[TowerTalk] Fwd: Re: Shunt Fed Boom, dangers of RF in shack?

[Joe Subich, W4TV]

 > Mike, do you have line chokes on the coax feed lines? It appears to me
 > that when you use the boom as ana antenna, the coax feed line for the
 > yagi will radiate and have RF on its shield outside.

In addition to chokes, I would make sure the shield of any feedline is
connected to the MAST (for boom fed yagis) or the ground system (for
shunt fed towers) and would disconnect any feedline that involved a
DC grounded (e.g. beta, hairpin or T match) element when the boom/shunt
feed was in use.  Without special care, the higher frequency feedline
will be tapping the active (fed) element at a high voltage point.

73,

    ... Joe, W4TV


On 9/10/2010 3:15 PM, Dan H wrote:
>
>
> -------- Original Message --------
> Subject: 	Re: [TowerTalk] Shunt Fed Boom, dangers of RF in shack?
> Date: 	Fri, 10 Sep 2010 07:17:39 -0700
> From: 	Dan H<n5ar at air-pipe.com>
> To: 	Mike&  Coreen Smith<ve9aa at nbnet.nb.ca>
>
>
>
>    On 9/9/2010 3:51 PM, Mike&   Coreen Smith wrote:
>>   Hi Guys n' Gals
>>
>>   I've shunt fed the boom on several antennas years ago and want to try it
>>   again, but I am "gun shy" now.
>>
>>   Last winter I was using a short shunt fed tower on 160m, running 1200w in
>>   some 160m contest and fried a bunch of stuff in my rig.  All that's on this
>>   tower was the SF for 160m and a couple 6m beams.  The rig was an IC-736 with
>>   2 coax feeds............Yup, you guessed it.......one for HF (which was on
>>   160m) and one dedicated to 6m..........Did I get RF in the shack from that
>>   or was that a coincidence?  BTW, I didn't blow a 6m RX preamp or anything. .
>>   . it was mostly the keying cct that was affected.
>>
>>   What I would like to try this time is to feed the booms on a couple med
>>   sized VHF yagis for the WARC bands.  I know the lengths and how to construct
>>   gamma matches etc.  That's not the issue.
>>
>>   I am leery about doing it for fear I smoke another radio.
>>
>>   Can someone pse explain to me what exactly happens to the "other (ie:
>>   original VHF) antenna" when you are feeding the actual boom of it and using
>>   it on another band.
>>
>>   I have read some of the older N4KG posts about feeding the boom of the
>>   TH6DXX on 40m but no details on what happens to all the coax lines in the
>>   shack.  In my case some VHF and some HF. . .
>>
>>   TNX !
>>
>>   Mike VE9 Antenna Antenna
>>   Mike, Coreen&    Corey Smith
>>   699 Rte 616 Keswick Ridge
>>   NB
>>   Canada
>>   E6L 1T1
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>>
> Mike, do you have line chokes on the coax feed lines? It appears to me
> that when you use the boom as ana antenna, the coax feed line for the
> yagi will radiate and have RF on its shield outside. I would think you
> need good line chokes at the yagi feed point built for both frequencies.
> I have written a short discussion of coax feed line chokes with links to
> K9YC and W1TTT who have good information on the topic.
> http://www.sdxa.org/?p=161
>
> 73, Dan, N5AR
>
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