TV Antenna Troubleshooting

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Fading

Depending on the terrain at your location, there are often two paths for the signal to follow to your antenna.

Signal path diagram

When the sun warms up the land, a warm air layer near the ground can add a third path. The warm air causes a bending called refraction, which is identical to the “mirage effect".

Signal path diagram

Which of these paths will be the strongest is hard to predict. The ground-reflecting path is usually weak if the reflection point is forested. The bent path can be enhanced by focusing, making it stronger than the direct path.

These paths will add together at your antenna, and considering phase, subtraction is a possibility. Whether subtraction occurs depends on the length of the bent path. Since the warm air layer is always either growing (sun up) or shrinking (sun down) this path length is always changing. For UHF the path length need change by only 300mm to turn addition into subtraction. If both path signals are about the same strength, your DTV channel will dropout momentarily. If you see two dropouts that were N minutes apart then you will probably continue to see dropouts every N minutes. This is fading. There is no cure for it, but a stronger antenna will make it less likely.

The above is but one scenario for fading. There are many variations on this depending on the terrain.

Overload

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.

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, the signals of all channels are distorted, 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 the cause.

An attenuator is a resistor network that can be used to reduce the gain of an amplifier. An attenuator is often the simplest solution to overload.

If you are close to an FM station, there might be a narrow range between too much and too little gain. You can make that range larger by using an amplifier with an FM trap or by using a more directional antenna. See “Nulls in radiation pattern”.

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. The latter are more effective. If the FM station is close enough you might need more than one.

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