GROL Antennas and Transmission Lines 2 — Questions and Answers
Question 1: What does a standing wave ratio (SWR) of 1:1 indicate on a transmission line?
- Perfect impedance match with maximum power transfer and no reflected power (Correct answer)
- Total reflection of power at the antenna feedpoint
- Half of the transmitted power is reflected back to the transmitter
- The transmission line is open-circuited at the antenna end
Correct answer: Perfect impedance match with maximum power transfer and no reflected power
An SWR of 1:1 means the transmission line and antenna impedances are perfectly matched, so no power is reflected and all power transfers to the antenna.
Question 2: What is the characteristic impedance of coaxial cable most commonly used in commercial radio transmitter installations?
- 25 ohms
- 50 ohms (Correct answer)
- 300 ohms
- 600 ohms
Correct answer: 50 ohms
50-ohm coaxial cable is the industry standard for commercial RF transmitter feedlines due to its balance of low loss and adequate power-handling capacity.
Question 3: What happens to RF power when there is an impedance mismatch between a transmission line and an antenna?
- All power is dissipated as heat in the cable dielectric
- Standing waves are created and some power is reflected back toward the transmitter (Correct answer)
- The antenna radiates more power than was delivered to the feedline
- The operating frequency shifts to compensate for the mismatch
Correct answer: Standing waves are created and some power is reflected back toward the transmitter
Impedance mismatch causes a portion of the forward power to be reflected at the antenna feedpoint, creating standing waves and reducing radiated power.
Question 4: What is the velocity factor of a transmission line?
- The ratio of signal propagation speed in the line to the speed of light in free space (Correct answer)
- The ratio of transmitted power to reflected power on the line
- The frequency at which the transmission line is exactly one wavelength long
- The maximum RF current the line can safely carry
Correct answer: The ratio of signal propagation speed in the line to the speed of light in free space
Velocity factor is the ratio of signal speed inside the transmission line to the speed of light in vacuum, typically 0.66–0.95 for common coaxial cables.
Question 5: What is the primary risk of operating a transmitter into a high SWR load?
- The output frequency increases beyond the licensed bandwidth
- Excessive reflected power can overheat and damage the transmitter's final amplifier (Correct answer)
- The transmission line radiates RF energy, causing interference
- The antenna gain doubles, causing overmodulation
Correct answer: Excessive reflected power can overheat and damage the transmitter's final amplifier
High SWR returns significant reflected power to the transmitter, potentially causing excessive voltage or current that can destroy the final amplifier stage.
Question 6: An antenna tuner (matching network) is primarily used to:
- Change the resonant frequency of the antenna element
- Transform the antenna impedance to match the transmission line and transmitter output (Correct answer)
- Amplify the RF signal before it reaches the antenna
- Suppress harmonics radiated by the antenna
Correct answer: Transform the antenna impedance to match the transmission line and transmitter output
An antenna tuner uses inductors and capacitors to transform the impedance presented by the antenna to the value required by the feedline and transmitter, reducing SWR.
Question 7: Which transmission line type generally offers the lowest attenuation per unit length at UHF frequencies for high-power commercial use?
- RG-58/U flexible coaxial cable
- RG-8/U coaxial cable
- Open-wire parallel feedline (ladder line)
- Rigid coaxial line or waveguide (Correct answer)
Correct answer: Rigid coaxial line or waveguide
Rigid coaxial lines and waveguides offer the lowest loss per unit length at UHF/microwave frequencies, making them the choice for high-power broadcast and radar installations.
GROL Antennas and Transmission Lines 2
What does a standing wave ratio (SWR) of 1:1 indicate on a transmission line?