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(0 O \ CO ^ -\ IA o © EHF/SHF SATCOM ANTENNA AND RADOME TEST RESULTS

o. JL Allen L. Johnson U Wayne 0. Fischbach ^f Avionics Laboratory ^ Wright-Patterson AFB, Ohio 45433

ABSTRACT

v"—6" The current airborne EHF SATCOM system has a RF power output

of 100 watts and an antenna beaniwidth of less than 1 degree. As

the RF beam from the antenna passes through the dielectric radome

it will be refracted at each surface. The resulting change in

direction of the beam depends on the frequency, the dielectric

constant of the radome, and the shape of the radome surfaces.

Tests have been performed to evaluate the beam deflection at 20 and

44 Giga-Hertz. Other systems effects which were evaluated included

antenna beam distortion, RF heating, ana Radio Frequency

Interference between two antennas (SHF and EHF) under a single

radome. ^~

INTRODUCTION

Command post aircraft currently use satellite communications

(SATCOM) to achieve worldwide connectivity. Early SATCOM systems

employed the Ultra High Frequency (UHF) band, with simple

aerodynamic blade or overhead-looking antennas (Armstrong, 1978).

As the communications systems became more sophisticated, the use of

the microwave frequency bands became practical. Antennas now used

for SATCOM include Super High Frequency (SHF) and Extremely High

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frequency (EHF) dibhes, (Schultz, 1983). These antennas must be

placed under radomes to maintain the aerodynamics of the airborne

command post. The Avionics Laboratory has investigated the effects

of narrow-beam systems since it undertook the development of its

first microwave SATCOM system in 1969. We are currently flight

testir.n our fifth generation microwave SATCOM system, fJoyner,

1981).

Some of the operational command post aircraft are already

equipped with SHF SATCOM systems to allow operation with the 7-8

OigaHertz (GHz) Defense Satellite Communications System (DSCS II &

DSCS HO. With the advent of the MILSTAR system, which operates

at 20 and 44 GHz, these command post aircraft will require a raciome

capable of handling 7/8, 20, and 44 GHz: i.e. a tri-banu rariome.

A tri-band radome has been developed for the Avionics

Laboratory's SATCOM system aboard the C-135 testbed aircraft which

is maintained and operated by the 4950th Test Wing, (Joyner, 1985).

Evaluation of the parameters and effects of the C-135 radome is

necessary to validate models used to predict the radome

characteristics and reduce the risk associated with future tri-band

radome developments.

AM/ASC-30 SATCOM SYSTEM TESTS

The AN/ASC-30 EHF SATCOM system has a Radio Frequency (RF)

power output of 100 watts and an antenna beamwidth of less than 1

degree. As the RF beam from the antenna passes through the

-64-

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dielectric radome it is refracted at each surface. The resulting

charge in direction of the beam, called beam squint, depends on the

frequency, the dielectric constant of the radonie, and the shape ot

the radome surfaces. Tests have been performed to evaluate the

beam squint at 2U and 44 GHz.

Other systems effects which were evaluated included radome

attenuation, antenna beam shape, RF heating, Radio Freouency

Interference (RFI) between two antennas under a single radome, and

radome deflection in flight.

RF Beam Squint

Measurements of the RF beam squint were accomplished by

comparing the optimum antenna pointing angles with the radome in

place and removed. One practical problem in measuring the beam

squint of the AN/ASC-30 tri-band radome was the lack of a 20/44 GHz

satellite on-orbit to use as a signal source. In place of the

satellite, a simulator was used, consisting of a 2U GHz transmitter

and a 44 GHz receiver on the tower of building 620 at WPAFB, OH,

Figure 1. The AN/ASC-30 with its 44 GHz transmitter and 20 GHz

receiver was installed in aircraft C-135/372, which was located on

the ground at Patterson Field.

For the beam squint test the 20 GHz tower transmitter and 44

GHz aircraft transmitter were turned on and their antennas pointed

toward the respective receivers. The aircraft antenna was moved in

0.1° azimuth steps for ±2.5°, and 0.1° elevation steps fur ^1.0° of

-65-

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the calculated pointing angle from the aircraft to the tower. In

the tower and aircraft, the azimuth antenna beam patterns were

received, recorded, and correlated with the transmitter pointing

angles. The radome was then removed and the azimuth/elevation

steps repeated. Any difference in the pointing angle for the peak

of the beam was attributed to radome-induced beam squint. Due to

the complex curvature of the radome, various portions were measured

from the nose, to broadside, to the tail, Figure 2.

Radome Attenuation

It was possible to measure the absolute attenuation of the

radome at 20 and 44 GHz by comparing the received power with and

without the radome in the line-of-sight path, using the same test

configuration as for beam squint.

Antenna Beam Shape

The pattern of AN/ASC-30 EHF antenna is also of interest. The

beam squint measurements provided the information necessary to

characterize the affect of the radome and ribs on the

three-dimensional beam shape.

Raderre RF Heating

When an SHF transmitted beam passes through the radome, local

RF heating will occur. For the RF heatiny tests a special test

fixture was constructed to simulate a section of the C-135 aircraft

fuselage. The AN/ASC-18 airborne SATCOM antenna was mounted on the

fixture and the tri-band radome installed over the antenna,

•66-

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Figure 3. The AN/ASC-18 terminal, which was installed in the

nearby Rooftop Facility, has a power output of 10 kW in the SHF

band. To conduct the test, the SHF transmitter was turned on and

operated until the radome's inside temperature stabilizes. The

extent of the heating was measured by attaching a series of heat

tabs (which blacken at a specific temperature) around a 15 inch

diameter circle inside the radome in the area where the antenna

beam intercepted the radome. The test, procedures require the SHF

transmit power to be raised from a 2 kW level to a maximum of 10 kW

in four steps. At a 10 kW transmit power level, the uniform energy

2 density from the antenna would be 12.4 watts/inch . In the near

field, the energy distribution is not uniform and it was

anticipated that "hot spots" could occur with energy densities of

20 watts/inch . Tests with flat panels of the radome material

indicated that at 8 GHz the loss in the radome is approximately 2dB

(60% transmission). In a hot spot it was expected that 8

2 watts/inch could be dissipated in the radome skin. The maximum

temperature was determined for each power level by reading the

highest temperature heat tab which had fired (blackened).

RFI/EMI Between Two Antennas Under One Radome

For some airborne command post applications, two dish antennas

will be mounted under the same radome. One antenna system operates

at 7 and 8 GHz while the other operates at ?0 and 44 GHz. For the

RFI/EMI test, the 20 GHz noise out of the 8 GHz transmitter that

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couples into the 20 GHz antenna/receiver and the 7 GHz nurse frui..

the 44 GHz transmitter that couples into the 7 GHz antenna/receiver

were measured. Both antennas were mounted on the test fixture,

under the tri-band radomo. Receiver measurements were made as oath

transmitter was turned on. The antennas were positioned at various

geometries; both pointed tcward the same equal-angle reflection

point, both antennas normal to the radome wall, and the dtitennas

pointed almost directly toward each other.

Radome Deflection In Flight

The AN/ASC-30 system employs a 26" parabolic antenna that is

mounted on top of a C-135 test aircraft. An aerodynamic radome

covers the antenna to reduce the drag, Figure 4. Since the radome

has to pass three frequency bands (7-8 GHz, 20-21 GHz, and 43-45

GHz), a special wall thickness design was required. The resulting

thin-wall radome design did not have sufficient stiffness to meet

deflection limits during maximum aerodynamic loading. To improve

the stiffness, three ribs were added to the radome, Figure 5. The

final design was run on a NASTRAN computer model (WANG, 1984) to

calculate the radome wall deflection during en 8° aircraft

sideslip, the maneuver specified as providing the maximum dynamic

loading to the sides of the radome.

The tri-band radome was constructed by Hitco Inc. under1 a

subcontract with Bell Aerospace Corp. and the Raytheon Company.

The radome was then installed on the C-135/37? test aircraft and a

I "68-

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test, flown to measure the actual deflection (Hocutt, 19f:C>).

The tri-bard radome was modelled by Pell Aerospace Corp. us<ny

a NASTRAN finite element computer model. The radome surface was

dividea into 320 separate partis and pressure levels calculated for

the specific maneuver were applied to each panel. After several

iterations of different skin thickness and rib placement, a f,;nal

design was achieved with a skin thickness of 0.145 inches and rib

placement shown in Figure 6. The maximum deflection location and

value were predicted to be 0.251 inches at node 1056 with the 8°

sideslip maneuver, Figure 7.

To measure radome deflection, a fixture was constructed by the

4950t^ Tw to fit over the AN/ASC-30 antenna, Figure 8. Two linear

deflection transducers were mounted at. the edges of the fixture and

the deflection arm attached to buttons cemented to the inside* of

the radome. The buttons were positioned within a few inches of the

predicted maximum deflecticn point. The linear deflection

transducers were connected to c DC power source and to the

instrumentation system. The transducers had a travel of 4l.b

inches from their static position. The transducer position was

calibrated over the entire trcwel range and recoroed on a magnetic

recorder and a chan recorder to dn accuracy of 0.001 inches.

jrsTj suns

kaoome Beam Squint Measurements

A comparison of the antenna azimuth angle with the AN/ASC-30

tri-band radome both in place and removed providea a measurement, of

-69- *

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the beam soin'nt. A summary of the bean squint for vorious portions

of the radon« (azimuth tingles) is shown in Figure 9. One azimuth

sweep showing the worst beam squint 1? presented in Figure 10. The

beam squint appears to be approximately 0.4° over the nose and teil

and en the order cf 0.1° broadside.

Radome Attenuation Measurements

In an effort to evaluate the effect of radome thickness and

incidence angles on the radome attenuation, a 36-inch square flat

panel was constructed of the same material end thickne<~ as the

tri-band radome. Attenuation tests were conducted on the flat

panel. The results of tne 7.0 GHz and 44 GHz measurements are shown

in Figure 11.

Measurements of the actuai radome attenuation were made by

compering the received signal level with tnc AN/^SC-30 tri-band

radome in place and removed. A plot of the attenuation for various

portiens of the radome is shown in Figure lc. Thr radome

attenuation measurements are in close agreement with the flet penel

measurements.

Antenna Pattern

The 44 GHz pattern of the tri-band intern,: **as measured with

the radome in place and removed. The mainbeam portion of the

pattern is shown as a 3 dimensional plot, Figure 13. This data

shows the effects of the radome and ribs particularly on the

sidclobe distribution.

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fadome Heating

The AN/ASC-18 SW transmitter was first operoted at ? rW RF

power at 0 GHz. The ambient temperature was approximately 40°F

outside the radome and 50PF inside I he radome. After 20 minutes of

operation, the radome temperature stabilized at less than IGG°P at

the top and left side of the test circle, 105CF at the right sidet

and 1 lb T at the bottom. Following each power level setting, the

maximum temperatures were recorded, Table ].

TABLt 1 Temperature Distribution On Tri-Band Radome

Xmi_t J^owor Measurement 91 9cr 180u 270° Time

2 KW Temp Tabs "Thermocouple

lOO0^ 105 r 115°F lOO^F 20 Mir

s ;<w Temp Tabs Thermocouple

100° 100°

iib° 100°

130° 325°

100° 100c

lb Mir.

8 KW Temp Tabs Thermocouple

130' 135°

130° 170°

130° 180"

130° 135c

32 Min

!fl KW Temp Tabs Thermocouple

130° 176°

130" 180°

200° 188°

130° 176c

16 Min

With the transmitter operating at 5 kW of power for 15

minute«-, the maximum temperature recorded was 130°F. After

transmitting 8 kW for 32 minutes, the temperature tabs on the

bottom of the circle showed 130° but a thermocouple measured 1S0T.

The maximum power transmitted was 10 kW for 16 minutes, ulter which

the temperature tabs showed 200°F at the bottom of the circle.

Subsequent discoloring of the radome material indicated that the

-71-

internal plys of the radorne reached a temperature of approximately

500° F during the 10 kW run.

From the measurements, it appears that the current tri-band

radume design could withstand between 2 and 5 kW of transmit power

at 8 GHz without experiencing an excessive temperature rise. Power

levels above 5 kW are likely to cause an excessive temperature

build up in the laminated plys wh^ch could damage the radorne. If

the radorne loss were reduced, the transmit power could be increased

proportionally.

RFI/EMI Test

With the AN/ASC-18 transmitting at 8.0 GHz, the AN/ASC-30

system encountered no interference at 20.8 GHz except when the

transmit antenna was pointed at 225° azimuth where the mainbeam

just grazed the AN/ASC-30 antenna. In that configuration, the

worst case interference was wUh the AN/ASC-30 pointing at 180°

azimuth and 80° elevation. Approximately a 1 dB rise was noticed

in the noise floor of the AN/ASC-30 receiver when the AN/ASC18 was

transmitting 10 kW. It was postulated that the interference was

caused by energy coupling directly into the RF mixer and

downconverter that is mounted on the back of the dish, rethtr than

through the antenna feed. The RF mixer and downconverter were

removed, reinstalled, and all connectors properly tightened. A

retest showed only 0,1 dB rise in the noise floor. To determine

the effect of tne interference on link performance, data wus

-72-

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transmitted to the AN/ASC-30 system with the interference source un

and off. The Bit Error Rate (BF.R) performance of the system

appeared to be uneffected by the interference, Figure 14.

When tho AN/ASC-30 transmitted 100 watts at 44 GHz, no

interference was experienced in the AN/ASC-18's 7 GHz receive

system, regardless of the orientation of the two antennas.

Radome Deflection Flight Test

During the aircraft's tcke-off roll, at a speed of

approximately 90 knots, both sides of tha radome started to detlect

outward, Figures 15. As the aircraft reached take-off speed,

rotated, and continued tc accelerate, both sides of the radome

deflected outward approximately 0.C9 inches. The aircraft climbed

to an altitude of 10,000 feet and the outward deflection remained

constant during straight and "evel flight. The aircraft made <a

data run at 200 Knots Indicatsa Air Speed (KIAS) with a 5° side

slip. There was no noticeable change in the radome deflection.

Several runs were made at 1G,00Ü feet" altitude, 200 KIAS, with an

8° side slip. The instrumentation recorded a deflection of one

minor division (approximately C.04 inches) inward on both sides of

the radome during those runs. Figure 16.

During the aircraft's approach and landing the redon.e

deflection decreased on both sides as the airspeed decreased,

Figure 17. Shortly after touchdown the instrumentation showed the

radome had returned to its stetic width.

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It appears the aerodynamic pressure on the front and top of

the radome causes both sides to bow outward approximately 0.10

inches at the antenna station. The effect of the side-slip

maneuver appears to reduce the front and top pressures, allowing

both sides of the radome to move inward 0.04 inches. The effect of

the dynamic pressure on the sides of the radome appears to be so

small that it is masked by the change in front and top pressures.

The NASTRAN model which predicted an inward deflection of 0.25

inches during the side slip appears to be quite conservative. The

model was used previously to predict the deflection on a dual-band

radome (Severyn, 1978). A similar flight test of the dual-band

radome produced deflections which agreed quite closely with the

model (Cummins, 1978). The difference between the tri-band radome

prediction and the test results may be due to inaccurate pressure

distributions in the model or to the difficulty of correctly

modeling the stiffness of the ribs.

REFERENCES

Armstrong, E.L., Maj (USAF), "AFSATC0M," Air Force Magazine, pp 8C-85, July 1978.

Cummins, W.S., "Airworthiness Test of Dual Frequency SA1C0M Radome," 4950th TW, Wright-Patterson AFB, OH, 2 June 1978.

Hocutt, Capt A.M., "Tri-band Radome Test Plan," AFWAL TM-85-13, Air Force Wright Aeronautical Laboratories, Wright-Patterson AFB, OH, 27 June 1985.

Johnson, A.L., "Airborne EHF Satellite Communications Terminals," Submitted to IEE Proceedings F, Herts England, July 1985.

-74-

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Joyner, T.C., "AN/ASC-30 LHF/SHF Command Post SATCOM Terminal," NAECON Symposium, Vol 1, pp 1?4-1?7, Dayton OH, 19 May 1981.

Joyner, T.E., "AN/ASC-30 Upgrade SATCOM Moderate Gain Antenna/ RadoM? Subsystem," NAECON Symposium, pp 1192-1197, Dayton OH, 20 May 1985.

Schultz.J.B., "MILSTAk To Close Üanqerous C3I Güp," Defence Electronics, Vol 15 No. 3, pp 46-59, March 1983.

Severyn, 7., "Aerodynamic Evaluation of Dual Frequency Radonie," Flight Test Plan 78-4-3, 49b0th TW, Wright-Patterson AFB, OH, 5 May 1978.

Wang, T., "AN/ASC-30 Upgrade Radome Structural Analysis," Bell Aerospace Corp, Buffalo, NY, 13 December 1984,

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