Re: [RFI] high noise level...

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

Jim Smith wrote:

> Ian White, G3SEK wrote:
>
> > >
> > Another possibility is that it might be coming up the rotator cable, 
> > and crossing over to the antenna at the top of the tower. Remember, 
> > there is actually no "ground" at the top of the tower - there are only 
> > "common" connections.
> In all the stuff I've read about current chokes, feedline radiation due 
> to asymmetry, etc. I don't think I've ever seen this possibility 
> pointed out (or thought of it).  If coax can pick up RF and reradiate 
> it, why not the rotator cable?

I found this out with a clip-on meter. Using a gamma-matched 6m beam, 
there was RF on the outside of the coax *and* the rotator cable (didn't 
check which conductors, but who cares).

Changed to a balanced, floating feed with no connection to the boom. 
Result - RF gone from the rotator cable, and nearly gone from the coax. 
Added ferrite beads on the coax loop above the rotator (acting as a 
coupling loop to the driven element) - RF gone. A lot of the received 
noise had gone too.

One moral of that experience is that the "ground" at the center of a 
gamma match is a mass delusion. It's actually a direct connection for RF 
currents to flow into the boom, mast, rotator, rotator cable and...

> (A disturbing thought, why not the tower too? Pretty difficult to wind 
> it in a multiturn coil.)

Without the clip-on current meter, I probably wouldn't have suspected a 
thing.


--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                           Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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