Choosing a TV Antenna Mounting Site

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Is a higher antenna always better?

No, for the first house below:

Wavelengths

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

But after the signal has skimmed over several equal-height obstacles it will be necessary to go up about 5 wavelengths to find a full-strength signal, even if the transmitting tower can be seen from a lower spot.

For channel 2, five wavelengths would be 26m. A mast that long is impractical. But below 26m the signal strength is roughly proportional to the square of the height. Thus the rule of thumb: “Higher is always better” for VHF.

For the following text, “horizon” will mean the highest obstruction your antenna can see. The horizon could be the top of a house or distant hill. The top of a tree could be the horizon for UHF but not VHF-low.

The rules for UHF are a little more complicated than for VHF. UHF is more affected by obstructions and less affected by height. For UHF, 5 wavelengths is only about 3m. But a UHF antenna should be higher than this in these cases:

  1. If at all possible, get the antenna above any obstructions.
  2. If your horizon is less than 200m away then raising the antenna makes a significant difference. You would be a candidate for a tower. (When the horizon is farther than 200m, the benefit is usually too small to justify the effort a high mast requires.)

You will probably want to attach a VHF antenna to your chimney. That is also likely the best place for a UHF antenna. But if your chimney mount is still obstructed (by trees, etc.) then an unobstructed site closer to the ground will work better for UHF. The essential goal is to find a spot where your UHF antenna can see a distant horizon in the direction of the station.

Note that on the front of a hill, the antenna height often makes little or no difference (VHF and UHF):

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

Power lines

Power lines will reflect the signal. But that is just one reason to keep antennas away from power lines. If there is appliance RF noise in the power lines, the lines will transmit the noise to a close antenna.

Many people have been killed when their antenna fell into power lines.
 

Attic antennas

If an indoor antenna is not as reliable as you want, an attic antenna is the next step up. If you are in a neighborhood with moderately strong signals, an attic antenna might work. But you are wasting your time installing an attic antenna in a poor-signal neighborhood. Most successful attic antennas are within 15km of the transmitter. (30km often works if you are on a hillcrest.) The problems with attic antennas are:

  1. The antenna might not be high enough above obstacles outside the house such as trees.
  2. It is hard to estimate the signal loss caused by the wood and other construction materials.
  3. Metal objects in the attic can block the signal.

Estimating the signal loss in ordinary construction materials requires knowledge of their water content. Exceptions are aluminum siding, stucco (which has an embedded metal screen), and foil-backed insulation, all of which totally block all signals. Concrete and most bricks have moderate water content, but their thickness is enough to block all signals. In a desert, plywood becomes so dry that it causes no signal loss at all, even for UHF. In any other place, there will be some moisture. Exterior wood is generally always wet inside, especially in north facing surfaces. (Paint does not prevent this.) The amount of water varies with the weather. Dry asphalt shingles are mostly transparent to TV signals, but the way they overlap encourages water to persist between them. The vapor barrier is often wet on one side or the other. The bottom line is that there is no way to predict the signal loss in these materials. It is usually a mistake to point an antenna through a surface that gets totally wet in rain.

Metals reflect signals. A metal object 8 inches long is big enough to reflect UHF. Smaller objects, such as nails, are of no concern. Wires and metal pipes effectively reflect VHF, as do plastic pipes containing water. If these reflecting objects are positioned to the side, to the rear, above, or below the antenna, they will have little effect on it, provided they are not too close. These objects should be further away than 600mm for UHF, 1200mm for VHF-high, or 1800mm for VHF-low, and an even larger separation will help a little.(Some might wonder why these numbers are not proportional to the wavelength. It is because the lower frequency antennas are lower in gain. An antenna’s aperture depends on the gain as well as the wavelength.)

There should be no horizontal or diagonal wires or pipes in front of the antenna. A perfectly vertical metal vent pipe is invisible to TV signals, but its flashing at the roofline might not be.

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