TV Antenna Terms Glossary - G to P

Noise/Noise figure/Noise factor

For TVs there are two main sources of noise:

Atmosphere noise. There are many types of sources for this noise. A light switch creates a radio wave every time it opens or closes. Motors in some appliances produce nasty RF noise.
Receiver noise. Most of this noise comes from the first transistor the antenna is attached to. Some receivers are quieter than others.

Receiver noise dominates on the VHF and UHF bands, and atmospheric noise is usually insignificant.

The noise figure and noise factor are the same thing, but the noise figure is expressed in dB. Every amplifier has a noise figure. The noise figure must be subtracted from the antenna gain. Thus the noise figure tells how much of the antenna gain you are throwing away by not buying a quieter amplifier. The amplifier in question is the OTA (over the air) receiver or the mast-mounted amplifier, whichever the antenna connects directly to.

Nulls in radiation pattern

Nulls in the radiation pattern can be useful. If you rotate the antenna so that a null points toward an interfering signal, that signal is eliminated. Some interference situations that might benefit from this trick include:

Adjacent channel interference (a very strong station one channel up or down)
Co-channel interference (two weak stations on the same channel)
Multi-path interference (usually caused by the direct path being blocked)
A very close transmitter (a neighborhood FM station, police station, taxi company, etc.)
An industrial noise source (a factory, a clinic, a malfunctioning power transformer)

For example, the Channel Master 4228 has nulls on both sides at 30 and 90 degrees:

Channel master

Yagi/Corner-reflector antennas have no nulls. LPDA antennas have nulls at 90 degrees, but LPVA antennas have no nulls. To make rabbit ears have nulls (at 90 degrees) lower them into a straight dipole.

In a multi-path situation, a null will work if there is only one strong ghost. (Find an analog channel close in frequency and from the same direction. Examine it for ghosts.) If there are multiple very strong ghosts then a better approach is a very high-gain (very directional) antenna with little reception to the side or rear.

Omni-directional antenna - This is a non directional antenna. Its gain is low, but if you are in a city and surrounded by stations, this might be the perfect antenna.

The gain of these antennas is no better than that of a typical indoor antenna, but if you mount this unit in the attic or on the roof, you will likely see a considerable improvement. And you won’t have to re-aim it or endure dropouts when someone walks past the TV. Most people will benefit by getting an amplified version, but if you are in a very strong-signal neighborhood, that won’t be necessary.

An important exception is if tall buildings block some stations. In this case you will likely have multi-path interference. If you see strong ghosting on analog channels then you have multi-path and you are not a candidate for an omni-directional antenna. Most DTV receivers have trouble with multi-path, even if the signal is strong. You need a very directional antenna that can ignore signals coming from the wrong directions.

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