performance be achieved at the 
lowest cost. The major constraints on the a p i u r e  c h i p  
array gain (10 dBic), (3) array thickness (within 1 inch), (4) manufacturing cost ($lSOO.OO), (5 )  
multipath rejection capability and (6)backIobe level (I? dt3). 
( I )  tntcrsatelliic isolation (20 dB), (2) 
The  design philosophy i s  using minimum number oC etements and maximum usage of the 
aperture as well a5 avoiding the grating lobes. After the detail trade off analysis, a nineteen crossed- 
slot element array with triangular lattice arrangement was ctiosen. The clement spacing was 3.9 inch. 
Proper amplitude taper o f  three circular rings of rclaiivr r x w ~ s  -6dR, -5 2 dB, and .3 dB (starting from 
the outside going towards the center) iva\ rksed to rcthte thc sid~I,>be Lcvcl so that the required 20 dB 
intersatllite isolation cat1 bc mainkained. Array s i x  1 22 kctie\ vd~ich include$ the mounting space. 
Three-bit diode phabe shifter v m q  sele~tcd to steer thr :4~tenn,ci &am 
The bearnformcr network which consists of two major  corrrponents, an unequal nineteen way 
power divider and a three hit phase shifter, had been staccessfrrlfy developed on a 20.6 diameter 
microstrip plane, as dmm in Fit. Ire 3. The hcatnforrnrr n~ twork  will continue to present design 
challenge in the future for the low cost production with high performance and reliability. 
The unequal power divider consists of an equal seven way power divider followed by six
unequal three way dividers. The divider design is based on the Wilkinson divider concept which splits
the 50 ohm line into two or more quarter wave sections of the right impedance to preserve a 50 ohm
impedance match. Balancing resistors bridge each arm at the ends of the matching sections. These
resistors keep the match when outputs are not balanced. For unequal divisions additional quarter wave
sections are required. Computer optimization using the "Touchstone" program was used to design the
impedance and resistor values.
The three bit phase shifters were based on the switched line principle. This was selected to
give the least phase shift error over the onerating band frnm lfut'g MFI7 tn lfi(_ _!I-I77, rnmn_r_d tn that
of the hybrid coupled phased shifter. The switched line phase shifter projects an 11 degrees error while
the hybrid phase shifter projects a 19 degrees error over the same frequency band. The disadvantage
of the switched line design is that twice as many diodes are required. However, the switched line
configuration can be redesigned with the back to front pin diodes at common points. This leads to the
possibility of using monolithic diode pairs so that the cost of antenna system will be further reduced.
POINTING SYSTEM DESIGN
A complete hybrid mode pointing system, which uses an external rate sensor and internal
error signals to optimize the angle tracking accuracy over a wide of contingencies, has also been
developed. Basically, the fast response operations are controlled by the angle rate sensor while the
radio link corrects the rate sensor errors. Because the rate sensor bias error changes slowly, the radio
link data is averaged over a 30 seconds period before correcting the rate sensor. This will filter out any
attitude perturbations, shadowing or fading (multipath) effects in the radio link.
When the rate sensor is disconnected, the rate sensor correction is eliminated and the radio
link loop is automatically increased. This allows somewhat less accurate but still adequate satellite
tracking on pilot signal alone;however, it will no longer be able to follow vehicle turns during signal
outages.
The sequential lobing dither of the radio link is held to plus and minus 1.5 degrees under
normal signal strength conditions in order to have maximum intrersatellite isolation. The dither
magnitude is only increased if signal level drops. This is done automatically by sensing the strength of
the pilot signal. The dither frequency is a modifiable parameter which can be adjusted between 10 and
200 Hz.
The elevation angle of the antenna beam is controlled by searching for maximum signal
strength. Because of the wide antenna beamwidth and the generally slow variation of satellite elevation
angle, this search is done at rate of one incremental beam per second from 30 degrees through 70
degrees (from the array broadside) in increments of 5 degrees. The total number of incremental
elevation beam is nine. The low search rate also separates the elevation servo information from the
azimuth servo to prevent cross-coupling.
ARRAY ASSEMBLY
A total of five layers of substrate material board were used to construct the whole array which
consists of antenna array module and the beamformer module as shown in Figure 4. The antenna
array module consists of four boards - a top crossed-slot board, a stripline excitation feeder board, a
hybrid/corporate feed network board and a bottom ground board. The top two boards (same
thickness) of the crossed-slot array elements were etched on a single sided, copper clad teflon
fiberglass board. The thickness of the board were 0.09 inch and 0.125 inch for the breadboard unit #1
and #2 array, respectively. The thicker board used for the unit #2 is to increase the antenna
bandwidth. A set of plated through holes around the crossed-slots were required to form the cavity
wall for the crossed-slot element. The bottom two boards with thickness of 0.031 inch each constitute
225
the integrated stripline hybrid/corporate feed network. The beamformer board and the 0.125 inch 
stiffner plate were bonded together using silver-bonded epoxy to form the beam former module. 
t 
Fig. 4. A photo to show the five layers of MSAT-X array design 
The antenna array module were key together with the beamformer module and then were 
bolted together with the back cover leaving an air gap of 0.2 inch between the beamformer circuit and 
the cover. Figure 5 shows a final assembled MSAT-X array. A very thin (0.002 inch) painted 
polyurethane type radome was employed to provide protection against the enviroment. 
Fig. 5. Final assembled MSAT-X phased array 
The total thickness of the array unit #1 and #2 were 0.68 inch and 0.75 inch, respectively. The 
thickness of the final production array will be reduced to 0.625 inch when the 0.125 inch stiffner plate in 
between the beamformer board and the antenna array board are removed. 
D€UGlNAL PAGE IS 
'DE POOR QUALITY 226 
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MEASURED ARRAY PERFORMANCE
Extensive measurements were performed during the contract with JPL. Typical measured
array patterns are shown in Figures 6 and 7. Well behaved array patterns were observed in all the
measured data. Excellent axial ratio (< 5 dB) of the array pattern was achieved within 95% of the
required space coverage across the operating frequency band.
The measured array performance is summarized in Table 1. These results show that all the
requirements were satisfied except the gain. However, it is believed that the 10 dBic gain at 20 degrees
elevation angle will be realized in the future through the optimization of the crossed-slot excitation
feeder design and the beamformer aperture dktribution. Detail will ha di,:t.n¢_:pcl;n th.... t;_g.
t-- •
.z
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IIItlIIItltlIIItlItHIIllI_III_
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(a) 1545 MHz
(b) 1660 MHz
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(b) 1660 MHz
Fig. 6 Measured elevation array patterns
at scan angle of 40 degrees away
from array normal
Fig. 7 Measured elevation array patterns
at scan angle of 60 degrees away
from array normal
227
Table 1. Summary of Measured Array Performance
Measured Value
Unit #1 Unit #2 Spec
Gain at 20" elevation angle
(dBic) 1545 MHz
1660 MHz
Intersatellite Isolation (dB)
Bacldobe (dB)
Multipath Rejection (dB) (pattern
dropoff from 20" to 0 ° elevation
Antenna Thickness (inch) (Exclusive of
RF Connector)
9.2 (average) 9.3 (average) 10
7.7 (average) 8.3 (average) 10
26 26 20
12 (95%) 12 (95%) 12
<12(5%) < 12(5%)
>6 >6 6
0.68 0.75 < 1
CONCLUSIONS
An L-band car-top comformal phased array has been sucessfully developed for the mobile
satellite communications. The following conclusions can be drawn from the development work with
JPL:
(1) The printed cavity-backed stripline fed crossed-slot element is proven to be the best radiating
element for the MSAT phased array.
(2) The measured array gain at 70 degrees from the array broadside is 9.3 dBic and 8.3 dBic for
the frequency 1545 MHz and 1660 MHz, respectively. 10 dBic gain across the frequency band
is achievable in the future.
(3) Excellent intersatellite isolation (26 dB) was achieved which is better than the requirement (20
dB).
(4) The total thickness of the developed array is 0.68 inch and 0.75 inch for the unit #1 and #2,
respectively.
(5) The multipath rejection requirement was satisfied.
(6) The backlobe level is better than 12 dB in 95% of the required space coverage. The worst
backlobe is 6 dB when the array was scanned to 70 degrees from the array broadside at certain
azimuth angles.
228


MSAT-X electronically steered phased array antenna system

MSAT-X electronically steered phased array antenna system

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