Antennas and Feed Lines Flashcards

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

Answer: 73 ohms

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

What is the primary advantage of a Yagi antenna over a dipole for HF communication?

Answer: Directional gain and front-to-back ratio

A Yagi provides directional gain and improved front-to-back ratio compared to an isotropic or dipole antenna.

What happens to the impedance of a transmission line when it is terminated in a purely resistive load equal to its characteristic impedance?

Answer: It equals the characteristic impedance at all points

A line terminated in its characteristic impedance is a matched line, presenting that same impedance at every point along it.

What type of antenna produces a circularly polarized signal?

Answer: Helical antenna in axial mode

A helical antenna operating in axial mode produces a circularly polarized wave, useful for satellite communications.

What is the velocity factor of a typical solid polyethylene coaxial cable?

Answer: 0.66

Solid polyethylene dielectric coaxial cable has a velocity factor of approximately 0.66, meaning signals travel at 66% of the speed of light.

What is the function of a balun at a dipole feed point when coaxial cable is used?

Answer: To prevent common-mode current on the coax shield

A balun suppresses common-mode (shield) currents that would otherwise cause the coax to radiate and distort the antenna pattern.

How does antenna height above ground affect the elevation angle of maximum radiation for a horizontal dipole?

Answer: Greater height lowers the main lobe angle

As a horizontal antenna is raised higher above ground, its angle of maximum radiation decreases toward the horizon, favoring DX communication.