VHF and UHF

The RF and microwave spectrum can be subdivided into many bands of varying purpose, shown below.

radiospec

On the lower frequency end, VLF (Very Low Frequency) tends to be used in submarine communication while LF (Low Frequency) is generally used for navigation. The MF (Medium Frequency) band is noted for AM broadcast (see posts on Amplitude modulation). The HF (shortwave) band is famous for use by HAM radio enthusiasts. The reason for the widespread usage is that HF does not require line of sight to propagate, but instead can reflect from the ionosphere and the surface of the earth, allowing the waves to travel great distances. VHF tends to be used for FM radio and TV stations. UHF covers the cellphone band as well as most TV stations. Satellite communication is covered in the SHF (Super High Frequency) band.

Regarding UHF and VHF propagation, line of sight must be achieved in order for the signals to propagate uninhibited. With increasing frequency comes increasing attenuation. This is especially apparent when dealing with 5G nodes, which are easily attenuated by buildings, trees and weather conditions. 5G used bands within the UHF, SHF and EHF bands.

Speaking of line of sight, the curvature of the earth must be taken into account.

los

The receiving and transmitting antennas must be visible to each other. This is the most common form of RF propagation. Twenty five miles (sometimes 30 or 40) tends to be the max range of line of sight propagation (radio horizon). The higher the frequency of the wave, the less bending or diffraction occurs which means the wave will not propagate as far. Propagation distance is a strong function of antenna height. Increasing the height of an antenna by 10 feet is like doubling the output power of the antenna. Impedance matching should be employed at the antennas and feedlines as losses increase dramatically with frequency.

Despite small wavelengths, UHF signals can still propagate through buildings and foliage but NOT the surface of the earth. One huge advantage of using UHF propagation is reuse of frequencies. Because the waves only travel a short distance when compared to HF waves, the same frequency channels can be reused by repeaters to re-propagate the signal. VHF signals (which have lower frequency) can sometimes travel farther than what the radio horizon allows due to some (limited) reflection by the ionosphere.

Both VHF and UHF signals can travel long distances through the use of “tropospheric ducting”. This can only occur when the index of refraction of a part of the troposphere due to increased temperature is introduced. This causes these signals to be bent which allows them to propagate further than usual.

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