RF Transmission & Antenna Systems Flashcards

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

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

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.

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

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

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.

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?

Answer: 30 dB

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.

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

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

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.

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

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

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.

What is the significance of the FCC's 60 dBu and 54 dBu contours for Class C FM stations?

Answer: They define the protected service contour and the interference contour respectively

For FM stations, the 60 dBu contour defines the protected service area, while the 54 dBu contour defines the interference boundary — no new station may cause interference within another station's 60 dBu contour.

Which measurement technique is used to determine the antenna VSWR of an in-service broadcast antenna without taking the transmitter offline?

Answer: A directional wattmeter measuring forward and reflected power in the transmission line

A directional wattmeter (Bird or equivalent) inserted in the transmission line measures forward and reflected power simultaneously during operation, allowing VSWR calculation without interrupting service.