TV Antenna Basics

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Baluns

A balun is an adapter that adapts a balanced line to unbalanced line. If a balanced transmission line (such as twinlead) is connected directly to an unbalanced line (such as coaxial cable) the two lines become a long-wire antenna, which is undesirable for VHF and UHF. All baluns are passive bi-directional devices. They are usually above 90% efficient. There are two types:

4-to-1 balun  -  This will connect 300-ohm twinlead to 75-ohm coaxial cable.  This balun is usually a ferrite transformer.

1-to-1 balun  -  This will connect a 75-ohm balanced load to 75-ohm coaxial cable.  This balun is often just some ferrite beads slipped over the coax.

Signal Amplifiers, Preamplifiers, part 2

There are two types of signal amplifiers:

  • Preamplifiers (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.)

Never feed an amplifier output directly into another amplifier. There should always be a long cable between the preamplifier and the distribution amplifier. Placing the two amplifiers close together can cause overload and/or oscillation.

A mast-mounted amplifier’s most important characteristic is its noise level, usually specified by the noise figure. But many manufacturers don’t take this number seriously. If it is given at all, it is often wrong. If all makers don’t do them right then comparison-shopping is not possible. The author is inclined to rate amplifiers for their noise figures as follows:

0.5 dB superb (anything better runs into thermal atmospheric noise)
2.0 dB excellent
4.0 dB fair
6.0 dB poor
10 dB awful

The noise figure is a number you must subtract from the antenna’s gain. The noise figure tells how much of the antenna’s gain you are throwing away by not buying a quieter amplifier. This loss is gone and cannot be made up later.

Receiver overload

Signal amplifiers are supposed to be linear. That is, the output is a magnified but otherwise unaltered version of the input. But too much signal can make an amplifier non-linear, usually clipping off the tops and bottoms of the sine waves. When this happens, all channels are affected, not just the one that is too strong. In fact, the too strong signal is usually not a TV station. A close FM station or police station is more likely.

If you add a good amplifier to your antenna system and your results get worse instead of better then you have overload, and you need to reconsider more carefully what you are doing.

Overload never causes any equipment damage.

An attenuator is a resistor network that can be used to reduce the gain of an amplifier. 6 dB attenuators are available at Radio Shack. 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.

If you are close to an FM station, there might be a narrow range between too much and too little amplifier gain. (Too little gain = dropouts, too much gain = overload.) You can make that range larger by using an amplifier with an FM trap or by using a more directional antenna. VHF preamplifiers usually include FM traps that can optionally be disabled. Freestanding FM traps are also available. FM traps can either cover the entire FM band or can be single frequency traps that you tune to the offending station. If the FM station is close enough you might need more than one FM trap.

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