OK to use TV coax for microphone?

Sorry, he normally resides in my killfile where he belongs, but every now and then I have to reconfigure something and he gets out.

He's back in there with a shot of Ketamine now.

d

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That may be the case over there (although I doubt it). Over on our side of the earth, RF cables, and especially those intended for television use are 100% shielded. Because if they are not, it starts causing problems. Problems for the cable operators when RF leaks (in either direction) between the TV station on channel X and the over-the-air signal of the same station on the same frequency. This is a chronic problem in some of the poorer-maintained systems.

And problems for individuals even with just a run of several meters to a rooftoop antenna because of the increased susceptability to RFI, particularly in remote regions where the signal is weak to start with.

RF coax made for cable TV use have foil shields plus a few strands of wire braid around the foil. In fact many of them even have a double foil shield.

Don't mistake the wire braid (which may, indeed be only

10% coverage) for the actual outer shield of the coax which is the active portion.

And out in remote rural areas, RF signals are of the same order.

Yup, but the stuff we are discussing here has just a very loose braid. Have a look at the second pic on this page - low loss TV coax.

d

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Pearce Consulting
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What is that stuff good for?! I've never seen anything like that over here. It can't be useful for low-level signals (receiving, audio, etc.) any place where there is any RFI.

OTOH, they make an intentionally "lossy" coaxial cable (one brand name is "Radiax") which is used for distributed Tx/Rx (such as running a cable through a tunnel to provide cell service underground, etc.) That stuff has holes all along the length to deliberately leak RF along the way.

Over there? Over here we find lots of 72 ohm tv coax on sale in the diy stores which has very poor screening. In fact I'd say the majority of terrestrial tv antennas in the Uk are wired with it.

Well yes, cable tv operators even over here use much higher quality coax.

I don't think any regular in this ng is going to make a mistake like that.

Most areas here (UK) have excellent analogue signal strength.

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It is good for absolutely nothing, and nowadays resides only in cut price electrical stores and old boxes in attics.

I know Andrew's Radiax very well - I've specified it for distributing VHF in tunnels.

d

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Pearce Consulting
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coax doesn't shield in that way. It works by cancellation and eddy current effects duh!

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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. 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 shielded (apart from a small break to prevent the shorted turn) and works on the magnetic component of the received signal alone.

Twisted pair inside a single screen deals with all these potential problems.

d

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Pearce Consulting
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Radial field in the coax resulting in cancellation.

that's all true, i have built several loops of that type for HF.

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.

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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

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

Yep. Way back in the 1960's, I was installing 2way radios in various vehicles. Someone found some RG-58/u, with a solid center conductor. I was told to use it. About 100 vehicles later, someone declared that solid coax was a bad idea. Management decided that it would replace all the coax, but only after evidence of the first failure. So, we waited, and waited, and waited. I was long gone when one of the coax cables finally failed, about 5 years or 150,000 miles later.

Fast forward to the early 1990's and cellular telephone installs. At the time, the average cell phone was big, boxy, heavy, and ugly. The connector of fashion was the TNC. Once again, some dealers were installing solid center conductor coax in the vehicles. They would last about a month and then break. Huh? Solid center conductor RG-58/u works in 100 buses for 5 years, but not in a passenger car for a month? Something is obviously different.

It's the connector. In the 1960's we soldered everything. The common PL-259 and UG-175 adapter was as sloppy a connection that could have been mis-designed. 30 years later, everyone was crimping the connectors. The breakage was always at the connector. The crimper was dinging the center conductor. The sharp edge on the rear of the center pin did the rest. Soldering the center pin and the longer distance between the crimp and the point at which the cable will begin to flex (UHF versus TNC) did much to prevent breakage.

All RG-6/u "type" cables are not all solid center conductor. Belden Brilliance 1694F is flexible.

at about $0.70/ft. However, it cannot be used with Type-F connectors. The data sheet mumbles: "1694F is intended for use with connectors designed for 1694A" which is not very helpful.

The foil outer shield usually tears in a flex test long before work hardening does anything to the center conductor.

I've watched cable flex testers locally:

(Bring hearing protection). These simulate normal cable bending on telephone headsets. The flexing is far more radical than anything that would be encountered with RF coax cable. I asked if they had ever tried flexing RF type coax cable. As I vaguely recall, neither solid or stranded center conductor coax cables survived for very long, but I don't recall the numbers. I'll ask.

Incidentally, there's also the tiny triboelectric noise created by the mylar coated insulated shield, rubbing against the dielectric:

Good microphone cable should be "bonded" (i.e. glued) to prevent this admittedly tiny noise problem. You're not going to find that in commodity RF coax cable.

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The RF-type foil-with-some-braid coax (LMR400 is one type) is quite popular, and does seem to provide good shielding. However, there are high-frequency (UHF/VHF) applications in which it has a rather evil reputation. In particular, most repeater operators I know avoid it like the plague when it comes time to run their primary feedlines.

Practical experience seems to suggest that this sort of construction is prone to broadband-noise problems when used for duplex applications... e.g. in a repeater where you're transmitting 25 - 100 watts up the cable in one direction, and also trying to receive a microvolt-level signal on a nearby frequency at the same time in the other direction.

The culprit seems to be the fact that the foil and braid don't make perfect contact throughout the cable - they're not (and cannot be) soldered together, and the contact between them is simply a mechanical-pressure contact which is imperfect. There seems to be an irregular make-and-break effect - I've heard it called "micro-arcing" - which causes some small amount of the transmitted energy being rectified and spread around the spectrum as broadband noise. Some of this noise ends up on the repeater's receiver frequency, and cannot be filtered out at the receiver... and this competes with the incoming signal and can swamp it out (a form of receiver desensitization).

It doesn't take much of this noise to be a problem... I figured out last year that in our repeater application (35 watt transmitter on

145.27 MHz) the transmitter is putting out literally a quadrillion times more power than the receiver is picking up from a hand-held radio out at the edge of our service area. Even a tiny fraction of the transmitter power, rectified into noise, can wipe out the desired signal.

The remedy for this, in practice, is to use a different type of cable... one without the foil-and-braid shield construction. One choice is a good "double braid" shield (which as Michael indicates, tends to use a silver-plated braid). An even better choice is heliax, which uses a seamless corrugated-copper shield.

Both are expensive - Andrew 1/2" heliax is edging up towards $3/foot these days... and both are too stiff to use as microphone cable :-)

The foil-and-braid cables seem to be fine in simplex RF applications, where you aren't trying to receive and transmit through the same cable at the same time - the amount of broadband noise being generated is inconsequential in simplex use.

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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 shielding 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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It's fine for connecting a cheapo mic so long as it's kept stationary - as the coax is nearly rigid and non-pliant compared to good mic cable. If your mic o/p is balanced, use a 3-conductor [screen + twisted pair] cable and bin the coax! Any 2-wire cable (co-ax or not) will do for an aerial feed, if it's kept short! Jim

>

"Richard Arse Licker Crowley"

** Gawd -

what a low life piece of autistic scum this human turd is.

.... Phil

Tee Hee white it will be with a couple of strands in the middle and no braid worth talking about ...and a moulded plug at each end..........gosh you techy guys will argue about anything! ......

Impedances are almost never matched, source to load, or source to transmission line to load in audio. Telephone lines are the only case that comes to mind. Mics, speakers, preamps to amps -- just not done. In short, we don't match source to cable, nor cable to load. Nor source to load. Not only that, the impedance of a typical cable, such as rg-59, though 75 Ohms above 100KHz, is much much higher in the audio range.

Earl

Same here. *shrug* ?

What kind of microphone?

Most good microphones need 2-conductor plus shield cable. Most coax has only one conductor, plus shield.

There are two kinds of coax, 50 ohm and 72 ohm. What is the impedance of the coax, and what is the required impedance for your application? In short lengths it won't matter, but for like 30' or more, it might.