Propagation Problems at VHF and UHF

As long as the power you transmit is sufficient, you can communicate at VHF over line of sight links without too much problem. However, in reality, we all know that signals can, in fact, travel much further.

Before we get into this, however, we need to very briefly look at one further example of how radio waves behave at VHF and UHF.

How VHF signals bounce
At VHF signals can be reflected off any plane surface such as a building, a large metal structure or a hill. They can also be reflected, for example, around a valley if you are transmitting from one hill top to another.

Reflected signals can mean reception out of line of sight, which can be a good thing. But they can also cause interference since the two signals arrive at the receiver separated by a tiny delay as one of them travels further to reach its target.

You simply need to know that signals can bounce in this way at VHF and that the higher the frequency; the easier signals can be reflected.

Radio shadows
A hill or building will shade radio signals in a similar way in which it would act as a shade for a beam of light.

However, signals can bend at the edge of the hill or other sharp object and thus be received (at reduced signal strength) on the other side.

Again this is propagation of a signal that exceeds the line of sight. It is also how many of us receive signals from our local VHF domestic stations or, as an alternative, an explanation of sometimes why we can't!

The above is a rather abbreviated and simple explanation of a complex effect but it will illustrate the way that signals behave at relatively short distances. With this out of the way we can, next week, go on to look at real long distance communication which represents the fun part of VHF DXing.