Radio propagation at VHF is mainly influenced by physical objects, such as terrain and ground cover (clutter), and tropospheric factors, principally refraction. The primary mode of propagation for VHF radio waves is through direct waves. These travel in a straight line, becoming weaker as distance increases. Transmitting and receiving antennas must be able to 'see' each other for communications (Line of sight) to be effective, so antenna height is critical in determining range.
The straight line joining the sender and the receiver must be relatively free of obstacles.
Low level flight reduces greatly the range of reception (because of mountains, hills, etc).
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