In article ,
Paul Burridge k wrote:

I recall a story from many years ago - possibly an urban myth -
where some guy stuck a pin through a ham's coax feeder and thereby
took him off air/blew up his rig etc. Given that RF shorts are a
totally different kettle of fish from DC shorts, I'm just wondering
how feasible from a technical perspective this reported act of
sabotage is.

"Pinning" a coax has a long history in the mythos of RF... I've heard
stories about it for years, usually involving somebody pinning the
coax of an obnoxious CB operator.

I'm no expert on transmission lines, but it strikes me that the
efficacy of such a stunt depends to a great extent on the point in the
line where the pin is inserted as related to the wavelength of the
transmitted signal.

Well, an effective short at point along the coax is going to cause a
complete reflection at that point, and a very high SWR on the line.
This may appear to the transmitter as a short, as an open, or as an
intermediate resistance with a boatload of reactance, depending on the
distance from the transmitter to the short.

A well-designed modern transmitter/amplifier may survive this sort of
nasty load well enough, through e.g. voltage and current sensing
circuitry which feed back to the bias or ALC circuit, and reduce the
power to avoid overcurrent or overvoltage damage, and/or through the
use of internally-ballasted RF finals transistors with a big safety
margin.

A cheap amplifer (such as many of the "multiple pill" not-so-"linear"
amplifiers I see being sold to the CB-cowboy market) could very easily
leak out all of its Magic Blue Smoke quite quickly, working into
this sort of load.

We all know short and open stubs are used as
matching elements at the higher frequencies, so it's implicit that
just sticking a pin in anywhere isn't necessarily going to adversely
affect the efficiency of an antenna system, unless one hits a node at
the frequency of operation.

Not so, I believe. Remember, what you're doing is creating a
trivially-short, shorted "stub" across the line. The pin itself will
present a low-R, low-Z impedance - most of the power flowing up the
line from the transmitter will go into this impedance, and very little
will flow up the remainder of the line to the antenna.

Radiated power will drop very sharply, and the transmitter/amp is
likely to indicate its distress in one way or another.

--
Dave Platt AE6EO
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