Choosing a TV Antenna Mounting Site

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Ground Reflections

Often the signal waves are angled downward slightly, usually the result of diffraction over an obstacle in the distance. If there is mostly-flat ground in front of the antenna, the ground reflection can be efficient.

Instantaneous voltage diagram:

Instantaneous voltage diagram

These two waves pass through each other without affecting each other. But the antenna responds to the instantaneous sum of the two overlapping waves. Where the two waves subtract, there will be places where reception is very weak.

Average power diagram:

Average power diagram

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

The result is a striped region of alternating strong and weak layers parallel to the ground. Thus there are cases where lowering the antenna might put it in a stronger signal. So, while the following technique is unorthodox, the result is very credible:

Beetle Bailey
© King Features Syndicate. Reproduced here with permission.

Unfortunately a strong spot for one channel can be a weak spot for a different channel, so compromise might be necessary.

The ground doesn’t have to be as flat as you might guess for these layers to form. Weeds, shrubs, and trees are mostly transparent to VHF. A surface wet from rain will usually be 100% reflective.

Average power diagram for UHF:

This layering problem is greatest for UHF. The distance from a very strong spot to a very weak spot can be as little as three feet. But weeds might save you. Stand where the antenna will go and look toward the ground in the direction of the transmitter. If you see weeds or shrubs or trees, you are OK. If you see lawn or dirt or pavement then you likely have some layering.

An antenna has an aperture, over which all incoming signal is collected. In this diagram the aperture is positioned to collect signal from two layers. But adjacent layers always have opposite polarity, and subtract. Thus this antenna is picking up no signal at all (assuming a 100% efficient ground reflection).

When layering is present, if a larger antenna is necessary, choose an antenna whose aperture is wider, not taller. Otherwise you may find the new antenna works no better than the old one.

The ground reflection can be very helpful. Assume the power in the incident wave is P. If the reflection is 100% efficient, you might expect the power in the overlapped area to be 2P. But instead it will vary from 0P to 4P. (Power is the square of voltage. Where the voltage doubles, the available power goes up by 4.)

If you can put the antenna in the most intense spot, it will collect 4 times as much signal as with no ground reflection (assuming a 100% efficient ground reflection).
(Please note the channel number and corresponding frequency used in these diagrams relate to U.S. channels, but are still relative.)

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