The formula (feet)=234/MHz for a quarter wavelength is just a rough starting point.
The resonant length is affected by many variables, like conductor diameter and
the placement of nearby conductors within 2 wavelengths, including other antennas and the earth.
It usually comes down to:
measure in place, then take down and adjust length. Repeat.
See the effect of height above earth (roof ground plane) for vertical and horizontal antennas
in figure 1 of chapter 3 of the ARRL antenna book.
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N0YWB
N0YWB
