The propagation of VHF and UHF radio waves is a fascinating subject. Several things can affect these signals.
Line of sight: VHF and UHF signals travel in straight lines, so they require a clear line of sight between the transmitting and receiving antennas. Buildings, hills, and other obstacles can block, reflect or weaken the signals.
Atmosphere: The atmosphere affects the propagation in various ways. Rain and snow can absorb or scatter radio waves, while ionization in the upper atmosphere can reflect them back to Earth.
Frequency: Higher frequencies, such as those used for UHF communication, have shorter wavelengths and are more easily absorbed by obstacles. Lower frequency VHF communication, can travel longer distances and can penetrate some obstacles.
The height and orientation of the aerial: Higher antennas can transmit signals further, while directional antennas can focus signals in specific directions. This one is pretty obvious.
VHF signals are often used for long range communication and in situations where obstacles are present. UHF signals are used for short range communication and in situations where line-of-sight is available.
VHF and UHF radio waves can propagate through the atmosphere by a phenomenon known as ducting. Ducting occurs when a layer of warm air is sandwiched between two layers of cooler air. This can create a duct or tunnel-like channel in the atmosphere that allows radio waves to travel much further than they would normally.
Ducting is most commonly observed over water, where the air above the water is cooler than the air over land. When the conditions are right, VHF and UHF radio waves can travel along this duct and be received over much longer distances than would normally be possible. Ducting can also occur over land, but it is less common because the temperature differences between the ground and the upper atmosphere are generally less pronounced.
The strength of the ducting effect can vary greatly depending on the specific atmospheric conditions. For example, if the temperature difference between the warm air layer and the cooler air layers is too small, or if the duct is too shallow, radio waves may not propagate very far. On the other hand, if the temperature difference is very large, or if the duct is very deep, radio waves can travel hundreds or even thousands of miles. Because ducting is highly dependent on specific atmospheric conditions, it is difficult to predict when and where it will occur.
Tropospheric propagation, also known as tropo, is a mode of propagation that occurs within the lowest layer of the Earth’s atmosphere, called the troposphere. This mode of propagation is most commonly associated with VHF and UHF radio waves, although it can also affect lower frequency radio waves.
Tropo is caused by changes in the density and temperature of the troposphere, which can bend and refract radio waves as they travel through it. This bending effect can cause radio waves to follow the curvature of the Earth’s surface, allowing them to travel much further than they would normally.
The strength of the tropo effect can vary greatly depending on a number of factors, including the temperature and humidity of the air, the height of the radio antennas, and the frequency of the radio waves. In general, higher frequencies are more susceptible to tropo effects than lower frequencies.