Re: [IRCA] AC twin lead or speaker wire for lead in.

[Nick Hall-Patch <nhp@xxxxxxxx>]

At 12:38 18/08/2007, you wrote:


>That's exactly what I thought.  When the dummy antenna was connected to the
>receiver, no signals at all.  Put in the 150' of RS coax, and viola!  Just
>like a BOG of 150'.  However, when I swapped out the RS stuff for an
>equivalent length of brand new Belden RG-6 and new connectors - and got no
>change, then it became a head scratcher.
>
>As I said, I am puzzled as to how signals get in a shielded cable.  The
>Belden has 100% foil shielding plus an aluminum braid drain.  Maybe the quad
>foil shield would be better.  At this point I'm not sure it's worth the cost
>to buy a box and find out.
>

I'm no expert on this, but even with a termination at the antenna 
end, this problem can be observed,  and often better shielding won't 
help.  The nice long shield itself acts as an antenna; it has 
resistance and an induced current from your locals will develop a 
voltage potential across that shield from one end to the 
other.   Either a signal is induced in the center conductor, or your 
receiver now sees a differential voltage between the center conductor 
and shield (earth grounds aren't perfect),  but I'm afraid there is 
where I become vague.   You could look at:

http://www.w8ji.com/common-mode_noise.htm

to see if any of the ideas there help to explain the problem.

I didn't see that you were using an isolation transformer in your 
tests (which might have helped), but did see this:

>Another thing I tried is winding a dozen turns of RG-58 through an FT-290-75
>toroid core to make a choke.  Made no difference, grounded or not.

So you're aware of common mode problems, and I presume you meant 
running the shield to ground on one side or the other of the 
choke.  There can be a law of diminishing returns there, where the 
inductance of the wound coax resonates with the capacitance per 
foot  of the cable, and you can end up on the capacitive side of the 
resonant peak, but I don't know if you're there yet.  That's a pretty 
big ferrite core (I have no info on it), but the smaller FT240 size 
would be giving you several hundred microhenries of inductance with a 
dozen turns I think, and you'd have tens of picofarads of capacitance 
as well.  A thought.

best wishes,

Nick








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Nick Hall-Patch
Victoria, B.C.
Canada

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