CBT RF Transmission & Antenna Systems 3

Question 1

What is beam tilt in a broadcast antenna, and why is it used?

Accepted Answer

Directing the main lobe of the antenna pattern slightly below horizontal to improve ground-level coverage

Answer

Beam tilt directs the antenna's main radiation lobe slightly downward (typically 0.5–2°), compensating for the antenna's elevation above the service area and improving signal strength at ground level.

Question 2

What is the purpose of null fill in a broadcast antenna system?

Accepted Answer

To reduce the sharp signal nulls in the vertical radiation pattern that cause coverage gaps

Answer

Null fill redistributes a small amount of energy into the deep nulls of the vertical radiation pattern, reducing coverage gaps that would otherwise occur at certain distances from the tower.

Question 3

When two FM stations on the same frequency are co-located, what minimum carrier-to-interference ratio is generally required to protect the desired signal?

Accepted Answer

30 dB

Answer

FM co-channel protection requires approximately 30 dB carrier-to-interference ratio to prevent the interfering signal from degrading the desired station's service contour.

Question 4

What is a transmission line's velocity factor (VF), and how does it affect electrical length calculations?

Accepted Answer

VF is the ratio of signal speed in the line to the speed of light; electrical length = physical length divided by VF

Answer

Velocity factor (typically 0.66–0.88 for coax) indicates that signals travel slower in the line than in free space; the electrical length equals the physical length divided by VF, which is critical for phasing networks.

Question 5

What is the function of a hybrid combiner in a broadcast RF system?

Accepted Answer

To combine two transmitter outputs onto one antenna while isolating the transmitters from each other

Answer

A hybrid combiner allows two transmitters to feed a single antenna system with high isolation between the two transmitter ports, so a fault in one transmitter does not affect the other.

(CBT) Certified Broadcast Technologist Practice Test

(CBT) Certified Broadcast Technologist Practice Test

CBT RF Transmission & Antenna Systems 3