I have been faithfully using a coil of coax as a common mode choke for
a while without any idea if it is really doing any thing.

Googling turns up more url's than I can follow. I would appreciate any
suggestions as to how might measure the effects of the common mode
choke. 30 to 17 meters is my favored range of operation. I have a
pretty well equipted shack.
John Ferrell W8CCW

On Oct 8, 3:14*pm, John Ferrell wrote:
I have been faithfully using a coil of coax as a common mode choke for
a while without any idea if it is really doing any thing.

Googling turns up more url's than I can follow. I would appreciate any
suggestions as to how might measure the effects of the common mode
choke. 30 to 17 meters is my favored range of operation. I have a
pretty well equipted shack.
John Ferrell W8CCW

http://www.k1ttt.net/technote/airbalun.html

On Oct 8, 10:14*am, John Ferrell wrote:
Googling turns up more url's than I can follow. I would appreciate any
suggestions as to how might measure the effects of the common mode
choke.

This web page might save you some time and effort.

http://www.k1ttt.net/technote/airbalun.html
--
73, Cecil, w5dxp.com

On Fri, 08 Oct 2010 11:14:13 -0400, John Ferrell
wrote:

how might measure the effects of the common mode

Take one piece of wire;
loop it into a half-dozen loose turns (1" diam);
connect a Red LED to the wire ends;
place loop around coax;
slide it along the coax for a distance of something more than a
quarterwave length of the frequency under consideration.

How bright did the LED get anywhere along that length?

73's
Richard Clark, KB7QHC

John Ferrell wrote in
news
I have been faithfully using a coil of coax as a common mode choke for
a while without any idea if it is really doing any thing.

You would not be alone. They seem to be the most popular suggestion for
dealing with common mode current issues, but I suspect it is often a case
of "fools rush in where angels fear to tread". The anecodotal support for
Ugly Baluns is not convincing, and quantitive measurements explain why.

They are cheap, and it is the only factor I see consistently shaping
recomendations, cheap rather than effective in the application. Of
course, doing nothing is cheaper, and better value if it is as effective!

Googling turns up more url's than I can follow. I would appreciate any
suggestions as to how might measure the effects of the common mode
choke. 30 to 17 meters is my favored range of operation. I have a
pretty well equipted shack.

Well, I better not give you any more references to follow.

The so called 'Ugly Baluns' are so popular, and peform so poorly. They
achieve very high choking impedance around a narrow region about self
resonance. If you have a narrowband application (eg single band antenna)
AND arrange that self resonance at the band of interest, then they are
capable of excellent peformance (very high choking impedance).

If your need is broadband operation, look to ferrite cored baluns.

As far as measuring common mode current goes, people have described
common mode current meters, and some are available commercially. The
problem is that common mode current is a standing wave, and a single
measurement (at a single location) says very little.

The indirect effects of common mode current are not so easy the
'measure' as such, but you can asses the impact by things like
sensitivity of line VSWR to line length and placement / routing, RFI to
ham shack gear, other household equipment, neighbors equipment etc. You
might also try to assess degradation of ambient noise due to common mode
current. Distortion of transmitted pattern is more challenging to assess.

Owen

On Fri, 08 Oct 2010 11:14:13 -0400, John Ferrell wrote:

I have been faithfully using a coil of coax as a common mode choke for a
while without any idea if it is really doing any thing.

Googling turns up more url's than I can follow. I would appreciate any
suggestions as to how might measure the effects of the common mode
choke. 30 to 17 meters is my favored range of operation. I have a pretty
well equipted shack.
John Ferrell W8CCW

I have had some success with short pieces of copper tubing slipped over
coax. however, I have no way of checking it to measure how effective this
is

On Fri, 08 Oct 2010 11:14:13 -0400, John Ferrell wrote:

I have been faithfully using a coil of coax as a common mode choke for a
while without any idea if it is really doing any thing.

Googling turns up more url's than I can follow. I would appreciate any
suggestions as to how might measure the effects of the common mode
choke. 30 to 17 meters is my favored range of operation. I have a pretty
well equipted shack.
John Ferrell W8CCW

Here is one of the better papers I've found on this subject.

www.audiosystemsgroup.com/RFI-Ham.pdf

Danny, K6MHE

On Fri, 08 Oct 2010 11:14:13 -0400, John Ferrell
wrote:

I have been faithfully using a coil of coax as a common mode choke for
a while without any idea if it is really doing any thing.

Googling turns up more url's than I can follow. I would appreciate any
suggestions as to how might measure the effects of the common mode
choke. 30 to 17 meters is my favored range of operation. I have a
pretty well equipted shack.
John Ferrell W8CCW
Thanks for all the replies.
Some I have already seen and some are new to me.

Now to digest them and try a Lab project or two...
John Ferrell W8CCW

John Ferrell wrote in
news

Now to digest them and try a Lab project or two...
John Ferrell W8CCW

The challenge is to read the material and lose the stuff that is just
plain wrong. When you find content that is wrong, it damages the
credibility of the whole paper.

To demonstrate how poorly common mode current is understand, the
following is a quote from MFJ's web site page describing their MFJ-835
twin needle "RF IN-LINE CURRENT METER, BAL.LINE, 1-30MHZ, 3AMP":

"How do you detect imbalance in open-wire or ladder line? By measuring
and comparing each feedline wire current -- your feedline is balanced if
the currents are equal."

Well, the line is balanced if the currents are equal in magnitude and
opposite in phase (ie 180° out of phase). The MFJ-835 compares ONLY the
magnitude, and it is true that if there is zero common mode current, a
properly calibrated MFJ-835 will indicate balance, but it is NOT true
that if the MFJ-835 indicates balance (equal magnitudes) that there is
necessarily zero common mode current.

You will find articles on the net where authors have expounded the
perfection of their station using the MFJ-835 as proof of zero common
mode current, some even explaining how they continuously monitor balance
(apparently at a fixed point in the line). The instrument is not suited
to the job, and their results are flawed. The articles would make a good
read if they weren't flawed.

Owen

On Sat, 09 Oct 2010 20:26:29 GMT, Owen Duffy wrote:

John Ferrell wrote in
news

Now to digest them and try a Lab project or two...
John Ferrell W8CCW

The challenge is to read the material and lose the stuff that is just
plain wrong. When you find content that is wrong, it damages the
credibility of the whole paper.


Yes.

I think the write up on K1TTT's site will satisfy my curiousity and
find its way into my Permanent Reference File...
I still need a little more time.

BTW, I think my next lesson will be to understand Hairpin Matching. I
plan to start with the page on your website. If I get stuck I will
bring it back here!

So many fun things to learn, so little time!

John Ferrell W8CCW