TopBand: Baluns

[km1h @ juno.com]

On Mon, 25 Aug 1997 01:39:09 -0000 owner-topband@contesting.com writes:
>Dear fellow Top-Banders,
>
>testing my new antenna - an 83ft elevated vertical with folded
>elevated radials - I got a problem which I couldn't solve sofar.
>
>The vertical is driven via a current type balun - 100 no.73 ferrite 
>beds 
>slipped over a piece of teflon coax. The antenna itself is not
>connected to ground, instead the shield of the coax cable is
>grounded directly before going into the current balun.

I use an almost identical setup with a pair of verticals. I suspect that
the problem is in your selection of ferrites. While the 73 mix is good
for the purpose, the current handling capacity of those small beads is
very inadequate. 

Also, if I read your message correctly, the coax shield is connected
wrong ??
The shield should go thru the balun and connect to the radials plus DC
ground right at the feedpoint.  I would also suggest connecting a 1 mh to
2.5 mh RFC from the antenna feedpoint to a true DC ground. It will
protect somewhat from lightning and also help drain off percipitation
static charges. 
Shield current isolation, which has many other names, is important in
both of our setups. I use 6 large Radiokit A637 beads over each feedline
at the feedpoint and they do not get warm. With ferrites, at these
frequencies, performance is measured by the pound, kilogram, etc. . 
Addition of the beads also measurably improved the F/B ratio. The A637
beads are approximately 35mm long,  25mm wide and a 13mm center hole so
RG-213 and similar cables fit  very well. 

Please private E-Mail me for more details on the beads.

73....Carl   KM1H 



>If I penetrate the antenna with 500W for about 1 minute the balun
>becomes very hot (you can't touch it anymore) and the SWR is 
>increasing.
>
>Now my question - what can be done to reduce the power, the balun has 
>to
>absorbe? Would it be helpful, to install a voltage type balun
>(e.g. W6TC design) instead or a combination of both?
>
>Any suggestion is welcome. 
>Please respond direct to dl8wpx@indo.net.id.
>
>BTW - heard Mike, VK6HD working West Coast yesterday and 
>was at least able to copy some pieces of Tree's, N6TR signal.
>It hopefully won't take long anymore that the paths running
>further north will be open again ....
>
>Best 73,
>
>de Joerg, YB1AQS
>
>
>--
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>

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