OCF Dipole Vs G5RV Vs Folded Dipole

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DAVE CUTHBERT:
WW5AA: the more I look at them and use them the more I dislike the center fed resonant all band dipole - your 130' doublet. The input impedance is exceedingly high on even harmonics, the line VSWR is tremendous, and the impedance presented to the tuner can range from much too low to much too high; too low to too high for a balun to contend with.

They work,  but the efficiency when the tuner and ladder line losses are considered are lower than many imagine.

For some time now I have favored the OCF antenna. There is the tradeoff of line common-mode current but to condemn the antenna for this without an objective investigation - to reject it out of hand - has the look of superstition.

DAVE CUTHBERT:
Is the common center fed resonant doublet fed with ladder line free of common-mode feedline current? No.

If fed thru an ideal balun at the tuner it can be. But the balun presents different impedances to GND for each ladder line wire. The system is now imbalanced.

Is this old standby of an antenna just another myth?

Tom Rauch:
Quote from: WX7G on November 21, 2009, 08:06:05 AM

Is the common center fed resonant doublet fed with ladder line free of common-mode feedline current? No.

If fed thru an ideal balun at the tuner it can be. But the balun presents different impedances to GND for each ladder line wire. The system is now imbalanced.

Is this old standby of an antenna just another myth?


I think we have been kidding ourselves for years to the extent that we might think the system is nearly lossless or the resonant doublet antenna the ideal length for lowest loss, but still the loss is pretty low. A trap dipole, abandoned partly because of "lossy traps", probably had lower losses.

Still it works fine on the air if the proper feedline length is picked and the tuner has enough tuning range. With a multiband antenna all most really want is an antenna that works OK.

I tried an antenna with real open wire line here, but it was small conductors. It was that old real open wire line stuff with bare conductors and molded white plastic insulators. I'd guess about #18 conductors. It fed a dipole on 80 meters. It had about 1 dB of loss on 80 and 2 dB of loss on 40 just in the feeder.

While that is inconsequential in many cases, it is clearly not lossless.  The freedom to use it on multiple bands outweighed the slight loss, although now I'm considering a trap dipole to replace it.

DAVE CUTHBERT:
Tom W8JI, I was looking for a trap dipole the other day and could not find one. Have they all gone away?

I use the traditional center fed dipole with cheap 300 ohm TV twin lead for camping and sometimes at home. Not bad. When I calculate the line loss it is disturbing but I have to admit that had I not done this I'd never know the difference. I suspect the line loss helps tuners deal with this. I've never had a modern tuner that would not tune such an antenna.

The traps from a 4BTV might make a nice 40-10 meter dipole. Along the lines of the old Hygain trap dipole. Did the marketing folks kill off the trap dipole by harolding "we use no lossy traps [but we use lossy hunks of coax and other things we don't tell you about]?" Not to name names but GAP comes to mind here.

I became interested in the old Windom antenna a year ago and built the original single wire feedline version. Wrote an article on it for AntenneX. But no antenna survives for long around here with all the experimenting going on and I'm back to verticals. But back to what I started to say, OCF seems like the way to tame line VSWR.

Tom Rauch:
Quote from: WX7G on November 22, 2009, 08:17:08 AM

Tom W8JI, I was looking for a trap dipole the other day and could not find one. Have they all gone away?

It really is the weirdest thing. One trap antenna uses traps with very high L and low C, not necessarily a good thing, but refuses to call the self-resonant inductors "traps" because they don't have lumped capacitors.
http://www.alphadeltacom.com/dxdd_ii.htm

Even the Butternut vertical, that uses traps, denies the traps are traps. Check out the banner ads for one of the better trap verticals that has ever been sold, the Butternut!

Even when we have very good traps antennas people seem to do anything to avoid calling the trap a trap. Probably because of the undeserved bad reputation traps have.

But you are right. They are very few trap antennas now, and those still using traps deny the traps are traps.

Tom


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