Tag Archives: End Fed Half Wave Antenna

Is Trimming an Antenna Proportional to it Frequency?

table with ham radio equipment
Ham Radio equipment on a table.

Based on what I’ve read so far on antenna theory, the shorter the wire the bigger the band. Although, is cutting the wire proportionately linear? To test this idea, I’m going to adjust a 40 meter End Fed Half Wave Antenna (EFHW). The wire size to resonates 40 meters (7.2 MHz) is 65 feet; the height of a 5 story building! To calculate the wire size, I used the following formula.

L = 468/ƒ

Where L is the length of the wire in feet, ƒ is the frequency you want the antenna to resonate the best, and 468 is a constant with velocity and conversion factors. This is the same formula to calculate the length of a dipole. By placing 7.2 for the frequency, I calculate the size of the antenna wire to be 65 feet long.

For the wire, I used the sotabeams.com antenna wire (DEF 61-12 Pt 6). I approximately measured 65 feet and added approximately 1 foot more. Adding one more foot, gives me assurances that the antenna will resonate at a lower band so I may cut the wire shorter and adjust as needed.

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