SBCA Antenna Systems 4

Question 1

What is the purpose of a 'splash plate' or subreflector in a Cassegrain antenna system?

Accepted Answer

To redirect the reflected signal from the main dish back toward a secondary focus near the vertex

Answer

In a Cassegrain system, the convex hyperboloidal subreflector reflects energy from the main dish back through a hole at the vertex to a feed located behind the main reflector.

Question 2

Which specification defines the minimum angular separation an antenna can maintain between the desired satellite and an interfering adjacent satellite?

Accepted Answer

Half-power beamwidth (HPBW)

Answer

The half-power beamwidth (HPBW) is commonly used to characterize antenna selectivity, and the FCC uses antenna size and beamwidth requirements to enforce minimum orbital arc discrimination.

Question 3

An antenna's gain is measured at 45.5 dBi at 14.0 GHz. If the operating frequency drops to 12.0 GHz with the same dish diameter, what happens to the gain?

Accepted Answer

Gain decreases because the effective aperture area is inversely proportional to frequency squared

Answer

Antenna gain is proportional to (πD/λ)², and since λ increases at lower frequency, gain decreases when frequency drops while dish size remains constant.

Question 4

What is 'antenna depolarization' in the context of satellite communications, and what causes it?

Accepted Answer

Rotation of the signal polarization plane caused by Faraday rotation or rain

Answer

Depolarization is the conversion of polarized signal energy into the orthogonal polarization, commonly caused by rain (differential attenuation) or ionospheric Faraday rotation.

Question 5

In a receive-only C-band earth station, what is the primary reason larger dish sizes (e.g., 4.5 m vs. 1.8 m) are often preferred over smaller dishes?

Accepted Answer

Larger dishes provide higher G/T ratios necessary to overcome C-band's lower satellite EIRP levels

Answer

C-band satellites typically operate at lower EIRP than Ku-band, so receive-only terminals need larger dishes to achieve sufficient G/T for acceptable carrier-to-noise ratios.

(SBCA) Satellite Broadcasting and Communications Association Certification Practice Test

(SBCA) Satellite Broadcasting and Communications Association Certification Practice Test

SBCA Antenna Systems 4