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