How an OCF dipole works
A centre-fed half-wave dipole works on its fundamental and odd harmonics, but on even harmonics the centre is a high-impedance point. Moving the feed point to about a third of the way along puts it at a place where the impedance is moderate, roughly 150 to 300 Ω, on the fundamental and on the second, fourth and eighth harmonics. So an 80m OCF typically works on 80, 40, 20 and 10m, and often 17m and 6m, through a 4:1 or 6:1 balun. Bands at multiples of three times the fundamental, such as 30m and 15m on an 80m OCF, land on a high-impedance point at a third of the way along, so they need a tuner.
This design goes back to Loren Windom's 1929 antenna, which used a single wire feeder. Modern versions use coax and a balun, and are sometimes sold as the FD4 or Carolina Windom.
Building tips
- Use a good balun and a choke. An OCF is unbalanced by nature, so common-mode current on the coax is common. Add a choke a few metres down the coax as well as the balun at the feed point. The choke calculator helps.
- Cut long and trim. Start a little long and shorten both legs in proportion until the lowest band is resonant.
- Height matters. The feed impedance and the harmonic band matches shift with height. Check with an analyser.
For a simple single-band antenna, use the dipole calculator, and for an end-fed alternative, the EFHW calculator.
Questions
How long is an 80m OCF dipole?
About 40 m (131 ft) in total, with legs of about 27 m and 13 m for a 33% feed point.
Which balun does an OCF need?
Usually a 4:1, although some designs use 6:1 for a higher feed impedance. Use a current balun rated for your power, plus a choke on the coax.
Which bands does an 80m OCF cover?
Typically 80, 40, 20 and 10m without a tuner, and often 17m and 6m. 30m and 15m usually need a tuner.
Why does my OCF have RF in the shack?
Common-mode current on the coax. Add a choke at the balun and another where the coax enters the house.