Antenna Theory and Design Flashcards

What is the purpose of a loading coil in an electrically short antenna?

Answer: To resonate the antenna by canceling the capacitive reactance

A loading coil adds inductive reactance to cancel the capacitive reactance of an electrically short antenna, bringing it to resonance.

What is the gain of a large parabolic dish antenna primarily determined by?

Answer: The diameter of the dish relative to the wavelength

The gain of a parabolic dish is primarily determined by the ratio of the dish diameter to the operating wavelength.

What is an NVIS (Near Vertical Incidence Skywave) antenna optimized for?

Answer: Local and regional communications within about 300 miles

An NVIS antenna radiates at very high angles (near vertical) to use the ionosphere for regional communications within approximately 300 miles.

What is the characteristic of an antenna with a very high Q factor?

Answer: Narrow bandwidth and high efficiency for a resonant antenna

A high-Q antenna has a narrow bandwidth, meaning it is very frequency-selective and operates efficiently only over a small frequency range.

Which of the following describes an end-fire antenna array?

Answer: Maximum radiation is along the axis of the array

An end-fire array produces maximum radiation along the axis of the array, like a Yagi antenna.

What is the purpose of a balun when connecting coaxial cable to a dipole antenna?

Answer: To match the balanced antenna to the unbalanced transmission line

A balun (balanced-to-unbalanced transformer) prevents common-mode current on the coaxial shield by matching the balanced dipole to the unbalanced coax.

What is the approximate gain of a typical 5-element Yagi antenna over a dipole?

Answer: 6 to 9 dBd

A 5-element Yagi typically provides 6 to 9 dBd gain over a reference dipole, depending on element spacing and design.