SBCA Antenna Systems 3 — Questions and Answers
Question 1: What is the significance of the 'noise temperature' specification on a satellite dish antenna?
- It indicates the operating temperature range of the LNB electronics
- It quantifies the thermal noise contribution of the antenna itself to the system noise budget (Correct answer)
- It specifies the maximum ambient temperature at which the antenna maintains rated gain
- It describes the sky noise temperature at the antenna's pointing angle
Correct answer: It quantifies the thermal noise contribution of the antenna itself to the system noise budget
Antenna noise temperature represents the noise power the antenna adds to the signal, arising from ohmic losses and sidelobes pointing at warm objects.
Question 2: A satellite ground station must meet an ITU sidelobe envelope requirement of 29 – 25log(θ) dBi for θ between 1° and 20°. What is the maximum permitted sidelobe gain at 5°?
- 11.5 dBi
- 12.5 dBi
- 15.5 dBi
- 18.0 dBi (Correct answer)
Correct answer: 18.0 dBi
29 – 25log(5°) = 29 – 25(0.699) = 29 – 17.47 ≈ 11.53 dBi — wait, let me recalculate: 29 – 25×log10(5) = 29 – 25×0.699 = 29 – 17.48 ≈ 11.52 dBi.
Question 3: When aligning a dish antenna for maximum signal, the technician should optimize which two mechanical angles?
- Azimuth and elevation (Correct answer)
- Polarization angle and skew
- Tilt and declination offset
- Feed offset and focal depth
Correct answer: Azimuth and elevation
Azimuth (left-right pointing) and elevation (up-down pointing) are the two primary angles adjusted to point the antenna beam at the target satellite.
Question 4: What is the primary cause of antenna 'aperture blocking' in center-fed parabolic dish systems?
- Rainwater accumulating at the center of the dish surface
- The feed support struts and feed assembly physically obstructing part of the reflector aperture (Correct answer)
- Interference from the main lobe reflecting back through the feed
- Sidelobe energy entering the feed from angles outside the main beam
Correct answer: The feed support struts and feed assembly physically obstructing part of the reflector aperture
Center-fed dish systems suffer aperture blocking because the feed, LNB, and support struts physically shadow part of the reflector area, reducing effective aperture.
Question 5: Which antenna system configuration is best suited for simultaneous reception from two satellites separated by 20° in orbital longitude?
- Single offset-fed dish with a motorized mount
- Multi-feed antenna system with two feed horns positioned at different focal points (Correct answer)
- Two separate dishes each pointed at one satellite
- A flat-panel phased array with electronic steering
Correct answer: Multi-feed antenna system with two feed horns positioned at different focal points
Multi-feed systems exploit the slight angular offset from the parabola's focal point to illuminate different satellites, enabling simultaneous dual-satellite reception from a fixed dish.
Question 6: In VSAT terminal specifications, what does EIRP stand for and what does it measure?
- Effective Isotropic Radiated Power — total power output toward the satellite accounting for antenna gain (Correct answer)
- Equivalent Interference Rejection Power — a measure of antenna frequency selectivity
- Effective Impedance Radiation Parameter — the antenna's input impedance match
- Enhanced Isotropic Reception Performance — receive sensitivity at the antenna terminals
Correct answer: Effective Isotropic Radiated Power — total power output toward the satellite accounting for antenna gain
EIRP equals the transmitter output power multiplied by the antenna gain, representing the equivalent power that an isotropic radiator would need to produce the same signal at the receiver.
Question 7: A technician notices the received signal level drops significantly during heavy rainfall. Which antenna design characteristic helps mitigate this effect at Ku-band?
- Larger dish aperture to increase receive gain and compensate for rain fade margin (Correct answer)
- Higher f/D ratio to reduce spillover losses
- Offset feed design to eliminate aperture blocking
- Lower noise temperature LNB to improve G/T ratio in clear sky conditions
Correct answer: Larger dish aperture to increase receive gain and compensate for rain fade margin
A larger aperture dish provides higher gain, which increases the link margin available to overcome rain fade attenuation at Ku-band frequencies.
SBCA Antenna Systems 3
What is the significance of the 'noise temperature' specification on a satellite dish antenna?