[RFI] high noise level...

Ian White, G3SEK G3SEK at ifwtech.co.uk
Fri Sep 12 09:07:08 EDT 2003


Tom Rauch wrote:
>
>> 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?  (A disturbing thought, why not the tower too?
>>  Pretty difficult to wind it in a multiturn coil.)
>
>There is a reason you don't hear of that problem much, although I agree it
>is possible.
>
>The coax connects DIRECTLY to the antenna element.
>
>The Rotor and tower are only close by, and obviously are coupled to the
>antenna significantly less than the feeder. This is particularly true when
>the antenna is electrically balanced and symmetrical about the tower.
>
>This is why a tiny amount of common mode noise on the shield is a big
>problem, and other cables or the tower itself is not.

The stray currents always originate on the outer surface of the coax 
shield, at the feedpoint... but what happens then can depend on:

* What else the shield is connected to up there. If it's connected to 
the boom (as in a gamma match) then some of the shield current will flow 
into the boom, mast and tower, depending on what impedance those offer 
at the connection point.

* What else the coax is taped closely to. Over the whole height of the 
tower, there can be enough capacitance through insulated jackets to 
cause significant currents in other conductors.


-- 
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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