Multiband Off-Center-Fed Dipoles for 160 & 80 Meters

For many years, our club used G5RV antennas1 for Field Day, launching four of them around our usual site in Concord, Massachusetts. They usually worked on all bands with external tuners or rigs with broad range tuners. A few years back when some popular newer rigs came with built-in tuners that were limited to handling SWRs of less than 3:1, we had issues with getting them to tune the G5RVs. Another solution was needed. One of our members had the answer: Bob, KC1DSQ, determined that an off-center-fed (OCF) dipole with a unique capacitive load would give us low SWR, 3:1 or lower, on all HF bands. He built one for himself and joined with the other Bob, W1IS, to build two more for FD 2019. Later, we found that Serge Stroobandt, ON4AA, had come up with a similar solution 13 years earlier.2

Theory

An OCF dipole has some unique characteristics3 because the feedpoint doesn’t go through the wide swing of drive impedances that a center-fed dipole has if you try to operate it on higher bands. For example, the impedance of a center-fed dipole on its fundamental frequency is about 75 ohms, a close match for our 50-ohm rigs and feed lines. However, on the second harmonic (40 meters for an 80-meter dipole), the impedance is several thousand ohms. Feeding a dipole off the center tames the wild impedance swings as we go up the bands with the caveat that we use only the even harmonics … 40, 20, 15, and 10 meters for an antenna cut for 80 meters. They can be made to work on other frequencies, but a wide range tuner will probably be needed.

Off-center-fed dipoles provide multiband operation by exploiting the resonances on harmonics of the fundamental frequency. These resonances are enhanced by moving the feedpoint away from the center of the