Re: Shorting out a transmission line

In article <sa2rq1550aj5jqvnvvupco7jm1dt9ok1q4@xxxxxxx>,
Paul Burridge <pb@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> 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 <dplatt@xxxxxxxxxxxx> AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
.

Relevant Pages

  • Re: Shorting out a transmission line
    ... Inside the coax cable are two conductors carrying current, the inside of the shield and the outside of the center conductor. ... At the point where you insert the pin, the current has two possible paths: it can continue down the cable as it normally does, or it can return to the other conductor via the pin. ... That depends on the transmitter and where the pin is inserted. ...
    (rec.radio.amateur.antenna)
  • Re: High-Power RF Multiplexer
    ... That would be about one-fourth of my 25 Watts, ... Is the RF on Coax, ... insert some DC at the transmitter and pick it off at the switch? ... Just make sure to connect it so the power goes ...
    (sci.electronics.design)
  • Re: High-Power RF Multiplexer
    ... That would be about one-fourth of my 25 Watts, ... Is the RF on Coax, ... insert some DC at the transmitter and pick it off at the switch? ... Just make sure to connect it so the power goes ...
    (sci.electronics.design)
  • Re: IR mains switch
    ... The major drawback is that it's yet another remote control amongst all ... I sampled a few seconds of demodulated output of their transmitter into a PC par port pin, ... Then playing back by connecting the serial port RTS pin to the power of the 430 MHz module, ... RS23w RTS has enough power to power those transmitter modules. ...
    (sci.electronics.design)
  • Re: IR mains switch
    ... The major drawback is that it's yet another remote control amongst all ... I sampled a few seconds of demodulated output of their transmitter into a PC par port pin, ... Then playing back by connecting the serial port RTS pin to the power of the 430 MHz module, ... RS23w RTS has enough power to power those transmitter modules. ...
    (sci.electronics.design)