Of Center Fed Dipole - OCD
2021-01-27 for much of my ham radio life I've heard about OCD, Off Center Fed Dipole. But I never really grasped what they were or why one might want one. But a couple months ago (before the bottom dropped out of the temperature and the snow started flying here in Wisconsin!) I got interested in them. After some research it seemed that their main claim to fame is that, like the End Fed half Wave antenna, they offer multiband performance. Just like the EFHW, if one is cut for 40 meters it can also be used on the harmonic bands, 20, 15 and ten meters without a tuner. So I decided to make one.
You need two things, a half wave wire cut for the lowest frequency band you intend to use which is then cut at the 1/3 point; and a 4:1 balun. So a month ago I cut a 68 foot wire for 40 meters and then cut it at 23 feet from one end. I had on hand an LDG 4:1 balun so I hurriedly strung it up as an inverted vee and to my pleasure noted that the SWR was very good on 40, 20 and 15 meters needing no tuner. I made a few contacts with it noting that it acted very much like my EFHW. My curiosity was satisfied so I disassembled it and went on to other things.
But I'm into light weight antennas for portable use so I ordered the 4:1 balun kit from QRPGuys and built it. Then today, even though we just had 5 inches of new snow and the temperature was in the 20s, I decided to put it up again using the QRPGuys balun. My driveway was just long enough for me to get it up without tromping in too much snow. I connected about 65' of RG-174 coax, which I ran through a window to my Xiegu g90 inside. SWR was 1:1 on 40 and 20, 2:1 on 15 meters. I didn't look at 10 meters.
It worked! I had two contacts with SKCC stations on 40 meters, one 80 miles away and the other 650 miles distant. Both gave me good reports. Both were strong in here. My venture was successful.
Observations? An OCD antenna is a good, relatively simple, multiband antenna that does not require a tuner. I did some quick comparisons with my permanently installed G5RV and MFJ EFHW. Performance seemed about the same on all three. Not needing a tuner is an advantage. And perhaps the 4:1 balun as opposed to a 49:1 transformer for an EFHW is an advantage. The only real disadvantage compared with an EFHW, is that in an inverted vee setup, the coax must run to the top of your support pole rather than just to one end near the ground as with the EFHW. Potentially that requires more coax and perhaps a stronger support pole. Otherwise it is a toss up.