On Tue, 05 Apr 2011 11:00:18 -0700, Richard Clark
wrote:

On Tue, 05 Apr 2011 10:58:25 -0400, John Ferrell
wrote:

Another possible obvious symptom is you are moving a transmission line
while keyed down, and you notice your SWR shifts at the same time. Is
it because of a faulty connection? Sometimes yes, but we are talking
about CM; and when it is CM, then that shift in SWR is because a tuned
circuit (the line is now part of a dynamic system) has been changed.


I had not thought of that. I will have to check that out.

Hi John,

Another method (that I have not tried, and may be apocryphal) is to
take large piece of tinfoil in your hand (the tin foil is gripped
around the line) and move it along the line while watching the SWR
meter.

The tin foil is used as a means to increase YOUR surface area to the
line because you are the major conductor that is moving. You have
diverted the Common Mode path away from where it was going, by moving
into your hand, through your body, to your feet, and ground.

One very sure method is to insert an additional length of transmission
line in series (will require a barrel connector). Some lengths are
more dramatic in the shift of SWR than others (again, this is about
wavelength).

This method has been reported to us here more than several times -
unfortunately the reporters hadn't connected the dots to recognize
this was a confirmation of CM.

It seems to me that I might get an indication of current on the
outside using a Grid dip meter as a detector at various points on the
transmission coax.

Get a split core ferrite. Snap it around the coax. Add a short length
of wire through the core. Connect the ends of the wire to an LED (now
a complete circuit). Move the core/LED along the line during
key-down. Did the LED glow?

Make sure to test the LED first to see if it glows (I can't imagine
why it would not). Test the LED after this survey to see if it glows
(I can imagine that CM current could have fried it). Play with adding
more turns of the wire through the split core.

You have a simple portable RF transformer/indicator.

73's
Richard Clark, KB7QHC
When TV was in in its infancy and in Ham TV we frequently used 300 ohm
twin lead transmission lines. The effect was dramatic when you wrapped
foil around the twin lead and moved it up an down the feedline. If a
half wave of line was accessible, You had the full range of tuning.

I also have a meter stick with a pair of wires on which I can slide a
shorting bar. The usual indicator is a 50 micro amp meter with a 1n34
germanium diode and either a dipole or sense loop attached. Nearly all
of that work was done at 440 mhz.

I will think about that LED indicator. Usually I keep the power low
when doing this sort of thing. The threshold required to trigger the
led. Also, once it triggers the abrupt transistion would probably
affect the operating conditions. Perhaps it is time to increase the
power a bit for such experiments...

John Ferrell W8CCW