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 to be effective, so antenna height is critical in determining range.
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This is the result of the movement of cool, humid air shortly after sunset when the ground air cools more quickly than the upper air layers . The same action may take place in the morning when the rising sun warms the upper layers.
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