HAM Antenna Theory and Design 4

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?

Accepted Answer

Cardioid (heart-shaped) pattern

Answer

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?

Accepted Answer

The physical length of the antenna relative to the wavelength

Answer

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?

Accepted Answer

Broader frequency coverage with relatively consistent performance

Answer

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?

Accepted Answer

The ratio of the effective aperture to the physical aperture area

Answer

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?

Accepted Answer

Phased array with a bidirectional broadside pattern

Answer

Two verticals fed 180° out of phase and separated appropriately create a bidirectional broadside pattern perpendicular to the line joining the elements.

HAM - Radio Extra Class Test Practice Test

HAM - Radio Extra Class Test Practice Test

HAM Antenna Theory and Design 4