Re: OK to use TV coax for microphone?

In article <481DD149.56BFCCC6@[EMAIL PROTECTED]
>,
Michael A. Terrell <mike.terrell@[EMAIL PROTECTED]
> wrote:

>   You can slap RG- numbers on anything.  It stopped being a military
>standard a long time ago.  Belden, Alpha, Times wire websites should
>have some good white papers.  RG meant 'Radio Guide', and all early coax
>was braided copper ****elding. Their is still some RF coax made this way,
>but it uses teflon and silver plated copper and is VERY expensive.
>
>http://www.belden.com/pdfs/Cable101/****elding.pdf
page 32 states 'Braid
>is for low frequency, foil for high frequency'.

The RF-type foil-with-some-braid coax (LMR400 is one type) is quite
popular, and does seem to provide good ****elding.  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 ****eld construction.  One
choice is a good "double braid" ****eld (which as Michael indicates,
tends to use a silver-plated braid).  An even better choice is heliax,
which uses a seamless corrugated-copper ****eld.

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.

-- 
Dave Platt <dplatt@[EMAIL PROTECTED]
>                                   AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
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