Aerospace and Electronic Systems - October 2018 - 36

Beam Shaping of X-Band Stepped Choke Ring Antenna for LEO Satellites

Figure 3.

The |S11| of the antenna for the thin and thick probe (see Figure 4).

trapezoidal septum with its two parallel sides of 22 mm and
10.03 mm, respectively.

ANTENNA OPTIMIZATION
The antenna parameters are optimized to achieve five required characteristics of the antenna. A return loss better than 20 dB at 10.5 GHz
is defined as the matching goal. From a radiation point of view, the
maximum gain at θ equal to 62.5° in both ϕ planes of 0° and 90° with
the goal of 6 dBi is set. The circular polarization of the antenna is
tested by setting the axial ratio (AR) to be better than 5 dB in both ϕ
plane cuts of 0° and 90° within the θ range of 0° to 62.5°. The final
obtained parameter values are presented in Table 1. As the result, the
|S11| of better than −15 around 10.5 GHz is achieved, as shown in
Figure 3. However, to improve the return loss bandwidth and make
it symmetric around the center frequency, a thickened probe, shown
in Figure 4, is used. The |S11| results with the reflection coefficient of
better than −20 dB at 10.5 GHz is shown in Figure 3.
As can be seen in Figure 5, the AR of the antenna without the
septum is below 5 dB within the whole 62.5° range in both plane
cuts of ϕ equal to 0° and 90°. As shown in this figure, unfortunately, the AR of the antenna with the septum becomes deteriorated
in ϕ equal to 90° due to the septum's asymmetrical structure in
addition to its limited distance from the choke edge. The obtained
AR of better than 7.5 dB corresponds to less than 0.7-dB polarization loss that is acceptable in the link budget calculation. The final

Figure 4.

Thickened probe used to achieve wideband performance for |S11|.

Figure 5.

Simulated AR of SCRA alone (not installed on the satellite body).

obtained radiation pattern is illustrated in Figure 6. The values of
gain in 62.5° and 70° are 2.6 dB and 0.2 dB in the ϕ = 0° plane, respectively, and 4 dB and 1.5 dB in the ϕ = 90° plane, respectively.

BODY EFFECT
As expected from the beginning, the placement of antenna on the
satellite body will change its characteristics, especially its radiation pattern. The structures of the LEO satellite body with both
solar panels are shown in Figure 7a. The antenna is placed in the
middle of one of the body's face and near to one of the edges,
while the solar panels are closed and then simulated in the CST
Microwave Studio. It is clear that the parasitic effect of the body is
negligible on |S11|. This effect, however, is more important when it
comes to the radiation pattern, shown in Figure 8. As can be seen,
the value of gain is 4.9 dB and 1.4 dB, in both ϕ = 0° and ϕ = 90°
planes, respectively, with the presence of a satellite mock-up. The
body effect on the radiation pattern, where the closed solar panels
are, is symmetrical in the ϕ = 0° plane and has slightly increased
the gain. However, due to the presences of an asymmetrical limited
ground plane in the ϕ = 90° plane, there are oscillations in this
plane around 60°. The same effects can be seen in the AR of the
antenna where it has drastically been increased from 70° upward,
as shown in Figure 9.

Figure 6.

Simulated directive gain pattern of SCRA alone.

IEEE A&E SYSTEMS MAGAZINE

OCTOBER 2018

Aerospace and Electronic Systems - October 2018

Table of Contents for the Digital Edition of Aerospace and Electronic Systems - October 2018