Re: OK to use TV coax for microphone?

On Sun, 04 May 2008 18:24:15 +0100, Dead Paul <dead_paul@[EMAIL PROTECTED]
>
wrote:

>On Sun, 04 May 2008 15:17:37 +0000, Don Pearce wrote:
>
>> On Sun, 04 May 2008 11:15:45 -0400, "Michael A. Terrell"
>> <mike.terrell@[EMAIL PROTECTED]
> wrote:
>> 
>>>Dead Paul wrote:
>>>> 
>>>> no pickup inside coax.
>>>
>>>
>>>   Really?  Then you don't know much about Coaxial cables, shielding
>>>effectiveness, signal ingression and a dozen other topics.  In other
>>>words, no single shield is 100% effective.
>> 
>> And of course unless it is made of soft iron or mumetal, it doesn't
shield
>> magnetically at all.
>
>coax doesn't shield in that way. It works by cancellation and eddy
current
>effects duh!
>
Coax used normally deals with magnetic fields by having effectively no
magnetic loop area - the two conductors are running along an identical
centre line so there is no magnetic loop to pick up a signal. As soon
as you run a signal along two pieces of coax in differential mode, the
area encompassed by the resulting loop is susceptible to magnetic
pickup. Depending on how you deal with the screens, you may have a
shorted turn to help out, but maybe not. Think of how a direction
finding loop antenna works - it is fully shielded (apart from a small
break to prevent the shorted turn) and works on the magnetic component
of the received signal alone.

Twisted pair inside a single screen deals with all these potential
problems.

d
-- 
Pearce Consulting
http://www.pearce.uk.com