Homebrew Cobra doublet antenna

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Dale Hunt:
Quote from: WX7G

WB6BYU, changing the wire spacing from 1" to 1' I get results just about like yours. And yes, 1" violates the NEC segment length rule.

That tells us a lot, actually.  It means that the actual resonances are highly dependent on the wire spacing
(and thus the impedance of the stubs) and probably the insulation between them.  That tells me there are
fairly high voltage differentials between the wires (on some frequencies), so dielectric losses may be high.
(That matches the experience of a friend who used one a longer one on 160m, and then took it down because
it was so poor.)  The fact that copper losses can be so high even with #14 wire suggests that variants using
ribbon cable or TV twinlead are going to be even more lossy (again, at least on some bands.)

Perhaps most of all, it tells me that modeling, at least with NEC2, probably isn't going to be very accurate
for closely spaced wires in plastic insulation, though it may help to identify some trends in behavior.
Perhaps a NEC4 model will come closer, but I'm not impressed by the performance of the model with 1' spacing,
and I have to think that closer spacing, smaller wires and plastic insulation them will make the losses even higher.

Linear loading, especially at close distances is lossy, and lossier than proper coils.

A good doublet fed with balanced line will be far more efficient than one of these cobra antennas.  But to each his taste.

Robert Tadlock:
A followup on my experiment .  I used the home brew cobra flat top at a length of 100 feet to start with  and later cut it to 73 feet like the commercially built ones are and couldnt see any difference between the two lengths other than the tuner control positions were different on a manual tuner and the AH-4 coupler had no problems either . Feedline was about 60 feet and running 100 watts . Its not too bad an antenna if you are pressed for space and cant get up a full length dipole or doublet .

Bob  N7BDY 


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