TV Antenna Troubleshooting

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Interference

Common sources of interference include:

  1. Adjacent channel interference (a very strong station one channel up or down).
  2. Co-channel interference (two weak stations on the same channel).
  3. Multi-path interference (usually caused by the direct path being blocked).
  4. A very close transmitter (a neighborhood FM station, police station, taxi company, etc.).
  5. An industrial noise source (a factory, a clinic, a malfunctioning power transformer).
  6. Household appliances, including motors, light dimmers, and fluorescent lights.

You might need an antenna that can reject the interfering signal. See “Nulls in radiation pattern” below.

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.

Co-channel interference

When the station you want is not receivable because of another station on the same channel, you have co-channel interference. The interfering station can be very far away and very weak, yet it can contribute enough “noise” to make the close station hard to receive. The remedy is a new antenna that is both stronger in the forward direction and weaker in the interfering direction. See “Nulls in radiation pattern”.

Multi-path interference

This is a problem that, if severe, can prevent DTV reception even if the signal is strong. The signal is reaching the antenna by more than one path due to reflections off of structures, and diffraction around hills and sometimes trees.

There are two distinct categories of multi-path interference. The first is “short delay” multi-path, delays of less than about 20 nanoseconds. On analog channels there will be no visible ghosts.

  • Short delay multi-path

    This is always caused by something directly in front of the antenna. One common cause is a tree in front of the antenna. There will be chaotically overlapped signals behind a tree. This will mainly affect UHF reception. The solution is to relocate the antenna (or cut down the tree). If the antenna stays behind the tree, you will likely see dropouts on UHF channels when the wind blows. And that’s for strong-signal areas. In weak-signal areas you will likely get no UHF reception at all behind a tree.

    The other common cause is an irregular horizon line (structures and trees in the distance). These will cause overlapping fields, which will result in a regular pattern of strong and weak spots. For UHF, moving the antenna right or left three feet or so can make a huge difference. Moving the antenna is usually the solution. Unfortunately a strong spot for one channel can be a weak spot for a different channel. The same phenomenon happens for VHF, but since wavelengths are ten times as big, the strong and weak spots are ten time further apart, so moving the antenna to a strong spot is often too far to be practical. For VHF the solution is usually a bigger antenna.

  • Long delay multi-path

    This is caused by a large obstruction like a hill or a large building. If you tune in an analog channel close in frequency and from the same direction, you will see ghost images. The offending signals are approaching the antenna mostly from the sides, but also maybe from the rear. Actually all analog images have these ghosts, but without the direct path blocked they are too dim to see.

    One solution is to move the antenna to a new spot where signals from the offending directions are less strong. A move of 6m or more will likely be necessary.

    The other solution is to select an antenna with little or no reception in the offending directions. There are two workable strategies here. If the analog channels show one really strong ghost (and maybe a number of weaker ones) then selecting an antenna with a null will work. See “Nulls in radiation pattern”. Otherwise an antenna with minimum radiation to the side and rear is the way to go. The higher the antenna’s gain, the less reception it will have to the side.

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