The random wire antenna I put up at my new QTH has been less than ideal, good, but with some issues. I guess this was to be expected given the nature of the antenna and the fact that I didn't install a counterpoise or radial system. What further compromised the antenna is the fact that my shack is on the second floor. On certain bands, 15 meters especially, RF in the shack was causing issues. Specifically anytime I would transmit at any power level above about 5W my computer would lose connection with USB devices such as my external sound card and my CAT connection to my radio. There was another instance where my tuner would never quit its tuning cycle, it would continue to tune until I turned the radio off.
I'll post a summary sometime of what all bands I used on, but mostly it worked well enough. In search of some options to solve the RF in the shack problem I came across some info about off center fed (OCF) dipoles. I had heard of them before, and their predecessor the Windom, but I'd never given them much attention. To back up a little my antenna goals are two fold: 1)to have a multiband antenna and 2)that it work well on 80 meters for NVIS use. The plan was to eventually add an 80 meter dipole right above the random wire feed point to ensure a good solid 80 meter NVIS signal. Had I known that the OCF dipole was a multiband antenna I would have most likely put one up from the start.
The important part about an OCF dipole is where to feed it. Upon researching the antenna I came across an article by L. B. Cebik. The article describes two OCF dipoles, one at a fundamental frequency of 3.5 MHz and one at 1.75 MHz. Cebik goes to to describe how each antenna works and what the SWR curves look like when fed at different points. As it turned out my 220' random wire was the perfect length to the foot for one leg of a 275' 1.75 MHz OCF dipole fed at the 20% point. All I needed to do was add 55' wire off the other side for the other leg. The 20% point corresponds to a 200 ohm feed point impedance which requires a 4:1 balun to match with 50 ohm coax feed line, something I already had been using with the random wire. Converting my random wire to an OCF dipole was a simple as disconnecting the side of the balun that I had grounded and connecting a 55' wire to get to 275' overall length. It took a few hours to get it all done, including cutting a few limbs off of a tree for the far end of the 55' leg, and then i was on the air.
I modeled the antenna in the demo version of EZNEC and it compares fairly well to what my antenna analyzer shows. Initial results have shown a signal report that bests my random wire on 80 meters and no issues on any other band. So far, so good. As long as I don't run into any issues this will probably be my primary antenna for a while. I'll post more about how it is doing once I've had some more time on the air with it.
