Remember that VHF radio is line of sight. Because of that a higher antenna is always better than a lower one for range. Antenna gain helps, but primarily in terms of making the transmission clearer and letting you pick up weaker signals. All the gain in the world won't help much if your antenna is low though. Range is proportional to the square root of the antenna height. So doubling your antenna height will increase range by about 40% The range for communication is determined by the heights of both antennas involved. Coast Guard antennas tend to be relatively high, which is why you can talk to them at considerable ranges.
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