CBT RF Transmission & Antenna Systems 2 — Questions and Answers
Question 1: In broadcast engineering, what does ERP stand for and what does it represent?
- Effective Radiated Power — the actual power delivered to the antenna terminals
- Effective Radiated Power — transmitter power multiplied by antenna gain relative to a dipole (Correct answer)
- Equivalent Resonant Power — power at resonance frequency of the antenna
- Effective Resonant Power — power reflected by the transmission line
Correct answer: Effective Radiated Power — transmitter power multiplied by antenna gain relative to a dipole
ERP (Effective Radiated Power) is the product of the transmitter power fed to the antenna and the antenna's gain referenced to a half-wave dipole, representing the apparent power radiated in the maximum direction.
Question 2: What is the primary function of a circulator in a high-power broadcast transmitter output stage?
- To boost the carrier frequency before reaching the antenna
- To protect the transmitter PA from damage caused by high reflected power (Correct answer)
- To combine signals from two separate transmitter chains
- To convert horizontal polarization to circular polarization
Correct answer: To protect the transmitter PA from damage caused by high reflected power
A circulator directs reflected power (from antenna mismatches) into a dummy load, protecting the transmitter's final amplifier stage from potentially damaging high VSWR conditions.
Question 3: How does antenna height above average terrain (HAAT) affect FM broadcast coverage?
- Higher HAAT decreases coverage due to increased atmospheric absorption
- Higher HAAT increases coverage by extending the radio horizon (Correct answer)
- HAAT only affects AM coverage, not FM
- Higher HAAT requires reduced transmitter power by FCC rule
Correct answer: Higher HAAT increases coverage by extending the radio horizon
FM propagation is essentially line-of-sight, so greater HAAT extends the radio horizon and increases the geographic coverage area significantly.
Question 4: What is the purpose of pressurizing a coaxial transmission line with dry nitrogen?
- To increase the dielectric constant and reduce signal velocity
- To prevent moisture ingress that would increase line losses and cause arcing (Correct answer)
- To raise the characteristic impedance from 50 to 75 ohms
- To lubricate the inner conductor for easier installation
Correct answer: To prevent moisture ingress that would increase line losses and cause arcing
Dry nitrogen pressurization keeps moisture out of the transmission line, preventing increased dielectric losses, corrosion, and potential arcing at high power levels.
Question 5: A broadcast antenna has a horizontal radiation pattern (HRP) that is not perfectly omnidirectional. What is this called?
- Elliptical pattern
- Directional antenna pattern (Correct answer)
- Null fill radiation
- Beam tilt profile
Correct answer: Directional antenna pattern
An antenna that radiates unequally in different horizontal directions is classified as a directional antenna, and its pattern must be FCC-licensed to protect co-channel and adjacent channel stations.
Question 6: What is the typical power loss per 100 feet for 1-5/8 inch Heliax coaxial cable at 100 MHz?
- Approximately 0.1 dB per 100 feet
- Approximately 0.5 dB per 100 feet (Correct answer)
- Approximately 2.0 dB per 100 feet
- Approximately 5.0 dB per 100 feet
Correct answer: Approximately 0.5 dB per 100 feet
1-5/8 inch Heliax (Andrew LDF7-50A type) exhibits approximately 0.5 dB per 100 feet at 100 MHz, making it well-suited for long tower runs in FM broadcast applications.
Question 7: What FCC rule governs the maximum allowable antenna height and power for full-power FM broadcast stations?
- 47 CFR Part 15
- 47 CFR Part 73 (Correct answer)
- 47 CFR Part 97
- 47 CFR Part 74
Correct answer: 47 CFR Part 73
47 CFR Part 73 (specifically Subpart B for FM) governs all technical standards for full-power FM broadcast stations including maximum ERP, HAAT, and antenna requirements.
CBT RF Transmission & Antenna Systems 2
In broadcast engineering, what does ERP stand for and what does it represent?