On Sun, 03 Apr 2011 10:40:45 -0700, Richard Clark
wrote:

On Sun, 03 Apr 2011 10:19:34 -0400, John Ferrell
wrote:

What I need to know is
1- When to use them. RF in the Shack is pretty obvious

Hi John,

Sometimes it is not always that obvious, but it can be made obvious. I
say this because the symptoms do not always draw our attention to the
problem arising from Common Mode (CM) current.

One possible obvious symptom is that you can feel the current on the
shielded components of your system - like a microphone shell when your
lip brushes along it (which can sometimes give a nasty shock); or when
you pull a connector and get that tingle of feeling (which might kill
you if conditions were worse). "Obvious" can vary from subtle to
deadly.


Been there, done that...
although my particular case was trying to cathode modulate a home brew
final about 50 years ago...

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.

* * * * * * * * *

Let's set aside in-the-shack symptoms and look at the antenna end of
what might be obvious.

For beam antennas, through careful testing with known remote stations
(beacons, for instance) you find that you have high sidelobes, almost
no back rejection, and a broad response off the front. The
transmission line coming down is acting like a vertical antenna, and
is feeding its drive into the antenna feed point (two antennas in
parallel). That extra antenna is picking up stations that would
normally be suppressed by beam design.

Some folks never notice this - and it shows how benign CM can be (it
is there full blown, but it is NOT obvious at all).

For wire antennas, you cut and cut and trim and trim your antenna to
frequency, but even though it is book perfect (using the usual
formulas for length), it is wildly out of whack and a bitch to tune
with your tuner. Your dipole without choking is actually a tripole:
two horizontal elements with a third vertical element (which is
variable in length and relationship to ground/environment as it swings
in the wind). You raise it higher to see if that does anything, and
the tune shifts to the moon because the third element has gotten
longer.


You have just described my recent attempt on 160 Meters.
I will put it on my "TO DO" list and try again.

Again, some people never notice this, we have seen them; others do
notice it and fulminate against the dipole as being a useless
invention where their endfire (which suffers the same issues, but is
magical by comparison) antenna is the cat's meow.

More could be said....

2- When not to use them. There must be some negatives.

That is a natural question, but I cannot think of any down-side at
all.

3- "Compare and Contrast" the Inductor-capacitor choke with the
Ferrite Choke(W2DU). Are they electrically equivalent?

They are NOT electrically equivalent. The ARE functionally equivalent
- that is, they both suit the same purpose of introducing a very hi Z
into the current path of CM.

The coil of transmission line is, on first pass approximation, an
inductor whose Z climbs with applied frequency. The presumption is
that it can only get better for the higher bands. This works to a
point, but the same coil of TL also has loop-to-loop capacitance, and
end-to-end capacitance. These two capacitances, at some frequency,
become resonant and offer you a tank circuit. This works even better
to isolate the current at the drive point from the rest of the line.
However, a tank is not a wide band device by definition. It can
exhibit an extremely hi Z at one frequency, and often high enough Z in
neighboring bands (hence it is useful if you understand this). As you
go higher in frequency, and for the very large coils suggested
earlier, then the capacitances begin to dominate and actually kill any
sense of choking.

I thought this was a likely scenario but you are the first I have
encountered that passed that along. Somewhere I read about a fellow
who was using spool of coax in the shack for a choke on 160M...
Well intentioned, but clearly not really a solution!


The ferrite choke is a resistor, plain and simple. This means it is
wide-banded - although its frequency characteristic is not universally
applicable (this widebandedness usefully covers two, maybe three
octaves). The formulation of the ferrite determines the coverage, and
so you can either throw more beads into the mix, or use beads of
different formulations to create a more flattened frequency response
(over decades of frequency change).

4- Are either of them a multi band solution?

Yes, as described above.

5- Are there Upper and lower frequency limitations for using them.

That too is described above.

As usual, a Google search provides an overwhelming set of responses
that most likely will cover the subject in greater detail than I can
digest.

Early in the list provided by the search I discovered
http://www.audiosystemsgroup.com/RFI-Ham.pdf

That may have all the answers but I have only glanced at the beginning
of the 66 page document.

It is a very good and broad discussion. The downside is its enormity.
However, this is not a simple topic and understanding comes slow.

The reason for that is that Common Mode analysis as distinct from
Differential Mode analysis immediately brings up the specter of WTF?
There are TWO currents flowing at the same time in the same circuit?

This perspective is usually very foreign and takes deep meditation to
come to grips with. It is difficult for the majority of the Pros too.

I am going to prompt readers for the next question:
"What is common mode current?"
If any one cares to indulge me by asking, we can go further, but the
material in this posting is enough to digest in one sitting.


It is you who are indulging me!

Google and other Internet resources are very helpful but they are a
poor substitute for direct answers to direct questions.

As the sailor told the Captain, "I won't abandon the ship until the
keel comes through the foredeck!"

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.

73's
Richard Clark, KB7QHC