  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. 