SBCA Antenna Systems 4 — Questions and Answers
Question 1: What is the purpose of a 'splash plate' or subreflector in a Cassegrain antenna system?
- To redirect the reflected signal from the main dish back toward a secondary focus near the vertex (Correct answer)
- To shield the feed horn from direct sun illumination
- To increase the physical aperture of the antenna beyond the main reflector diameter
- To reduce wind loading by breaking up airflow across the dish surface
Correct answer: To redirect the reflected signal from the main dish back toward a secondary focus near the vertex
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?
- Half-power beamwidth (HPBW) (Correct answer)
- First null beamwidth (FNBW)
- Sidelobe isolation angle
- Orbital spacing compliance threshold
Correct answer: Half-power beamwidth (HPBW)
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?
- Gain increases because lower frequency signals penetrate the atmosphere better
- Gain decreases because the effective aperture area is inversely proportional to frequency squared (Correct answer)
- Gain stays the same because it depends only on the physical dish size
- Gain increases by approximately 1.3 dB due to reduced atmospheric absorption
Correct answer: Gain decreases because the effective aperture area is inversely proportional to frequency squared
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?
- Complete signal loss caused by the antenna pointing away from the satellite
- Rotation of the signal polarization plane caused by Faraday rotation or rain (Correct answer)
- Impedance mismatch between the feed and LNB that reduces signal amplitude
- Mechanical misalignment of the dish that causes polarization skew errors
Correct answer: Rotation of the signal polarization plane caused by Faraday rotation or rain
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?
- C-band frequencies require physically larger antennas to maintain structural integrity in wind
- Larger dishes provide higher G/T ratios necessary to overcome C-band's lower satellite EIRP levels (Correct answer)
- C-band LNBs have higher noise temperatures requiring larger apertures to compensate
- Regulatory requirements mandate minimum dish sizes for C-band TVRO reception
Correct answer: Larger dishes provide higher G/T ratios necessary to overcome C-band's lower satellite EIRP levels
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.
Question 6: What does the 'figure of merit' G/T represent in a satellite receive system?
- The ratio of antenna gain (dBi) to system noise temperature (K) in dB/K (Correct answer)
- The gain-to-threshold ratio that defines minimum acceptable signal level
- The ratio of transmitted gain to received power for link budget calculations
- The efficiency factor comparing theoretical to measured antenna performance
Correct answer: The ratio of antenna gain (dBi) to system noise temperature (K) in dB/K
G/T (gain-to-noise temperature ratio) in dB/K is the key figure of merit for a receive system, combining antenna gain and system noise temperature into a single performance metric.
Question 7: A satellite dish is installed at a site with 35°N latitude targeting a satellite at 101°W longitude. The site longitude is 87°W. Which tool is most appropriate to calculate the required azimuth and elevation angles?
- A spectrum analyzer set to the satellite's downlink frequency
- A satellite look-angle calculator using the geographic coordinates of both site and satellite (Correct answer)
- A signal-to-noise ratio meter calibrated for the target satellite's EIRP
- A polarization angle reference chart based on local magnetic declination
Correct answer: A satellite look-angle calculator using the geographic coordinates of both site and satellite
Look-angle calculators use site latitude/longitude and satellite orbital longitude to compute precise azimuth, elevation, and polarization skew angles.
SBCA Antenna Systems 4
What is the purpose of a 'splash plate' or subreflector in a Cassegrain antenna system?