bgkoe: The 300 ohm side of the transformer attaches to the ocfd dipole elements and the 75 ohm side of the transformer attaches to the 75 ohm coax. If using a balun with twin-lead on one side and a threaded F-connector on the other, the twin-lead is the 300 ohm side. If you have a "cube" type of balun, the screw terminals are the 300 ohm side and the push-in pin (or threaded f-connector) is the 75 ohm side.
I think part of the confusion in our descriptions is that "ocfd" is not very meaningful anymore.
1) A half-wave dipole can be fed at the center directly via 75 ohm coax. You can also move the feedpoint away from the center of this half-wave dipole and use a commonly available 4:1 impedance-tranformer + balun to make it convenient to attach to coax. Ordinarily there is no need to do this as a center connection directly to a half-wave is the most convenient.
2) The wacky "OCFD" dipole is a multiband setup that *requires* the use of an offset feed, and a very special length of each of the legs to get the compromise working on a very wide bandwidth. As you move up and down in frequency, the actual impedance at the feedpoint varies quite a bit, so the antenna is often not really at 300 ohms but above it and below it all the time. The 4:1 transformer just helps bring that variation at the antenna terminals down to an acceptable level for the impedance of the coax, but it never is truly a 300-75 ohm transformation. But- it is close enough for receive-only situations.
Note that due to the extreme non-symetry of the two element lengths in the "ocfd" multiband antenna, about the only thing the tv-type of 300-75 ohm baluns do is provide impedance transformation of 4:1, however any attempt to do any balanced-to-unbalanced conversion is totally swamped. Thus there is hardly any balance at all, and the outer skin of the braid is part of the antenna, unless you choke it for real with perhaps a large handful of broadband ferrites. Or maybe choke it for just one band with a few turns of coax around a 2.5 - 3" plastic bottle, or a pawsey-stub.
3) I recently built a full-wave vertical dipole designed for single-band use, which is also offset. But it is not really an "ocfd" dipole. In this case, the 75 ohm coax was attached directly between the 1/4 wave point and the 3/4 wave point at the "current node". Normally you'd think that an impedance transformer is needed there, but due to the exact lengths of the legs, and since it is designed for single-band use, it is not needed. A choke balun was added since it too is extremely non-symetrical.
Man, I can see where "ocfd" is going to get confusing since they don't exactly share the exact same impedance rules when you slide the feedpoint around on a half-wave
prcguy: Hey, that's a great idea concentrating on 153.3 and 283.3 and seeing how it multibands together. It will be interesting to see what the look angles are on those bands, since the ocfd does leave a little to be desired the higher you go in frequency - so does the look angle. Perhaps your version will do better - I'll check it out. Nice tip!
