[AMRadio] Hint for shielded audio cable

Jim candela jcandela at prodigy.net
Sat Apr 8 21:21:18 EDT 2006


   This is good stuff, a real ham making do, and it could be argued, doing
it better while doing it cheaper. I wonder however with that picture from
CAT-5 remnants, what is the capacitance per foot? Many coax cable types come
in at about 30 pf / foot for comparison.

   Now with single sided shielding of a cable we get two results:

1.) No ground Loop
2.) An end that where the shield is hot (not terminated) when the shield
length is an odd multiple of a quarter wavelength. For example, 19" at abt.
144 Mhz. This assumes that there is RF floating around at 100+ Mhz.

   You can get away from the hot shield by grounding both ends, and if the
cable is short, and terminated within the same chassis, a ground loop may
not hurt, and the electrostatic shielding of the shield will be superior. If
there are ground currents within the chassis, the ground current with the
ground loop could raise holy hell with say 60/120 hz entry into an audio
chain by transformer action. You can eliminate the loop by using a second
shield wire insulated from the first where each shield wire is grounded on
opposing ends. Now you can have your cake and eat it too.

Keep in mind that shielding for E-Fields and H-Fields have different
techniques. So far we have only discussed shielding for E-Fields. A twisted
pair wire shields for H-Fields, and a shielded twisted pair works for both.
Go a step further, and the ultimate is a twisted pair with two shields as
described above. In the absence of a twisted pair approach, you sometimes
see in radios, a steel spring of abt. 1/4" diameter soldered to a chassis at
multiple points with a wire threaded through the spring. This steel spring
is a good E and H field barrier, so long as the multiple groundings don't
create too bad of a ground loop. This approach works best when the wire run
is only a few inches long. The approach also has very low capacitance per

Good work John


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of John E. Coleman (ARS
Sent: Saturday, April 08, 2006 5:04 PM
To: 'Discussion of AM Radio'
Subject: [AMRadio] Hint for shielded audio cable

	Have you ever needed a small piece of shielded cable for under
chassis work and didn't wont to cut a piece of that expensive RGxxx that is
real small and difficult to work with.  Well here is what I do.

	I'm a computer tech and occasionally have a need to do some cabling
work.  There is always a scrap or two of CAT5 cable here and there because
I'm a packrat as well.  If you pull the individual wires out you have some
nice small hookup wire and if you need to shield one just take a long piece
and wrap it around a short piece and you have shielding.

	Now, this is not a transmission line and should be grounded at one
end only.  It is just a static shield to reduce capacitive coupling into the
audio wire.  The same should be true if using coax for this purpose.  You
only ground one end.  I now use this home brewed stuff for about every
shielding application I need under an audio chassis.  And it makes for real
pretty colorful work.  You can wrap the solid wire on the outside or the
striped wire can be the shield.  This gives you 8 colors to work with. The
link below shows the orange pair used in this way.


John, WA5BXO

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