Re: OK to use TV coax for microphone?

Dave Platt wrote:
> 
> 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.


   50 ohm hardline is popular too. Some guys even make 50 to 75 ohm
matching networks and use surplus CATV trunkline.

-- 
http://improve-usenet.org/index.html


Use any search engine other than Google till they stop polluting USENET
with **** and junk commercial SPAM

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm