HAM Antennas and Feed Lines 5 — Questions and Answers
Question 1: What is the radiation pattern of an isotropic antenna?
- A figure-eight pattern in the horizontal plane
- A cardioid pattern with a defined null
- A perfect sphere — equal radiation in all directions (Correct answer)
- A toroidal pattern with a null off the ends
Correct answer: A perfect sphere — equal radiation in all directions
An isotropic antenna is a theoretical point source that radiates equally in all directions, forming a perfect sphere.
Question 2: What is the significance of the 1:1 SWR condition on a transmission line?
- All transmitted power is reflected back to the source
- Maximum power transfer occurs with no reflected wave (Correct answer)
- The line radiates exactly half the input power
- The line impedance equals free-space impedance (377 ohms)
Correct answer: Maximum power transfer occurs with no reflected wave
A 1:1 SWR means the line is perfectly matched — all forward power is absorbed by the load with no reflection.
Question 3: A log-periodic dipole array (LPDA) antenna is best described as:
- A narrow-band, high-gain directional antenna
- A broadband directional antenna with moderate gain across a wide frequency range (Correct answer)
- An omnidirectional antenna with high gain at all elevation angles
- A circularly polarized antenna for satellite operation
Correct answer: A broadband directional antenna with moderate gain across a wide frequency range
An LPDA is a broadband directional antenna providing consistent moderate gain across a wide frequency range, widely used for HF communication.
Question 4: What is the effect of ground conductivity on the performance of a quarter-wave vertical antenna with radials?
- Higher conductivity increases the antenna's resonant frequency
- Higher conductivity reduces ground losses and improves radiation efficiency (Correct answer)
- Higher conductivity narrows the antenna bandwidth
- Ground conductivity only affects antennas operating above 30 MHz
Correct answer: Higher conductivity reduces ground losses and improves radiation efficiency
Higher ground conductivity reduces the resistive loss in the ground return current path, improving the efficiency and gain of a vertical antenna.
Question 5: What is the advantage of using a terminated folded dipole (T2FD) antenna?
- Very high gain over a narrow bandwidth
- Broadband operation with low SWR across a wide frequency range due to the terminating resistor (Correct answer)
- Perfect circular polarization across all HF bands
- Zero radiation resistance for purely reactive operation
Correct answer: Broadband operation with low SWR across a wide frequency range due to the terminating resistor
The T2FD uses a terminating resistor to achieve a broad, low-SWR frequency range at the expense of efficiency, making it useful for wideband receive or transmit applications.
Question 6: What is the main advantage of a rhombic antenna over a Yagi for long-distance HF communication?
- Higher gain per element used
- Much broader bandwidth covering multiple HF bands (Correct answer)
- Easier rotation for beam steering
- Lower profile, requiring less real estate
Correct answer: Much broader bandwidth covering multiple HF bands
A rhombic antenna is inherently broadband, covering many HF frequencies with useful gain, unlike the narrowband Yagi.
Question 7: What occurs when a transmission line is exactly one half-wavelength long, regardless of its characteristic impedance?
- The line presents a short circuit to the source
- The input impedance equals the load impedance (the line is transparent) (Correct answer)
- The line transforms the load to its complex conjugate
- The SWR at the input doubles compared to at the load
Correct answer: The input impedance equals the load impedance (the line is transparent)
A half-wavelength transmission line repeats the load impedance at its input, making it electrically transparent — the input impedance equals the load impedance.
HAM Antennas and Feed Lines 5
What is the radiation pattern of an isotropic antenna?