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?
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If you can get hold of some better cable do so. You have two problems here - first coax isn't balanced, which is sort of bad news for tiny signals such as you get from a mic. And secondly, to compound that, if you strip back a little of the outer sheath, you will find that the screen is 90% holes - in other words, the cable is just about unscreened.
As for using it for UHF TV, go ahead - that is what it is meant for, and 10 meters is just about the maximum run length before you need to think about using something better.
d-- Pearce Consulting http://www.pearce.uk.com
In message , Chris Siz writes
While you might get away with using your cable, good screening/shielding becomes increasing important at the lower frequencies, as the interfering signals (on the outside of the coax) penetrate more easily through the outer shield. Well-screened audio would be better. In many cases, 'flimsy' screening may be adequate at UHF (and the cable may have low loss) but, again, well-screened cable will be better for preventing interference. Maybe it's a case of try it and see what happens?
-- Ian
I would have thought the converse to be true, surely the "holes" in the screening braid would be less of a problem at audio frequencies than at UHF? I am thinking in terms of wavelength.
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.
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. If the antenna is 300 ohms, the cable is not suitable unless matched with a balun or other impedance matching device.
Isn't coax kind of rigid? Will the microphone need to move around?
In message , Bob Eld writes
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.
As it's TV coax, it's unlikely to be 50 ohms.
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
600 ohms is dead and buried, for the audio world.The only time it is mentioned is in data sheets, "will drive XXdB into 600R" etc
martin
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 shield 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 shield. 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 shielding.
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.
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Not necessarily in fact a lot of TV coax is entirely copper.
Yes, the laws of nature decided that. Higher voltages can spark through it and TV RF can
Rubbish.
Rubbish again. Where do you get this knowledge from?
Steve
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What planet do you live on? I have never seen TV coax as you describe. Coax with copper-plated steel inner conductor is mostly used as data cable (ethernet).
73 de G3NYY-- Walt Davidson Email: g3nyy @despammed.com
The stuff with foil is higher quality, used mostly for satellite dish feeder. TV coax is just a copper inner with about half a dozen fine strands for a screen.
d-- Pearce Consulting http://www.pearce.uk.com
75-ohm with copper-coated steel center conductor is often used for outdoor cable-company coax runs. The steel core gives it enough tensile strength to allow long runs to hang between poles (and between pole and house) without stretching. This stuff is often a "hardline" coax, with an aluminum outer shield... very low RF leakage, which is an important issue for cable companies.
Indoor TV coax is sometimes RG-59, sometimes RG-6. It usually seems to have a solid-copper center conductor (to allow for termination via a crimp-on F connector). The shield may be anything from one layer of braid with poor coverage, up to a quad shield (two layers of braid, two layers of foil). It may be fairly stiff.
Coax which is optimized for microphone usage tends to be relatively flexible (stranded center conductor, compliant jacket), and I understand that the better varieties are designed to minimize the generation of triboelectric noise (which occurs when the cable is moved, as a result of static or piezoelectric effects).
Using TV coax for a microphone ought not to be harmful, but it's probably not the best choice, and I'd do it only if a limited budget made it impossible to get something better suited to the purpose.
Useful reference:
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Quite so. The concept of matching a microphone's impedance to get the best power transfer has not been practiced for a considerable time. Modern dynamic mics expect to look at a "bridging" impedance e.g. around 10K for a mic having a Z of 150-200 Ohms and to operate as a voltage, rather than a power source. The characteristic imedance of the feeder (over a few feet) may thus be disregarded.
Les.
No they expect to see 1K2 to 2K not 10K and that is what most pro mic pres provide. 10K input impedance is for bridging line levels.
Cheers
Ian
You want to see the crap which passes for rg58 at maplin electronics.
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Yes, and i've seen rg58 like that at maplin electronics!
Just for your information :-)
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It's all I've ever seen for RG-6 TV cabling. Try sanding the tip to see if it turns silver, or use a magnet.
It's really a TV-only thing. Copper plated steel wire won't work for many applications.
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Indeed. Take it out with the rest of the trash.
Looks like a reference to some hi-Z cable that is made with condctive fabric (or other such substance) to reduce the cable microphonic tendencies due to the triboelectric effect of some kinds of dielectric insulation.
Not enough details to answer your question adequately. First of all, the kinds of microphones used for audio applications (reinforcement, recording, etc.) are usually balanced and require cable with two inner wires and an outside shield/screen. If you are trying to use a balanced mic, then the cable is unsuitable because it is unbalanced.
So, the biggest question is WHAT microphone? Equally helpful would be to reveal what it plugs in to.
"impairment" of the audio signal is unlikely to occur, and is probably not an issue here.
Not really important to the question. It wouldn't make much difference if it were 1 m or 1Km
Note that even if you need unbalanced cable for your unidentified microphone, typical RF/antenna cable is very poorly suited for terminating in typical audio- type connectors. It is frequently aluminum and unsuitable for any kind of soldering, etc.
Insufficient details to answer that question, also. There are hundreds of different kinds of antenna- type (RF) coax. Some of it might be perfect for UHF TV (whatever that means in your area?) And other kinds may be completely a disaster.
Again, "interference" is not the most likely mode of failure of using the wrong kind of cable.
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