IEEE - Aerospace and Electronic Systems - May 2020 - 32

Multifrequency Spaceborne Deployable Radiometer Antenna Designs

Figure 10.
Phase centers' measurement of the horn. (a) Horn under test. (b)
Plumb used for phase centers' test.

Figure 13.
Far-field patterns of the antenna at 18.7 GHz.

Figure 11.
Far-field phase pattern of the feed at 18.7 GHz.

Figure 14.
Far-field patterns of the antenna at 23.8 GHz.

Figure 12.
ACMR OA under test.

a better S11 curve (generally below À20 dB) comparing
with the V-polarization wave in most of the frequency
band (18-40 GHz). However, in a low working frequency
band, the S11 result of the H-pol is À19 dB at 18.7 GHz.
The S11 of V-pol have values of À16 dB (18.7 GHz),
À28 dB (23.8 GHz), and À15 dB (37 GHz). The isolation
between the vertical and horizontal polarization is better
than À40 dB.

ACMR OA HORN PHASE CENTERS' MEASUREMENT
The far-field testing system is utilized to measure the
phase centers of the horn at all three frequency bands (see
32

Figure 10). Phase centers are measured by monitoring the
phase patterns while adjusting the horn's position back
and forth on the positioner. The far-field phase pattern at
18.7 GHz is depicted in Figure 11. It can be seen that
phases remain relatively constant from À20o to 20o when
the horn is mounted at the phase center. The distances of
the phase centers to the horn aperture by measurement are
13.5, 19, and 22 mm for 18.7, 23.8, and 37 GHz, respectively. It is also clear that although curved profile horn is
used, the discrepancy of the phase centers at tri-band is
unavoidable. Tradeoff is made that the least position offset
in wavelength should be found between the phase centers
and dish focal point at all frequencies.

ACMR OA PATTERNS MEASUREMENT
Antenna after assembly is shown in Figure 12, and the patterns are shown in Figures 13-15. The antenna patterns as
well as beam alignment are measured by the far-field system.
It is clear that the antenna has SLLs about À37 dB and the
CP levels with À23 dB for 18.7 GHz, the beam width is
about 1.40o, and the average SLLs are about À55 dB in wide
angle range. The patterns have SLLs of À36 dB and CP levels of À34 dB, respectively, for 23.8 GHz, the beam width is
about 1.25 , and the average SLLs is about À60 dB in wide-

IEEE A&E SYSTEMS MAGAZINE

MAY 2020



IEEE - Aerospace and Electronic Systems - May 2020

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