Choosing a TV Antenna Mounting Site

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Diffraction

Diffraction is the ability of a wave to bend around into the shadow formed by an obstruction. It doesn’t matter whether it is an absorbing or reflecting obstruction. Most OTA (over the air) viewers depend on diffraction for their reception. The only exceptions are:

  1. Where the transmitting tower can be seen.
  2. Sometimes in cities with tall buildings reflection is more effective than diffraction.

VHF Diffraction

VHF Diffraction

The direction the signal is moving is always perpendicular to the wave fronts. Thus if an antenna is mounted in the shadow of a building, the antenna should point at the top of the building, because that is where the wave is coming from.

Low frequencies diffract efficiently, but VHF diffracts poorly. UHF is another ten times worse.

UHF Diffraction

UHF Diffraction

(Please note the channel number and corresponding frequency used in these diagrams relate to U.S. channels, but are still relative.)

These diagrams use linear shading and thus are perhaps overly pessimistic. Reception might be possible where these diagrams show no signal. (Logarithmic shading would convey more optimism.)

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