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How to Use a 40 Meter Dipole on 15 Meters

The length of a wire antenna is directly proportional to its wavelength and inversely proportional to the frequency, upon which it is to be used. Antennas must be resonant with their transmitted frequency to maximize their efficiency but, they may also resonate on harmonics of their fundamental frequency. Since 21 megaHertz (MHz) or 15 meters is the third harmonic of 7 MHz (40 meters), a 40 meter dipole will resonate on 15 meters. Use your 40 meter dipole on the 15 meter amateur radio band. You may need to tweak and tune it for optimal performance and lowest standing wave ratio (SWR).

Things You'll Need

40 meter band dipole antenna
Standing wave ratio (SWR) meter
Transmission line (balanced or coaxial)
Trans-match antenna tuner
Amateur radio transmitter or transceiver
Wire cutters (optional)

Instructions

- 1
  Connect the 40 meter dipole to the SWR meter's output and connect your transmitter to the input of of the SWR meter.
- 2
  Transmit using low power on 40 meters while switching between forward and reverse settings on the meter to find the frequency range where the lowest SWR is obtained on 40 meters.
- 3
  Multiply the center frequency (lowest SWR) times three to predict the harmonic frequency where you should find a resonance match on 15 meters.
- 4
  Switch your transmitter to the 15 meter band and look for minimum SWR by testing across a range of frequencies in the 15 meter band. The low-SWR range should be wider than it was on 40 meters because 21 MHz is a higher frequency and has a shorter wavelength.
- 5
  If you find that your lowest SWR on 15 meters is on the center frequency of the range you wish to transmit on, do nothing more and enjoy your two-band antenna as is.
- 6
  If your desired frequency is lower than the antenna's resonant (low-SWR) range and your desired 40-meter center frequency is also too high, you may wish to snip 1/2 inch at a time off each end of your dipole and recheck for SWR resonance each time as you shorten the antenna in small increments.
- 7
  If your desired operating frequency is higher than the antenna's 15 meter SWR or if you are satisfied with the 40 meter resonance of your antenna, connect your antenna to a trans-match antenna tuner and tune for a 1:1 matching SWR.

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