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.
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. 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. What I mean is, IOW, you won't
successfully short out coax at RF unless you stick the pin in at an
appropriate point. Of course, I might be full of crap on this one as
antennas have never been my strong point. Can anyone enlighten me?

btw: this is for academic discussion only! I've no beef against any
amateur and have been one myself for over 20 years.
--

"What is now proved was once only imagin'd" - William Blake

Inside the coax cable are two conductors carrying current, the inside of
the shield and the outside of the center conductor. The current on one
of those conductors travels to the antenna, and an equal current returns
on the other 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. The fraction which goes each
way is determined by the impedance of each path. A pin is electrically
very short at frequencies at which the coax can be effectively used, so
it has negligible reactance. Assuming that it's making good contact with
both the shield and center conductor -- which it might not be -- the
resistance will also be small. So it makes a good RF short circuit.
Therefore a large fraction of the current will return via the pin rather
than going on down the cable. So the first effect will be that it will
greatly reduce the amount of power which reaches the antenna to be radiated.

What will happen to the transmitter? That depends on the transmitter and
where the pin is inserted. If the cable didn't have any loss and the pin
had zero resistance, the transmitter would see a pure reactance. That
is, what it would see would look like a pure L or C, with the value and
sign depending on the pin's position relative to the transmitter. In
practice, the pin will have some resistance and the cable will have some
loss, so the transmitter will also see some amount of resistance, the
amount again depending on the pin position, as well as the cable loss
and pin resistance. I suspect that most modern 100 watt-class solid
state transceivers would probably just shut down their output stage and
not be permanently damaged, but I'd rather not experiment with my own
rig. The result might be more spectacular with a tube type linear with
pi network output. But again, it would depend on the design of the
transmitter and the particular impedance it sees.

Roy Lewallen, W7EL

Paul Burridge 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.
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. 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. What I mean is, IOW, you won't
successfully short out coax at RF unless you stick the pin in at an
appropriate point. Of course, I might be full of crap on this one as
antennas have never been my strong point. Can anyone enlighten me?

btw: this is for academic discussion only! I've no beef against any
amateur and have been one myself for over 20 years.

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
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!

I leave only this.

At VHF and up it's common to use a shorted 1/4 wave section for
second harmonic suppression at the output. Very effective and dirt
cheap. The finals are not the least bit bothered.

If a short appeared near a 1/4 wave node at operating frequency
it might go unnoticed.

Allison
KB!GMX

wrote:
I leave only this.

At VHF and up it's common to use a shorted 1/4 wave section for
second harmonic suppression at the output. Very effective and dirt
cheap. The finals are not the least bit bothered.

If a short appeared near a 1/4 wave node at operating frequency
it might go unnoticed.

I'm afraid it wouldn't go unnoticed. The transmitter would see an open
circuit, instead of the proper load of typically 50 ohms. The effect on
the transmitter would be the same as disconnecting the feedline at the
transmitter.

Roy Lewallen, W7EL

On Sat, 24 Dec 2005 15:40:25 -0800, Roy Lewallen
wrote:

wrote:
I leave only this.

At VHF and up it's common to use a shorted 1/4 wave section for
second harmonic suppression at the output. Very effective and dirt
cheap. The finals are not the least bit bothered.

If a short appeared near a 1/4 wave node at operating frequency
it might go unnoticed.

I'm afraid it wouldn't go unnoticed. The transmitter would see an open
circuit, instead of the proper load of typically 50 ohms. The effect on
the transmitter would be the same as disconnecting the feedline at the
transmitter.

Roy Lewallen, W7EL

I did use the word "might" rather than will.

Actually it depends on the real life characteristics of the short. If
it were a perfect short (in theory) yes. But if there is any varience
from that it's going to be harder to predict. Likely it world look
more like a higher impedence, but not completely. In all likelyhood
the parameter that needs to be know more than any one
its frequency. At 432 it's impact would be very different than say
7.2mhz.



Allison

On Sun, 25 Dec 2005 00:35:08 GMT,
wrote:

On Sat, 24 Dec 2005 15:40:25 -0800, Roy Lewallen
wrote:

wrote:
I leave only this.

At VHF and up it's common to use a shorted 1/4 wave section for
second harmonic suppression at the output. Very effective and dirt
cheap. The finals are not the least bit bothered.

If a short appeared near a 1/4 wave node at operating frequency
it might go unnoticed.

I'm afraid it wouldn't go unnoticed. The transmitter would see an open
circuit, instead of the proper load of typically 50 ohms. The effect on
the transmitter would be the same as disconnecting the feedline at the
transmitter.

Roy Lewallen, W7EL

I did use the word "might" rather than will.

Actually it depends on the real life characteristics of the short. If
it were a perfect short (in theory) yes. But if there is any varience
from that it's going to be harder to predict. Likely it world look
more like a higher impedence, but not completely. In all likelyhood
the parameter that needs to be know more than any one
its frequency. At 432 it's impact would be very different than say
7.2mhz.



Allison

A short across the transmission line will have much the same effect at
432 as it will at 7 mhz.
What you are thinking about is a shorted stub attached to the
transmission line or output of the transmitter. A shorted 1/4 wave
length stub at the operating frequency placed across the transmitter
output will present a high impedance at the operating frequency and
will not be noticed by the transmitter. But at the second harmonic of
the stub it will be a 1/2 wave shorted stub which will present a short
at the output of the transmitter at the 2nd harmonic frequency.

The shorted stub would still allow energy to flow to the antenna
normally. But shorting the transmission line would not no matter where
it was.

73
Gary K4FMX

I leave only this.

At VHF and up it's common to use a shorted 1/4 wave section for
second harmonic suppression at the output. Very effective and dirt
cheap. The finals are not the least bit bothered.

True, but that's not the situation we're dealing with here.

If you place a shorted quarter-wave section directly at the
transmitter's terminals, in parallel with the antenna feedline, then
the transmitter "sees" the impedance of the feedline (its usual load)
in parallel with the impedance of the shorted stub (very high). The
net impedance is that of the load (the admittance of the shorted stub
is nearly zero) and the transmitter does not "notice" the presence of
the shorted stub.

That's not the situation which occurs if the feedline itself is
shorted 1/4 waveline towards the load.

In that situation, the *only* thing that the transmitter will see is
the shorted quarter-wavelength "stub" between itself and the short.
The impedance at the point of the short is nearly zero - it's the
impedance of the short itself, in parallel with the impedance of the
antenna as seen when looking up the remainder of the feedline. No
matter what the antenna's impedance is, the very low impedance of the
short itself is going to dominate the parallel combination. The
resulting near-zero-ohm combination will be transformed, by the
quarter-wavelength distance back to the transmitter, so that it
appears as an open circuit to the transmitter.

The transmitter cannot, in effect, "see past the short circuit" to the
antenna itself.

The same is true no matter how far up the feedline from the
transmitter the short/pin happens to be. At the point of the short,
the impedance is going to be nearly zero, and this near-zero impedance
will be transformed to some other value on the same very-high-SWR
circle (neglecting consideration of feedline loss, of course).

No matter where you pin the coax, the transmitter is going to be
unhappy.

If a short appeared near a 1/4 wave node at operating frequency
it might go unnoticed.

Different situation, I'm afraid.

If you have an antenna analyzer, try it out for yourself. Take an
arbitrary-length section of RG58 with a 50-ohm load at one end and a
BNC at the other. Run it into a BNC "T". Out the other leg of the T,
run an adjustable length of RG-58 to the antenna analyzer. You ought
to measure 50 ohms in this situation.

Now, stick a short directly across the third branch of the T connector
("pinning" the coax, so to speak), and see what your analyzer tells
you. It may read high-Z, or low-Z, or intermediate-Z with a lot of
reactance... but it'll be a high indicated SWR, and it won't be
anywhere near 50+j0.

Then, disconnect the antenna from the "T". The impedance and
indicated SWR won't change significantly.

Try changing the length of the RG58 between the "T" and the analyzer.
You'll get a different Z value with the short in place (whether the
antenna is or is not attached) but it'll still have a really high SWR,
no matter what coax length you choose.

--
Dave Platt 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!

On Sat, 24 Dec 2005 20:44:34 -0500, Gary Schafer
wrote:


I did use the word "might" rather than will.

Actually it depends on the real life characteristics of the short. If
it were a perfect short (in theory) yes. But if there is any varience
from that it's going to be harder to predict. Likely it world look
more like a higher impedence, but not completely. In all likelyhood
the parameter that needs to be know more than any one
its frequency. At 432 it's impact would be very different than say
7.2mhz.



Allison

A short across the transmission line will have much the same effect at
432 as it will at 7 mhz.
What you are thinking about is a shorted stub attached to the
transmission line or output of the transmitter. A shorted 1/4 wave
length stub at the operating frequency placed across the transmitter
output will present a high impedance at the operating frequency and
will not be noticed by the transmitter. But at the second harmonic of
the stub it will be a 1/2 wave shorted stub which will present a short
at the output of the transmitter at the 2nd harmonic frequency.

If it were a "perfect" short yes.

For real life the short have real impedence between center conductor
and shield. As frequency goes up a .2" peice of wire accumulates
enough real resistance and reactance to be a factor at high VHF and
uhf.

My favorite filter for 2m is found on the ARRL.com TIS site. it's
made with series and parallel shorted sections operating as tapped
resonant circuits. The stubs are only something like 2" and for 2m
thats about 13" short of 1/4 wave. Just shows what happens when
a transmission line stops being simply that.

The shorted stub would still allow energy to flow to the antenna
normally. But shorting the transmission line would not no matter where
it was.

Yes and No. See above.

Allison
Kb!gmx

On Sun, 25 Dec 2005 01:51:01 -0000, (Dave Platt)
wrote:

I leave only this.

At VHF and up it's common to use a shorted 1/4 wave section for
second harmonic suppression at the output. Very effective and dirt
cheap. The finals are not the least bit bothered.

True, but that's not the situation we're dealing with here.

If you place a shorted quarter-wave section directly at the
transmitter's terminals, in parallel with the antenna feedline, then
the transmitter "sees" the impedance of the feedline (its usual load)
in parallel with the impedance of the shorted stub (very high). The
net impedance is that of the load (the admittance of the shorted stub
is nearly zero) and the transmitter does not "notice" the presence of
the shorted stub.

That's not the situation which occurs if the feedline itself is
shorted 1/4 waveline towards the load

Only if the short is perfect or the frequency is low enough.
Even for shorted stubs doing it right requires a bit of effort
as even a loop at the end causes interesting effects.

In that situation, the *only* thing that the transmitter will see is
the shorted quarter-wavelength "stub" between itself and the short.
The impedance at the point of the short is nearly zero - it's the

Nearly zero is not zero and it's not a constant with frequency.
At 3.5mhz I will agree with you. At 350mhz it's going to show
something different. At 3500mhz, who knows?

impedance of the short itself, in parallel with the impedance of the
antenna as seen when looking up the remainder of the feedline. No

However is it parallel or is it some complex reflection of the length
to the antenna.

matter what the antenna's impedance is, the very low impedance of the
short itself is going to dominate the parallel combination. The
resulting near-zero-ohm combination will be transformed, by the
quarter-wavelength distance back to the transmitter, so that it
appears as an open circuit to the transmitter.

The transmitter cannot, in effect, "see past the short circuit" to the
antenna itself.

The same is true no matter how far up the feedline from the
transmitter the short/pin happens to be. At the point of the short,
the impedance is going to be nearly zero, and this near-zero impedance
will be transformed to some other value on the same very-high-SWR
circle (neglecting consideration of feedline loss, of course).

No matter where you pin the coax, the transmitter is going to be
unhappy.

If a short appeared near a 1/4 wave node at operating frequency
it might go unnoticed.

Different situation, I'm afraid.

If you have an antenna analyzer, try it out for yourself. Take an
arbitrary-length section of RG58 with a 50-ohm load at one end and a
BNC at the other. Run it into a BNC "T". Out the other leg of the T,
run an adjustable length of RG-58 to the antenna analyzer. You ought
to measure 50 ohms in this situation.

Now, stick a short directly across the third branch of the T connector
("pinning" the coax, so to speak), and see what your analyzer tells
you. It may read high-Z, or low-Z, or intermediate-Z with a lot of
reactance... but it'll be a high indicated SWR, and it won't be
anywhere near 50+j0.

Will it be greater tha say 500+janything? Depends on frequency.

Actually using a tee and doing that creates a working stub that is
tuned in the low microwave region and something reactive below
that. In fact the open Tee at high uhf is also a trouble maker.

Then, disconnect the antenna from the "T". The impedance and
indicated SWR won't change significantly.

But t does, depending on frequency and line length on the other side
of the short. Try it for an exact 1/2wave from source to load and pin
at 1/4wave point. the reflected impedences at both ends will be
in play. The pin is only a perfect short at very low frequencies
as you go up the "short" gets "longer" and behaves as a reactance.
It's a complex circuit and there are lumped analogs.

Try changing the length of the RG58 between the "T" and the analyzer.
You'll get a different Z value with the short in place (whether the
antenna is or is not attached) but it'll still have a really high SWR,
no matter what coax length you choose.

Its easier to vary F than change length. But yes you will see varying
impedences (R+J) all over the map.

I keep saying a perfect short would behave as stated. Most real
world components like that pin would be a pain at 3.5mhz and
something else at 432mhz.

Allison
KB!gmx