Ham Radio Technician Test Antennas & Feedlines 5 — Questions and Answers
Question 1: What is the main advantage of a beam antenna (such as a Yagi) over a dipole for VHF weak-signal work?
- Higher gain in a specific direction improves both transmit and receive performance (Correct answer)
- Beam antennas require no feedline matching
- Beam antennas are omnidirectional and cover all directions simultaneously
- Beam antennas eliminate multipath fading
Correct answer: Higher gain in a specific direction improves both transmit and receive performance
A beam antenna's directional gain increases effective radiated power on transmit and reduces noise from unwanted directions on receive.
Question 2: Which feedline type is immune to common-mode RF currents without additional chokes?
- Open-wire ladder line (Correct answer)
- RG-8 coax
- RG-58 coax
- Twinlead (300 ohm)
Correct answer: Open-wire ladder line
Open-wire ladder line is balanced and symmetrical, so it does not support common-mode currents the way coax can.
Question 3: What is the purpose of antenna radials on a ground-mounted vertical?
- They provide a low-loss RF ground return to complete the antenna circuit (Correct answer)
- They increase the antenna's resonant frequency
- They act as parasitic directors for additional gain
- They physically support the vertical element
Correct answer: They provide a low-loss RF ground return to complete the antenna circuit
Buried or on-surface radials create a low-resistance ground system that minimizes losses in the soil and completes the antenna circuit.
Question 4: If you double the number of radials on a ground-mounted vertical from 8 to 16, what is the expected result?
- Reduced ground losses and improved antenna efficiency (Correct answer)
- Doubled antenna gain in all directions
- Increased SWR requiring re-tuning
- No practical difference below 30 MHz
Correct answer: Reduced ground losses and improved antenna efficiency
Doubling radials reduces ground resistance and ground losses, increasing the power actually radiated by the antenna.
Question 5: What does 'dBd' mean when specifying antenna gain?
- Gain relative to a half-wave dipole reference antenna (Correct answer)
- Gain relative to an isotropic radiator
- Gain measured in decibels of power
- Gain measured over a ground plane
Correct answer: Gain relative to a half-wave dipole reference antenna
dBd means decibels of gain compared to a half-wave dipole; dBi compares to an isotropic radiator (dBd = dBi − 2.15).
Question 6: What is a common cause of RF feedback (RF in the shack) in a station with a coax-fed dipole?
- Common-mode current flowing on the outside of the coax shield back into the shack (Correct answer)
- Excessive antenna height above ground
- Using RG-213 instead of RG-58
- Operating the antenna on a harmonic frequency
Correct answer: Common-mode current flowing on the outside of the coax shield back into the shack
When coax is connected directly to an unbalanced dipole without a balun, RF can flow back on the shield outer surface into the operating position.
Question 7: What is the typical characteristic impedance of open-wire (ladder line) feedline?
- 300 to 600 ohms (Correct answer)
- 50 ohms
- 75 ohms
- 93 ohms
Correct answer: 300 to 600 ohms
Open-wire ladder line has a characteristic impedance of roughly 300–600 ohms depending on conductor spacing and wire diameter.
Ham Radio Technician Test Antennas & Feedlines 5
What is the main advantage of a beam antenna (such as a Yagi) over a dipole for VHF weak-signal work?