Re: OK to use TV coax for microphone?

In article <Xns9A948106DBE84451E7A@[EMAIL PROTECTED]
>,
 Chris Siz <default@[EMAIL PROTECTED]
> wrote:

> On Sat 03 May 2008 19:05:41, Kevin McMurtrie wrote:
> 
> > In article <Xns9A936FA2EF13F451E7A@[EMAIL PROTECTED]
>,
> >  Chris Siz <default@[EMAIL PROTECTED]
> wrote:
> > 
> >> I am in the UK and have a 10m length of some cheap TV aerial
> >> coax.  It came from a discount store as a TV coax extension
> >> cable.  I measure the cable diameter as 4.8mm.
> >> 
> >> (1) Is it ok to use this sort of coax for a microphone?  I don't
> >> want an impaired audio signal.  The length I need to use is 3 to
> >> 4 metres. 
> >> 
> >> (2) Is it ok for UHF TV or is it actually quite low grade coax
> >> and prone to interference or mess up the aerial signal?
> > 
> > There are a few differences.  TV coax will work in some conditions
> > but not generally.
> > 
> > TV coax is very brittle.  The inner wire is copper-plated steel
> > and the outer ****eld is aluminum wire and aluminum foil.  It will
> > quickly crack where it meets the connectors.
> > 
> > TV coax may not pass small audio signals well because of its
> > aluminum ****eld.  Aluminum is extremely reactive so it is always
> > coated with a thin oxide layer.  Higher voltages can spark through
> > it and TV RF can capacitively couple through it.  Microphone
> > signals might become distorted.  Cable for lower frequencies uses
> > copper ****elding. 
> > 
> > Good microphone and instrument cable has an insulation that drains
> > away static electrical charges.  RF coax can contain electrical
> > charges in the insulation that causes it to act like condenser
> > microphone. 
> > 
> 
> I am the OP and as you can probably tell I am no electronics or 
> radio expert.  However even my limited knowledge struggles to 
> believe some of the points you have made.  
> 
> Forgive me if it's more obvious to others but your's is not some 
> sort of funny posting is it?

It's not a funny posting.  TV signals are high frequency RF and they 
behave very differently from audio.  The requirements for good TV 
performance are very different from microphone cable.

TV needs:
- Very constant RF impedance to reduce signal reflections and smearing.
- Low absorption of very high frequencies in the insulation.
- High conductance along the surface of the center wire.
- Low inductance and capacitance.
- Stretch resistant enough to hang from antennas and rooftops.

Music cable:
- No oxide layers.
- Low capacitance.
- Insulation that does not produce electric noise on impact.
- Resistant to being stepped on and bent.
- Very high ****elding coverage to avoid noise pickup from nearby power 
cables.

You can buy general purpose coax that works perfectly for both audio and 
RF.  It's a bit pricey.  Since you mentioned it being cheap TV coax, I 
doubt it's a good performer outside of TV frequencies.

The real answer is to TRY IT.

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