OCF Trap Dipole - towertalk Forum at eHam.net

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eHam.net Forum : towertalk : OCF Trap Dipole

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OCF Trap Dipole	Reply
by N4PSE on April 29, 2006	Mail this to a friend!
I have a 170 ft 80/160 mtr trapped dipole that I'd like to try feeding off center like a "windom" to make it fit my property. I could feed it with either ladder line or coax. Any thoughts? Think it will play like a typical OCF dipole with multiband resonance points? Should I feed it at the typical windom off center point according the the standard formula?

RE: OCF Trap Dipole	Reply
by K4SAV on April 29, 2006	Mail this to a friend!
"Think it will play like a typical OCF dipole with multiband resonance points?..." No. If you feed it without a current mode balun, the feedline will be part of the antenna, and the resonant points will be totally random. You will also have RFI in the shack. I don't know of anyone that sells a high impedance balanced-to-balanced current mode un-un which you will need for ladder line. Off-center-fed dipoles and end fed long wires are responsible for more RFI problems than any other thing. Jerry, K4SAV

RE: OCF Trap Dipole	Reply
by K4SAV on April 30, 2006	Mail this to a friend!
small typo... make that a high impedance balanced-to-balanced current mode bal-bal

RE: OCF Trap Dipole	Reply
by WB6BYU on May 2, 2006	Mail this to a friend!
I presume the only reason for feeding it off-center is because the shack is closer to one end, since the antenna length won't change either way. The antenna resonances will be at the same frequencies regardless of how you feed it. That will be on 160 and 80 meters, and various other points throughout the HF spectrum. But there is no guarantee that the resonant points will be in the ham bands (as they would be with a windom) because the effect of the traps will vary considerably with frequency. With a Windom the feedpoint is at about the same point in the standing wave on each band, so the impedance is relatively constant. This will not be the case for the trapped dipole, however, so there will be a wider variation in SWR for a given feedpoint. It won't even work well for just 80 and 160m, because the current distributions on those bands are not the same as they would be on a non-trapped, full-length wire. So you certainly could try hooking up a 4 : 1 balun at some point off-center, and with a good tuner you probably could get a low SWR on most HF bands. But it may not be very efficient.

RE: OCF Trap Dipole	Reply
by WA1RNE on May 11, 2006	Mail this to a friend!
Don't mess with the feedpoint. Get the feedpoint up as high as you can and positioned wherever that highest point puts it. Then bend the ends down or off to the side. This will be a far better alternative than OCF. 73, Chris

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