Radio wave propagation (part 2)
Radio signal propagation in the VHF. UHF, SHF and EHF - range
VHF : Very High Frequency 30 MHz - 300 MHz
UHF: Ultra High Frequency 0,3 GHz - 3 GHz
SHF: Super High Frequency 3 GHz - 30 GHz
The topic is complex and multifaceted. The explanations given in this chapter are roughly simplified.
In the VHF range one speaks only of the direct wave - there is no longer any distinction between ground and sky waves. Ultra-short waves are only reflected in the ionosphere or atmosphere when "radio weather phenomena" occur very rarely. However, signals in the ultra-short wave range and in the microwave range can be reflected by mountains, rock faces and other surfaces. This can increase the range. The frequencies from approx. 100 MHz have proven to be particularly favorable for this purpose. In a forest, high frequencies are more likely to be absorbed than low frequencies. This means that signals in the shortwave range penetrate a forest more easily than cell phone signals. The same applies to the reflection of radio waves: while rock faces can reflect signals, forests tend to absorb signals. A shortwave antenna can hang between trees.
PMR radios that operate at a relatively high frequency should be used from high ground in the forest. If a remote antenna is provided in the VHF or UHF range in order to achieve a greater range, it should be mounted as high as possible - e.g. on a treetop.
It also makes a difference whether the receiving or transmitting antenna is "lying down" or "standing up". In the first case one speaks of horizontal polarization, in the second case of vertical polarization. Vertical polarization is more likely to be reflected, which is often associated with poorer reception quality, since the superposition of direct and reflected waves can lead to signal distortions. If the signal is reflected between the transmitter and the receiver on an object (mountains, cliffs, buildings, ...), "twisting" in the plane of polarization can also occur. Basically, the antenna polarization should match that of the incoming wave in order to avoid signal loss. Even if the transmitter and receiver are in sight, both must have the same polarization.
Loose snow shields radio waves to a small extent. Mobile phones and radios can be used in igloos and snow caves, and VHF and DAB + radio programs can also be received.
Something else changes with increasing frequency: namely the free space attenuation. The higher the frequency, the lower the range, all other things being equal.