Choosing a TV Antenna Mounting Site

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Horizon-induced Multi-path

The following four diagrams are identical except for the view rotation.

Rays from the transmission tower come to earth after passing over a horizon ridge. This ridge could be a tree line 50m away or a mountain ridge 5km away. The rays diffract at the ridge, staying in a plane perpendicular to the ridgeline. The result is often overlapping rays.

Overlapping fields will result in weak signal spots and strong spots arranged in a regular pattern.

Horizon-induced muti-path

For UHF the strong and weak spots are often 1.5 to 6m apart. If you are in a neighborhood with overlapping fields, moving your antenna a few feet can make a huge difference in signal strength. The chimney might seem like the perfect site, but if the chimney is in a weak spot then the chimney is a mistake.

To make matters worse, the pattern of strong and weak spots will be different for different frequencies. You will want to find a spot that is strong for all the channels you want. But such a spot might not exist above your roof. In this case you must search for a spot that is the best compromise for your must-have channels. In the worst case you might need two antennas and a switch.

To make matters even worse, you will not likely discover that you are in such a neighborhood until after you have purchased and installed the antenna. To prove that you have strong and weak spots, you move the antenna (leftward and rightward, higher and lower) while keeping it perfectly pointed at the signal and watching the DTV signal strength indicator. (What? Your TV is not on the roof? Well maybe with some cordless phones you can get your wife to help you.) It is hard to keep a large antenna pointed correctly while devoting half of your attention to not falling off the roof, but a smaller antenna might not achieve a digital lock.

At this point a professional installer starts to look like the smart choice. But will he stick with it, or will he too quickly declare further improvements impossible and walk away? He will hesitate to raise his estimate, but he will not work at a loss.

These problems are UHF problems. VHF does the same thing, but with strong and weak spots 15 to 60m apart they are not very evident and there is usually not much you can do about them.

Non-uniform fields

Overlapping fields result in non-uniform fields: layered and continuously varying. An antenna in a non-uniform field doesn’t perform quite like one would guess. Normally an antenna captures all the radiation within its aperture. But in a non-uniform field some signal gets rejected.

Many people in this situation conclude they need a bigger antenna. But a bigger aperture gathers signal from a larger area, and this larger area is usually even more non-uniform, causing greater signal rejection. Many people have switched to a larger antenna and found no improvement.

There is more than one way to explain this counter-intuitive result. (These explanations sound totally different, but they are equivalent.) Probably the simplest explanation is that the antenna’s beam width gets narrower as the aperture gets bigger. The bigger antenna is now so directional that it cannot be pointed directly at both sources that produce the overlapped field.

A solution to this dilemma is an asymmetric aperture. Choose an antenna whose aperture is large in the direction of the layer, but small in the direction across the layer. Stacked antennas will have such an asymmetrical aperture.

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