Antenna Theory and Design Flashcards

What is the radiation resistance of a half-wave dipole antenna in free space?

Answer: 73 ohms

A half-wave dipole in free space has a radiation resistance of approximately 73 ohms.

What is the effect of adding a ground plane to a quarter-wave vertical antenna?

Answer: It provides a low-impedance return path and creates an electrical image of the antenna

A ground plane provides a low-impedance return path and acts as a mirror, creating an electrical image that completes the half-wave dipole equivalent.

What is the input impedance of a folded dipole antenna compared to a standard dipole?

Answer: Four times the impedance (about 292 ohms)

A folded dipole has approximately 4 times the impedance of a standard dipole, or about 292 ohms, due to the transformer action of the folded element.

Which antenna type uses a driven element, a reflector, and one or more directors?

Answer: Yagi-Uda antenna

The Yagi-Uda antenna uses a driven element, a slightly longer reflector behind it, and one or more shorter directors in front.

What is the typical front-to-back ratio of a well-designed 3-element Yagi antenna?

Answer: 10 to 15 dB

A well-designed 3-element Yagi typically achieves a front-to-back ratio of 10 to 15 dB.

What happens to the radiation resistance of an antenna as its length becomes much shorter than a half wavelength?

Answer: It decreases significantly

As an antenna becomes electrically short, its radiation resistance decreases significantly, making it less efficient at radiating power.

What is the polarization of the electromagnetic wave radiated from a vertical antenna?

Answer: Vertical polarization

A vertical antenna radiates vertically polarized electromagnetic waves, with the electric field oriented vertically.