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Coax Cobra wire antenna

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Old 03-10-2012, 09:13 PM
Davo
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Default Coax Cobra wire antenna


Does anybody still remember that design of a half wave end fed antenna for 11m that came out in the 70's or earlier? Made from a piece of RG58 coax cable one length no joins from the radio to the tip. Strip off the plastic shield and braid for 8'6" or so and make a choke of 3-5 turns (forget) on 6" diameter the same length down from where the braid stops.
If it was a vertical dipole you would fold the braid back.
Don't hear anything of this design any more but I used it in my youth with an AM 0.5w handheld and remember it working very well.
It still appeals to me as it is cheap, no connections, nothing to burn out and could be run inside a piece of tapered fibreglass to get it in the air. Rolls up real small and doesn't get tangled up like a dipole.
These days we could use ferrite baluns as the choke and RG8X for the cable. Could use cheap excess RG59 coax as well I guess. SWR at radio would only be 1.5:1 with RG59.
Nobody talks about this any more so I guess it had some drawbacks.
Any comments? great portable or emergency concept.
Biggest problem I see with this antenna is getting it in the air!
Thanks,
Dave


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Old 03-10-2012, 09:27 PM
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something like this ?

The Vertical Bazooka Antenna - Ham Radio Library

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Old 03-10-2012, 10:23 PM
Davo
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Default coax dipole

Very similar indeed but this one has the braid folded back which makes it a center fed dipole. The one I saw the braid was cut at the center point and just the radiator exposed. I would also have thought that folding the braid back was the way to go too...

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Old 03-10-2012, 10:26 PM
Davo
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This bazooka design saves wastage of coax in the coils. Neater in some ways.

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Old 03-11-2012, 07:44 AM
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Yes, it is still used, and the discussion comes up about fairly frequently - Vertical Coaxial Sleeved Dipole.
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Old 03-11-2012, 09:02 AM
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The one I remember was different. it did not have a sleeve. The braid was removed and not folded back.
rgds,
Dave

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Old 11-05-2013, 05:38 AM
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Is this the antenna you're talking about? I see a lot of antenna plans with the shield pulled back but this one looks a lot easier and I don't see what difference it makes if the shield is in or out of the outer jacket.

Thinking about making one of these. Right now I'm using a 1/4 wave ground plane in the attic, this one will be too tall and I'll have to hang it from a tree branch.

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Old 11-05-2013, 06:16 AM
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The antenna in the illustration is an 'Zepp' antenna. The antennas with the braid folded back down over the outer insulation of the cable is a center fed 1/2 wave antenna. They are not the same. The '1/2' wave is center fed with the coax running up inside the braid folded back on it's self. You would treat it like you would any other center fed 1/2 wave antenna. The 'catch' with them is that there is high voltage developed at the ends of a center fed 1/2 wave dipole, both ends. So, a choke at the point where the cable exits the braid is a very good idea. Keeps or reduces the affect of that high voltage on the feed line.
What about that vertical 'Zepp'? It has high voltages at the 'end' of the section of the feed line it's using as it's 'other half'. And the designer of that drawing has one there. I haven't used one but think I'll give it a shot to see what happens. I have a feeling that there will be some SWR/impedance matching problems...
- 'Doc

Last edited by W5LZ; 11-05-2013 at 03:03 PM.

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