CBT - Certified Broadcast Technologist RF and Antenna Systems Questions and Answers — Questions and Answers
Question 1: A broadcast station is experiencing periodic signal fade and unexpected long-distance reception of its VHF signal, particularly on hot, calm days. This phenomenon is most likely caused by which of the following?
- Ionospheric skip
- Tropospheric ducting (Correct answer)
- Ground wave propagation
- Multipath cancellation from a nearby structure
Correct answer: Tropospheric ducting
Tropospheric ducting occurs when a temperature inversion creates a 'duct' in the troposphere that can trap VHF and UHF signals, allowing them to travel much farther than the normal line-of-sight. This condition is often associated with stable, high-pressure weather systems, such as hot, calm days, and can cause interference from distant stations or unusual long-range reception.
Question 2: What is the primary function of a radome on a broadcast antenna?
- To amplify the transmitted signal
- To change the antenna's polarization
- To protect the antenna from environmental elements (Correct answer)
- To decrease the antenna's wind resistance by creating a vacuum
Correct answer: To protect the antenna from environmental elements
A radome is a structural, weatherproof enclosure that protects an antenna from environmental elements like ice, rain, wind, and debris. It is constructed from material that is transparent to radio waves, minimizing signal attenuation while safeguarding the physical integrity and performance of the antenna.
Question 3: An impedance mismatch between a transmitter's 50-ohm output and an antenna system results in a high Voltage Standing Wave Ratio (VSWR). Which of the following is a direct consequence of this condition?
- Increased antenna gain
- A portion of the RF power is reflected back to the transmitter (Correct answer)
- The frequency of the transmitted signal is shifted
- Improved signal-to-noise ratio at the receiver
Correct answer: A portion of the RF power is reflected back to the transmitter
A high VSWR is an indicator of an impedance mismatch, which causes a portion of the forward power traveling to the antenna to be reflected back down the transmission line toward the transmitter. This reflected power can potentially damage the transmitter, reduce the overall efficiency of the system, and lead to signal degradation.
Question 4: A broadcast engineer is selecting an antenna for a new FM station. The antenna specification sheet lists its gain as 5 dBd. What is the equivalent gain expressed in dBi?
- 2.85 dBi
- 5.00 dBi
- 7.15 dBi (Correct answer)
- 10.00 dBi
Correct answer: 7.15 dBi
The gain of an antenna can be expressed relative to a theoretical isotropic radiator (dBi) or a half-wave dipole antenna (dBd). A standard dipole antenna has a gain of 2.15 dBi. To convert from dBd to dBi, you add 2.15 dB. Therefore, 5 dBd + 2.15 dB = 7.15 dBi.
Question 5: For maximum signal transfer between a transmitting antenna and a receiving antenna in a line-of-sight system, which property of the antennas must be aligned?
- Bandwidth
- Physical height
- Voltage Standing Wave Ratio (VSWR)
- Polarization (Correct answer)
Correct answer: Polarization
Polarization refers to the orientation of the electric field of the radio wave. For optimal signal reception and maximum power transfer, the transmitting and receiving antennas must have the same polarization (e.g., both vertical or both horizontal). A mismatch in polarization can cause significant signal loss.
Question 6: A technician at a television station notices that during periods of heavy icing, the transmitter's VSWR reading initially improves (decreases) before eventually worsening (increasing) as the ice accumulates further. What is the most likely reason for this initial improvement?
- The ice acts as a lens, focusing the RF energy more efficiently.
- The ice changes the antenna's polarization to circular, which is more robust.
- The antenna was originally tuned for a frequency slightly higher than the operating frequency, and the initial ice accumulation effectively 'lengthens' the elements, bringing them closer to resonance. (Correct answer)
- The ice insulates the antenna, preventing power loss due to thermal radiation.
Correct answer: The antenna was originally tuned for a frequency slightly higher than the operating frequency, and the initial ice accumulation effectively 'lengthens' the elements, bringing them closer to resonance.
Broadcast antennas are often tuned to have their minimum VSWR at a frequency slightly above the actual operating channel. As ice builds up, it dielectrically loads the antenna elements, making them appear electrically longer. This 'lengthening' can shift the resonant frequency downward, moving the point of minimum VSWR closer to the operating frequency, thus temporarily improving the match before further accumulation makes the antenna electrically too long and worsens the VSWR.
CBT - Certified Broadcast Technologist RF and Antenna Systems Questions and Answers
A broadcast station is experiencing periodic signal fade and unexpected long-distance reception of its VHF signal, particularly on hot, calm days.
This phenomenon is most likely caused by which of the following?