Re: OK to use TV coax for microphone?

In message <S7_Sj.2243$506.1543@[EMAIL PROTECTED]
>, Bob Eld 
<nsmontassoc@[EMAIL PROTECTED]
> writes
>
>"Chris Siz" <default@[EMAIL PROTECTED]
> wrote in message
>news:Xns9A936FA2EF13F451E7A@[EMAIL PROTECTED]
>> 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?
>>
>
>Yes, it's OK to use coax for a mic cable assuming the mic is a single
ended
>connection (one wire) with ground. The biggest issue is the cable
>capacitance. Depending on the mic's impedance, this capacitance will
>attenuate high frequencies. A low impedance mike will work better than a
>high impedance mic. The accumulated capacitance in four meters of cable
will
>be a couple of hundred pico-farads, probably not a problem.

Out of interest, how does the capacitance of ordinary TV coax (which 
will be 75 ohms, of course) compare with 'proper' mic cable? Of course, 
a lot of audio is balanced (usually nominally 600 ohms), which makes 
things generally less prone to the pick-up of interference. Unbalanced 
connections via coax (even if proper screened audio cable) is 
less-tolerant.
>
>For UHF TV cables are usually 72 ohms. If your cable is 72 ohms, it's
>probably OK. If it is 50 ohms it will still work but may degrade the
picture
>quality because of reflections within the cable.
>Try and see.

As it's TV coax, it's unlikely to be 50 ohms.

>If the antenna
>is 300 ohms, the cable is not suitable unless matched with a balun or
other
>impedance matching device.

UK aerials are not 300 ohms (except for those indoor FM radio aerials 
made from 300 twin feeder). I believe that, these days, they are also a 
bit of rarity in the USA.
-- 
Ian