- Have on hand a ruler with centimeter markings (I use a retractable tailor tape measure)
- Open your calculator and do 300 / your_frequency (in mhz) = wavelength
- wavelength / 2 = how many meters long a half wave dipole would be. If this is too long, divide by 2 again to get the length of a quaterwave dipole in meters.
- Extend the length of your antenna (I use ANT500s from my SDRs) to one of those lengths using your ruler. You should prefer half wave, but quarter wave is fine too.
For example, I'm listening on the national calling frequency for APRS (144.390) i'd do the following:
- 300 / 144 = 2.08
- 2.08 / 2 = 1.04 (half wave dipole length, way too long for my ANT500)
- 1.04 / 2 = 0.52 (quarter wave dipole, 52 centimeters can be done easily)
- Measure out 52 centimeters on the tape measure, use it as a guide to extend the antenna to the proper height.