OFF-CENTER-FED or (OCF) DIPOLES!

Off-Center-Fed Dipole usual practice is to feed a ½ wave dipole in the center where the feed point impedance is low and makes a suitable match to coaxial cables.

The dipole will accept energy from a feed point anywhere along its length, however, assuming that the source is matched to the higher impedance that is presented away from the center point. If the feed point is moved away from the center of the dipole, the impedance rises because current is dropping while voltage is rising.

The off-center-fed dipole takes advantage of placing the feed point in a location along the dipole at which the impedance is similar on more than one band, generally in the neighborhood of 150-300 ohms. A suitable impedance matching device such as an impedance transformer is then used to reduce the feed point impedance to something closer to 50 ohm. The feed point impedance of the antenna varies with height above ground and so will SWR. Here is a diagram of a OCF dipole for 80 Meters, 40 Meters and 20 Meters on one antenna. The feed point is on the order of 150 to 200 ohms. A 4:1 impedance transformer at the feed point should offer a reasonably good match to 50 or 75 ohm line.