HAM Antenna Theory and Design 4 — Questions and Answers
Question 1: What is the radiation pattern of a two-element phased vertical array with elements spaced one-quarter wavelength apart and fed 90 degrees out of phase?
- Bidirectional figure-8 pattern
- Omnidirectional pattern
- Cardioid (heart-shaped) pattern (Correct answer)
- Cloverleaf pattern
Correct answer: Cardioid (heart-shaped) pattern
Two vertical elements spaced λ/4 apart and fed 90° out of phase produce a cardioid pattern with maximum radiation in one direction and a null in the opposite direction.
Question 2: What is the self-resonant frequency of an antenna primarily determined by?
- The feedpoint impedance
- The physical length of the antenna relative to the wavelength (Correct answer)
- The type of coaxial cable used
- The antenna's radiation resistance
Correct answer: The physical length of the antenna relative to the wavelength
An antenna's self-resonant frequency is primarily determined by its physical length, which must be an electrical multiple of the operating wavelength.
Question 3: What is the main advantage of a log-periodic dipole array (LPDA) over a single Yagi antenna?
- Higher gain on a single frequency
- Broader frequency coverage with relatively consistent performance (Correct answer)
- Smaller physical size for the same gain
- Better front-to-back ratio
Correct answer: Broader frequency coverage with relatively consistent performance
An LPDA covers a wide frequency range (often several octaves) with consistent gain and impedance, unlike a Yagi which is optimized for a single frequency.
Question 4: What is aperture efficiency in the context of a dish antenna?
- The ratio of the dish's physical area to the wavelength squared
- The ratio of the effective aperture to the physical aperture area (Correct answer)
- The percentage of radiated power that reaches the target
- The efficiency of the feedhorn illuminating the dish
Correct answer: The ratio of the effective aperture to the physical aperture area
Aperture efficiency is the ratio of the antenna's effective aperture to its physical aperture area, typically ranging from 50% to 75% for practical dish antennas.
Question 5: What type of antenna is formed when two quarter-wave vertical antennas are fed 180 degrees out of phase?
- Collinear array with gain
- Phased array with a bidirectional broadside pattern (Correct answer)
- End-fire array with high front-to-back ratio
- Parasitic array similar to a Yagi
Correct answer: Phased array with a bidirectional broadside pattern
Two verticals fed 180° out of phase and separated appropriately create a bidirectional broadside pattern perpendicular to the line joining the elements.
Question 6: What is the antenna gain of a half-wave dipole relative to an isotropic radiator (dBi)?
- 0 dBi
- 1.64 dBi
- 2.15 dBi (Correct answer)
- 3.0 dBi
Correct answer: 2.15 dBi
A half-wave dipole has 2.15 dBi gain over an isotropic radiator due to its figure-8 radiation pattern concentrating energy in two directions.
Question 7: What is the effect of increasing the diameter of a dipole antenna element?
- Increases the antenna gain proportionally
- Narrows the antenna bandwidth
- Broadens the antenna bandwidth and lowers the Q (Correct answer)
- Shifts the resonant frequency higher
Correct answer: Broadens the antenna bandwidth and lowers the Q
Using a larger diameter conductor lowers the antenna's Q factor and broadens its bandwidth, allowing operation over a wider frequency range.
HAM Antenna Theory and Design 4
What is the radiation pattern of a two-element phased vertical array with elements spaced one-quarter wavelength apart and fed 90 degrees out of phase?