SBCA Antenna Systems 2 — Questions and Answers
Question 1: What is the primary purpose of a feed horn in a parabolic dish antenna system?
- To amplify the received signal before it reaches the LNB
- To collect and focus the reflected signal from the dish onto the LNB (Correct answer)
- To protect the LNB from weather and physical damage
- To rotate the dish toward the correct satellite azimuth
Correct answer: To collect and focus the reflected signal from the dish onto the LNB
The feed horn collects the reflected electromagnetic energy from the parabolic dish surface and focuses it onto the LNB input.
Question 2: Which antenna parameter describes the ratio of power radiated in the direction of maximum radiation to the average power radiated in all directions?
- Gain
- Directivity (Correct answer)
- Efficiency
- Beamwidth
Correct answer: Directivity
Directivity is the ratio of maximum radiation intensity to the average radiation intensity, independent of antenna efficiency.
Question 3: A dish antenna has an efficiency of 60% and a physical aperture area of 1.5 m². What is its effective aperture area?
- 0.6 m²
- 0.9 m² (Correct answer)
- 1.2 m²
- 2.5 m²
Correct answer: 0.9 m²
Effective aperture = efficiency × physical aperture = 0.60 × 1.5 m² = 0.9 m².
Question 4: What does the term 'cross-polarization discrimination' (XPD) measure in a satellite antenna system?
- The isolation between co-polarized signals on adjacent satellites
- The ratio of desired polarization signal power to the undesired orthogonal polarization power (Correct answer)
- The angular separation between the main beam and the first sidelobe
- The difference in gain between the two output ports of an OMT
Correct answer: The ratio of desired polarization signal power to the undesired orthogonal polarization power
XPD quantifies how well an antenna discriminates between two orthogonal polarizations, which is critical for frequency reuse systems.
Question 5: In a satellite uplink system, which condition causes antenna 'gain compression' at high power levels?
- Thermal expansion of the dish surface deforms the parabolic profile
- The feed horn becomes saturated and begins reflecting energy
- The high-power amplifier (HPA) driving the antenna enters nonlinear saturation (Correct answer)
- Atmospheric absorption reduces effective radiated power below the theoretical maximum
Correct answer: The high-power amplifier (HPA) driving the antenna enters nonlinear saturation
Gain compression occurs in the HPA when input power is high enough to drive the amplifier into its nonlinear region, reducing effective output gain.
Question 6: What is the effect of increasing a parabolic dish's f/D ratio (focal length to diameter) on antenna performance?
- Increases beamwidth and reduces gain
- Makes the dish deeper and more susceptible to wind loading
- Results in a shallower dish with wider feed illumination angle requirements (Correct answer)
- Has no practical effect on gain or beamwidth
Correct answer: Results in a shallower dish with wider feed illumination angle requirements
A higher f/D ratio produces a shallower (less deep) dish, requiring a feed horn with a wider illumination angle to cover the full reflector surface.
Question 7: Which type of antenna mount allows a single actuator to track all geostationary satellites visible from a fixed location?
- Az-El (azimuth-elevation) mount
- Polar (hour angle-declination) mount (Correct answer)
- X-Y mount
- Fixed-elevation steerable mount
Correct answer: Polar (hour angle-declination) mount
A polar mount aligns one rotation axis with Earth's polar axis, so a single motor sweep follows the geostationary arc without requiring a second axis adjustment.
SBCA Antenna Systems 2
What is the primary purpose of a feed horn in a parabolic dish antenna system?