HAM Antenna Theory and Design 5 — Questions and Answers
Question 1: What is mutual coupling between antenna elements in an array?
- The impedance of the transmission line connecting the elements
- The electromagnetic interaction between antenna elements that affects their impedance and current distribution (Correct answer)
- The physical spacing between elements measured in wavelengths
- The phase difference between signals fed to each element
Correct answer: The electromagnetic interaction between antenna elements that affects their impedance and current distribution
Mutual coupling is the electromagnetic interaction between nearby antenna elements that changes their individual impedances and current distributions from the isolated element case.
Question 2: What is a beverage antenna and what is it primarily used for?
- A vertical collinear array used for VHF communications
- A long terminated wire antenna used for low-angle HF reception (Correct answer)
- A loop antenna used for direction finding
- A multi-element Yagi for 80 meters
Correct answer: A long terminated wire antenna used for low-angle HF reception
A Beverage antenna is a long (one to several wavelengths) wire terminated in its characteristic impedance, used for low-angle receiving on HF bands with excellent directivity.
Question 3: What is the significance of the E-plane and H-plane in antenna radiation pattern descriptions?
- E-plane contains the electric field vector and H-plane contains the magnetic field vector in the direction of maximum radiation (Correct answer)
- E-plane is the horizontal pattern and H-plane is the vertical pattern for all antennas
- E-plane indicates elevation gain and H-plane indicates horizontal gain
- E-plane is for transmit patterns and H-plane is for receive patterns
Correct answer: E-plane contains the electric field vector and H-plane contains the magnetic field vector in the direction of maximum radiation
The E-plane is the plane containing the electric field vector and the direction of maximum radiation, while the H-plane contains the magnetic field vector and maximum radiation direction.
Question 4: What is the purpose of stacking two identical Yagi antennas vertically?
- To double the front-to-back ratio
- To reduce the bandwidth of the combined system
- To increase gain by reducing the elevation beamwidth (Correct answer)
- To shift the operating frequency lower by sqrt(2)
Correct answer: To increase gain by reducing the elevation beamwidth
Stacking Yagis vertically increases gain (about 3 dB ideally) by narrowing the elevation (vertical) beamwidth while maintaining the horizontal pattern.
Question 5: What is an isotropic radiator and why is it used as a reference?
- A dipole antenna oriented vertically, used because it is easy to build
- A theoretical antenna that radiates equally in all directions, used as a universal gain reference (Correct answer)
- A directional antenna with 0 dB gain on the reference axis
- A real antenna used in anechoic chambers for calibration
Correct answer: A theoretical antenna that radiates equally in all directions, used as a universal gain reference
An isotropic radiator is a theoretical point source that radiates equally in all directions (4π steradians), used as a universal reference for expressing antenna gain in dBi.
Question 6: What is the effect of ground conductivity and permittivity on the radiation pattern of a low-frequency vertical antenna?
- Ground properties have no effect on vertical antenna patterns above 1 MHz
- Poor ground increases low-angle radiation and improves DX performance
- High-conductivity ground improves low-angle radiation efficiency and lowers the angle of maximum radiation (Correct answer)
- Ground only affects the antenna's bandwidth, not its radiation pattern
Correct answer: High-conductivity ground improves low-angle radiation efficiency and lowers the angle of maximum radiation
High-conductivity ground acts as a better reflector, improving low-angle radiation efficiency and pushing the angle of maximum radiation closer to the horizon, which benefits DX communication.
Question 7: What is the term for the ratio of the maximum radiation intensity of an antenna to the average radiation intensity over all directions?
- Radiation efficiency
- Effective radiated power (ERP)
- Directivity (Correct answer)
- Antenna factor
Correct answer: Directivity
Directivity is the ratio of maximum radiation intensity to the average intensity over all directions, and equals gain for a lossless antenna.
HAM Antenna Theory and Design 5
What is mutual coupling between antenna elements in an array?