"Ghosting" in Off-Air Reception

[SolidSignal_Mike]

What is ghosting?

Many structures can reflect TV signals much like they reflect light. This can lead to the TV receiver "seeing" more than one version of the TV signal. Ghosting results when a TV signal arrives at the receiver via more than one path. Low areas in a neighborhood are especially susceptible to ghosting.

If ghosting is caused by a single structure that creates one distinct ghost image, a medium or large multi-directional antenna may be useful with careful positioning to eliminate the reflected signal. However, the antenna may require different positioning for each channel.

Directional antennas are the most ghost-resistant antennas since they "see" in only one direction and have a tendency not to see the reflected ghost signal. The further away from structures the antenna is located the less likely a problem will occur. Many factors, however, such as the structure's total surface area, the direction it faces, and neighborhood terrain, will influence how much effect the structure will have on TV reception.

Which structures cause ghosting?

Nearby buildings that are higher than the TV antenna or block the antenna's view in the direction of the TV station's transmitter such as:

Church steeples
Apartment/condominium buildings
School buildings
Water towers
Industrial buildings
Office buildings
Warehouses
Large communications/radio/TV towers
Athletic field lighting towers
High tension power lines and towers

Which structures do not cause ghosting?

Trees and foliage aren't usually a problem, but in some cases foliage can absorb TV signals and reduce their strength. Therefore, the best antenna locations should avoid foliage in the direction of the TV transmitter. The following usually do not cause ghosting:

Other homes and wooden buildings that aren't taller than antenna location
Streetlight or utility poles
Cellular or PCS poles consisting of a single pole no higher than treetops
Ham radio antennas
Other TV antennas

ref: http://www.antennaweb.org