[Ian Jackson]

In message , Bob Eld
writes

"Chris Siz" wrote in message
...
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