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Telescoping brass tubing allows rapid lengthening or shortening of the total electrical length.
Works on Quads, Yagis, Delta loops etc.
This will tune as a 1/2 wave line from 27.8 to 29.0 Mhz.
Using this method one can optimize an antenna for max front/back or forward gain while safely standing at ground level.
Most amateurs who build antennas use formulas or computer programs to determine element size but never take the time and effort to adjust their antennas for maximum performance.