>In article <481DD149.56BFCCC6@earthlink.net>,
>Michael A. Terrell earthlink.net> 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 shielding. 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/Shielding.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 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.