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VHF Propagation Characteristics

Very High Frequency (VHF) is the part of the radio-frequency spectrum ranging from 30MHz to 300 MHz. VHF transmissions are commonly used for analog television broadcasts, FM radio broadcasts, amateur radio, marine navigation and air navigation in countries around the world. VHF transmissions typically propagate over relatively short distances allowing the transmissions to be targeted to specific broadcast areas without impacting unintended areas. VHF propagation characteristics explain how VHF signals can be confined to a limited area, but also offer an explanation as to how VHF signals can travel much farther than intended.

Lack of Ionosphere Reflection
- VHF transmissions are very suitable for short distance terrestrial communications because the ionosphere does not typically reflect VHF signals, resulting in transmissions being confined to the local area. VHF signals generally travel only slightly farther than the line of site from the transmitter.
Tropospheric Ducting
- Tropospheric ducting of VHF signals can occur parallel to and in front of cold weather fronts advancing through a transmission area. Ducts can form at approximately 150 miles in front of a cold weather front and VHF transmissions will propagate through the duct for ranges much farther than normal. This results in the VHF transmission interfering with VHF signals from other distant transmitters that otherwise would not be affected.
Sporadic E
- The Sporadic E propagation characteristic of VHF transmissions is very rare and involves the E-layer of the ionosphere. As of 2010 this type of propagation is not fully understood, but is believed to result from the formation of dense ionized patches in the ionosphere's E layer, resulting in VHF signals being reflected back in the same manner that high frequency (HF) signals are propagated. This condition can last from minutes to hours, and can cause VHF signals to be reflected hundreds of miles beyond the intended transmission area, unintentionally interfering with distant VHF broadcasts.
Polar Mesosphere Summer Echoes
- Polar mesosphere summer echoes are isolated to areas that experience extremely cold temperatures in summer, such as Antarctica. Polar mesosphere summer echoes result in signal power increases over most of the VHF range. Research indicates that polar mesosphere summer echoes are the result of partial reflection of the VHF signal by several layers of the ionosphere.

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