Re: OK to use TV coax for microphone?

On Sun, 04 May 2008 18:47:17 +0100, Dead Paul <dead_paul@[EMAIL PROTECTED]
>
wrote:

>On Sun, 04 May 2008 17:39:09 +0000, Don Pearce wrote:
>
>> 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, ****elding
>>>>>effectiveness, signal ingression and a dozen other topics.  In other
>>>>>words, no single ****eld is 100% effective.
>>>> 
>>>> And of course unless it is made of soft iron or mumetal, it doesn't
>>>> ****eld magnetically at all.
>>>
>>>coax doesn't ****eld 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.
>
>Radial field in the coax resulting in cancellation.
>
>
>> 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 ****elded (apart from a small break to prevent the
>> shorted turn) and works on the magnetic component of the received
signal
>> alone.
>
>that's all true, i have built several loops of that type for HF.
>> 
>> Twisted pair inside a single screen deals with all these potential
>> problems.
>
>In agree it's better than two lengths of coax as balanced line but didn't
>the discussion centre around using coax as audio lead? I'm sure anyone in
>their right mind would use audio cable if they had it available.
>

Yup, that's right where this all started. Should he use poorly
screened coax as a microphone cable (I wouldn't be worried about it at
line level). I think the general answer is that while he might just
get away with it, it isn't a good idea and some properly screened
audio cable would be better.

d

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