SBCA Antenna Systems 5 — Questions and Answers
Question 1: What is the primary advantage of a phased array antenna over a traditional parabolic dish for satellite communications?
- Lower manufacturing cost for equivalent aperture sizes
- Electronic beam steering without mechanical moving parts (Correct answer)
- Higher gain per unit aperture area than reflector antennas
- Better performance in rain fade conditions at Ku-band
Correct answer: Electronic beam steering without mechanical moving parts
Phased arrays steer their beam by adjusting the phase of signals at individual antenna elements electronically, enabling rapid beam repositioning without mechanical actuators.
Question 2: During antenna system commissioning, a technician measures a receive signal level 3 dB lower than the link budget predicts. Which is the LEAST likely cause?
- Pointing error resulting in the beam being off-peak
- Degraded LNB performance or incorrect LNB gain specification
- Polarization skew misalignment causing cross-pol loss
- The satellite's EIRP being 3 dB higher than specified in the link budget (Correct answer)
Correct answer: The satellite's EIRP being 3 dB higher than specified in the link budget
A satellite transmitting 3 dB more than specified would cause the received level to be higher than predicted, not lower — it is the least likely cause of a 3 dB deficit.
Question 3: What is 'antenna gain roll-off' in the context of wideband satellite systems?
- The reduction in antenna gain observed at frequencies near the band edge compared to the center frequency (Correct answer)
- The decrease in gain as the antenna ages and the surface degrades over time
- The loss of gain caused by incorrect feed horn aperture sizing
- The change in gain when the dish is tilted away from the optimal elevation angle
Correct answer: The reduction in antenna gain observed at frequencies near the band edge compared to the center frequency
Gain roll-off describes how antenna gain decreases toward the edges of the operating bandwidth, which must be accounted for in wideband link budget calculations.
Question 4: Which grounding practice is most critical for protecting a satellite dish installation from lightning damage?
- Connecting the coax shield to building ground at the receiver location only
- Installing a dedicated ground rod at the dish base and bonding all metallic components to a single-point ground (Correct answer)
- Wrapping the coax cable in conductive tape and running it through metal conduit
- Using a surge protector on the coaxial line between the LNB and receiver
Correct answer: Installing a dedicated ground rod at the dish base and bonding all metallic components to a single-point ground
Proper grounding requires a dedicated earth ground at the antenna structure and bonding all metallic components (dish, mast, coax shield) to a single common ground point.
Question 5: An antenna's beamwidth is measured at 0.7° at 14.25 GHz. A neighboring satellite is located 2° away in orbital arc. What is the approximate isolation advantage provided by the antenna's directivity?
- The antenna provides no discrimination since both satellites are within the main beam
- The interfering satellite falls outside the half-power beamwidth, providing significant sidelobe attenuation (Correct answer)
- The interfering satellite is within the first sidelobe, providing exactly 13 dB of isolation
- The 2° separation equals the null-to-null beamwidth, providing maximum antenna discrimination
Correct answer: The interfering satellite falls outside the half-power beamwidth, providing significant sidelobe attenuation
Since the 2° separation is nearly 3× the 0.7° HPBW, the interfering satellite falls well into the sidelobe region where antenna gain is substantially reduced.
Question 6: What is the purpose of 'autotracking' systems on large earth station antennas used for non-geostationary (NGSO) satellites?
- To automatically adjust LNB noise temperature based on sky temperature variations
- To continuously repoint the antenna to track the satellite as it moves across the sky (Correct answer)
- To automatically switch between primary and backup feed systems during signal degradation
- To optimize the polarization angle as the satellite's attitude changes
Correct answer: To continuously repoint the antenna to track the satellite as it moves across the sky
NGSO satellites move relative to the Earth's surface, so autotracking systems continuously monitor the beacon signal and servo the antenna mount to maintain peak gain alignment.
Question 7: Which material property is most important when selecting a radome to cover a satellite dish antenna installation?
- High thermal conductivity to prevent ice accumulation on the radome surface
- Low RF dielectric loss to minimize signal attenuation through the radome material (Correct answer)
- High reflectivity at the operating frequency to prevent energy from escaping the dish
- Mechanical transparency to allow visual inspection of feed alignment without removal
Correct answer: Low RF dielectric loss to minimize signal attenuation through the radome material
Radome materials must have low dielectric loss (low loss tangent) to avoid attenuating the RF signal as it passes through on both transmit and receive paths.
SBCA Antenna Systems 5
What is the primary advantage of a phased array antenna over a traditional parabolic dish for satellite communications?