Calculations for Estimating Resonant Length for a Single-Band,
Half-Wavelength Dipole Antenna


Option 1:
Play it Safe” formula
     (allows cutting down length for more precise tuning):
     (use this when it is impractical to add length to antenna elements during tuning)

     492 / freq (in MHz) = total length in feet of both dipole elements (end-to-end)
     246 / freq (MHz) = length of each half of dipole (quarter wavelength)

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Option 2:
Real World” formula
     (use when you can add or subtract length during tuning):
     (this is an estimate and frequently needs further tuning based on environment)

     468 / freq (MHz) = total length in feet of both dipole elements (end-to-end)
     234 / freq (MHz) = length of each half of dipole (quarter wavelength)

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Example - 15 Meter Band:
Applying “Real World” formula to estimate resonant length of a 15 meter dipole for voice transmissions in the General Class section of the band:

1.  Choose a target frequency roughly in the middle of the desired band segment:
      General Class 15 meter voice segment: 21.275 MHz – 21.450 MHz
      Target frequency  = 21.350 MHz

2.  Apply “Real World” formula for length of each half of dipole:
     234 / 21.350 = 11 feet  (132”) for each half of dipole

3.  For more precise tuning lengthen or shorten each side for minimum SWR
     (ideal SWR = 1:1. Anything under 2:1 is adequate, could damage radio if over 3:1)

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Example - 20 Meter Band:
Applying “Real World” formula to estimate resonant length of a 20 meter dipole for voice transmissions in the General Class section of the band:

1.  Choose a target frequency roughly in the middle of the desired band segment:
     General Class 20 meter voice segment: 14.225 MHz – 14.350 MHz
     Target frequency  = 14.280 MHz

2.  Apply “Real World” formula for length of each half of dipole:
     234 / 14.280 = 16.4 feet  (16’ 5” or 197”) for each half of dipole

3.  For more precise tuning lengthen or shorten each side for minimum SWR
     (ideal SWR: 1:1, anything under 2:1 is adequate, could damage radio if over 3:1)

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Note #1
: If the necessary physical length is impractical, add some inductance “loading” (turns of a coil) to compensate electrically for missing physical length. The less loading, the more efficient the antenna is.

Note#2: Impedance of antenna should be around 50 ohms. Some increases can be achieved by slightly off-center feeding of the two dipole halves (different lengths). Some reduction can be achieved by reducing height of antenna off ground. Major adjustments require an impedance matching device.




The information on this page is provided
courtesy of Pete Harris, KE6ZIW.

Images on this page are courtesy of Wikimedia.