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When building a single band coaxial cable fed HF dipole antenna, you may easily use the formula
L = 468/F
to determine the overall length of the wire (L in feet, F in MHz). The formula takes into
account the end-effect of the antenna wire to give you the overall length of the wire portion.
As an example, using the formula L = 468/F, the overall length of a half wave dipole for 3.800 MHz
would be L = 468/3.800 or 123.2 feet. Cut the overall length of the wire to 125 feet or more to include
the extra length for tuning. Then cut that in half to provide two dipole legs – each 62.5 feet long.
Use this theoretical formula length as a starting point to cut the wire, leaving about one foot extra on
each dipole leg for wire fastening and fine tuning adjustment to allow for anomalies and environmental
effects in your particular installation. Longer length dipole legs may be required for Inverted-V
antennas.
Both of the dipole leg ends should be measured and cut longer than necessary so you have enough wire
to wrap around the Balun terminals and End Insulators, plus a little extra for final tuning.
It’s always
easier to cut excess wire rather than having to solder on extra wire when tuning.
In this example, we are making a 15 meter flat top resonant dipole. The lowest frequency on the 15
meter band is 21.000 MHz.
Example: 21.000 MHz (bottom edge of 15 meters)
Overall Length = 22.29 feet
Leg 1 = 11.145 feet
Leg 2 = 11.145 feet
Overall length would be 22.29 - BUT we add some extra for wire fastening and tuning
Therefore; cut the legs to 13 feet each
Longer length dipole legs (up to 2 feet longer) may be required for Inverted-V antennas.
