TV Antenna Terms Glossary - A to F

Pages: 1 2 3 4 5 »» Next

Adjacent channel interference

When the station you want is not receivable because of a much more powerful station in the next channel above or below, you have adjacent channel interference.

When a signal is 10 to 15 times more powerful than that of an adjacent channel station, most receivers become unable to receive the weaker station. To receive a far away station, you might need to use a directional antenna to reduce the strength of a nearby adjacent channel station. See “Nulls in radiation pattern”. But if both stations are in the exact same direction you might be out of luck. There are some frequency selective filters that can “trap” out a channel, but they are seldom able to reduce an adjacent channel by more than half.

When adjacent channel stations broadcast from the same tower or adjacent towers they must have an agreement that neither will exceed 10 times the other’s power.

Antenna

A device for converting radio waves into an electrical signal, or visa versa. Common TV antennas are capable of transmitting as well as receiving. (TVs never transmit.)

Antenna Amplifiers

Many people think adding an amplifier to their antenna will improve the performance of the antenna. The truth isn’t that straightforward. There are two types of signal amplifiers:

Preamplifiers or Mast-mounted amplifiers

These should be mounted as close to the antenna as possible. Usually the amplifier comes in two parts:

1. The amplifier. This is an outdoor unit that is normally bolted to the antenna mast. It must have a very low noise figure, and enough gain to overcome the cable loss and the receiver’s noise figure.

2. The power module (power injector). This is an indoor unit that commonly lies on the floor behind the TV. It is inserted into the antenna cable between the amplifier and the TV. This module injects some power, usually DC, into the coaxial cable where the amplifier can use it. The power injector is the amplifier’s power supply.

Distribution amplifiers

These are simple signal boosters. They are often necessary when an antenna drives multiple TVs or when the antenna cable is longer than 45m. Distribution amplifiers don’t need to have a low noise figure, but they need to be able to handle large signals without overloading. Commonly, distribution amplifiers have multiple outputs. (Unused outputs usually do not need to be terminated.)

Antenna Aperture (capture area)

An antenna has an aperture area, from which it captures all incoming radiation. The formula for the aperture area of any TV antenna is A=Gλ2/4π where λ is the wavelength and G is the gain factor over an isotropic antenna (not dB).

Antenna aperture

Attenuator

This device will decrease the strength of the signals passing through it. A 6 dB attenuator will reduce a signal to one-half its original voltage (one-quarter of its power). It employs a resistor network designed to not cause any reflections in the transmission lines. Most attenuators will not pass power and thus must not be inserted between the mast-mounted amplifier and its power injector.

If an antenna system needs two amplifiers, where the output of one amp feeds into the other amp, too much gain (overload) can result and an attenuator is usually the simplest solution. If you don’t have two amplifiers, it is unlikely that you will ever need an attenuator.

Attic antennas

If an indoor antenna is not as reliable as you want, an attic antenna is the next step up. If you are in a neighbourhood with moderately strong signals, an attic antenna might work. But you are wasting your time installing an attic antenna in a poor-signal neighbourhood. Most successful attic antennas are within 20km of the transmitter. (30km often works if you are on a hillcrest.) The problems with attic antennas are:

1. The antenna might not be high enough above obstacles outside the house such as trees.

2. It is hard to estimate the signal loss caused by the wood and other construction materials.

3. Metal objects in the attic can block the signal.

Estimating the signal loss in ordinary construction materials requires knowledge of their water content. Exceptions are aluminum siding, stucco (which has an embedded metal screen), and foil-backed insulation, all of which totally block all signals. Concrete and most bricks have moderate water content, but their thickness is enough to block all signals. In a desert, plywood becomes so dry that it causes no signal loss at all, even for UHF. In any other place, there will be some moisture. Exterior wood is generally always wet inside, especially in north facing surfaces. (Paint does not prevent this.) The amount of water varies with the weather. Dry asphalt shingles are mostly transparent to TV signals, but the way they overlap encourages water to persist between them. The vapor barrier is often wet on one side or the other. The bottom line is that there is no way to predict the signal loss in these materials.

Metals reflect signals. A metal object 8 inches long is big enough to reflect UHF. Smaller objects, such as nails, are of no concern. Wires and metal pipes effectively reflect VHF, as do plastic pipes containing water. If these reflecting objects are positioned to the side, to the rear, above, or below the antenna, they will have little effect on it, provided they are not too close. These objects should be further away than 600mm for UHF, 1200mm for VHF-high, or 1800mm for VHF-low, and an even larger separation will help a little.(Some might wonder why these numbers are not proportional to the wavelength. It is because the lower frequency antennas are lower in gain. An antenna’s aperture depends on the gain as well as the wavelength.)

There should be no horizontal or diagonal wires or pipes in front of the antenna. A perfectly vertical metal vent pipe is invisible to TV signals, but its flashing at the roofline might not be.