CBT RF Transmission & Antenna Systems 1 — Questions and Answers
Question 1: What is the primary purpose of a transmission line in a broadcast RF system?
- To amplify the RF signal before it reaches the antenna
- To transfer RF energy from the transmitter to the antenna with minimal loss (Correct answer)
- To filter out unwanted harmonics in the transmitted signal
- To match the antenna impedance to the studio equipment
Correct answer: To transfer RF energy from the transmitter to the antenna with minimal loss
A transmission line's primary role is to efficiently transfer RF energy from the transmitter output to the antenna while minimizing losses due to reflection and attenuation.
Question 2: Which parameter describes the ratio of reflected wave voltage to incident wave voltage on a transmission line?
- Standing Wave Ratio (SWR)
- Voltage Reflection Coefficient (Γ) (Correct answer)
- Return Loss
- Insertion Loss
Correct answer: Voltage Reflection Coefficient (Γ)
The Voltage Reflection Coefficient (Γ) is the ratio of the reflected voltage to the incident voltage, and is the fundamental quantity from which SWR and return loss are derived.
Question 3: A broadcast FM antenna has a gain of 6 dBd. Approximately how many times greater is its effective radiated power compared to a dipole radiating the same transmitter power?
- 2 times
- 4 times (Correct answer)
- 6 times
- 8 times
Correct answer: 4 times
6 dBd corresponds to a power ratio of 10^(6/10) ≈ 4, meaning the antenna focuses energy to produce four times the ERP of a reference dipole.
Question 4: What is the standard characteristic impedance of most coaxial transmission lines used in broadcast RF systems?
- 25 ohms
- 50 ohms (Correct answer)
- 75 ohms
- 300 ohms
Correct answer: 50 ohms
50 ohms is the standard characteristic impedance for broadcast transmitter output and coaxial transmission lines, as it balances low loss and high power handling.
Question 5: What does VSWR of 1:1 indicate about a transmission line system?
- Maximum power is being reflected back to the transmitter
- The line is completely open-circuited at the load
- The load is perfectly matched and no power is reflected (Correct answer)
- The system is operating at exactly half its rated power
Correct answer: The load is perfectly matched and no power is reflected
A VSWR of 1:1 indicates a perfect impedance match between the transmission line and the load, meaning zero reflected power and maximum power transfer to the antenna.
Question 6: Which type of antenna is most commonly used for omnidirectional FM broadcast coverage?
- Yagi-Uda antenna
- Log-periodic antenna
- Circularly polarized panel antenna or batwing antenna (Correct answer)
- Parabolic reflector antenna
Correct answer: Circularly polarized panel antenna or batwing antenna
Circularly polarized panel antennas (such as batwing or Superturnstile designs) are standard for FM broadcast because they provide omnidirectional horizontal radiation and circular polarization as required by the FCC.
Question 7: What is the purpose of an antenna diplexer in a broadcast facility?
- To combine two transmitters operating on different frequencies onto a single antenna (Correct answer)
- To split one antenna feed into two separate transmission lines
- To match the transmitter output impedance to 600 ohms
- To filter AM harmonics from the FM transmission line
Correct answer: To combine two transmitters operating on different frequencies onto a single antenna
An antenna diplexer allows two transmitters operating on different frequencies to share a single antenna system without interfering with each other.
CBT RF Transmission & Antenna Systems 1
What is the primary purpose of a transmission line in a broadcast RF system?