[RFI] high noise level...
Ian White, G3SEK
G3SEK at ifwtech.co.uk
Thu Sep 11 23:28:48 EDT 2003
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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