Topband: Shunt-feeding

[jbattin]

----- Original Message -----
From: <k6se@juno.com>
To: <topband@contesting.com>
Sent: Tuesday, October 09, 2001 7:56 PM
Subject: Re: Topband: Shunt-feeding


> Tom, W8JI wrote:
>
> "Insulated elements can have an arcing problem when used on a shunt-fed
> tower. This is especially true when moisture is present, and the
> insulation poor (like fiberglass or PVC).  Voltage between the insulated
> element and the boom can be very high, and is as much a function of the
> height and other antennas on the tower as
> the power involved..so without modeling there is no accurate way to
> predict voltage."
> ==========
> What Tom says is very true.  PA3DZN had arcing from his insulated
> Force-12 CX-34 tribander 20-meter reflector to the boom when he
> transmitted on 160m to his shunt-fed tower.  I suggested that he ground
> the centers of all of the parasitic elements of the tribander to the
> boom, which he did and it solved his arcing problem (as well as gave him
> more top loading on 160m).  He didn't notice any difference in the
> performance of the tribander on 20, 15, and 10m.
>
> The highest RF voltage always occurs at the farthest end of an antenna
> from the feedpoint.  In the case of a shunt-fed tower with an HF Yagi
> with insulated elements used for top loading, this highest RF voltage
> point would be at the ends of the Yagi's boom (where the CX-34's 20-meter
> reflector is).  If the center of the elements are grounded, beam
> performance should not be much different and the highest RF voltage on
> 160m would now be at the ends of the Yagi's elements where no arcing will
> occur.
>
> 73, de Earl, K6SE
> _

I burned the feed line off my KLM 40 meter beam when it flashed over to the
boom.   I grounded all the elements, and solved the problem._
John K9DX_____________________________________________
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