Good stuff Chuck.
One very important thing to remember is that the common-mode current "sees"
a transmission line of unknown impedance. One conductor is the outside
surface of the coaxial line, and the "other" surface (could be multiple) is
whatever conductor is nearby...side of a house, fence, earth surface, etc.
So you can get all sorts of interesting impedance transformation
issues. For example, the "common" common-mode choke consisting of ferrite
beads surrounding the outer conductor of a coaxial line would "like" to see
a short circuit "behind" it...so that you have a large-ish series impedance
followed by a shunt short circuit...maximum rejection of signal. If the
coax is grounded/bonded 1/2 wavelength from the choke position, that will
do the trick...at that frequency (and all multiples). But at a frequency
where the grounded coax shield point is 1/4 wavelength away from the choke
point...the choke is much less useful. I have personally measured this
effect using a setup similar to yours.
Back in my DoD days when doing wideband HF engineering/testing, we would
always bond the coaxial shields to ground (where we could) at least every
1/10wl (at the highest frequency of interest) between exciter and
antenna. This kept the common-mode impedance low and greatly aided the
effectiveness of common-mode chokes used.
Unfortunately I have never done a CM/DM measurement like you did; actually
30dB sounds pretty reasonable to me at HF, but I have no basis for that
At 19:42 2003-06-25, Chuck Counselman wrote:
>At 1:32 AM +0000 6/26/03, Howard Klein K2HK wrote:
>>This is a very interesting question.
>It's nice to hear that someone else thinks it's interesting. :-)
>>I imagine that we haven't heard more about it is because the average ham
>>does not have the equipment to make the measurements.
>I agree. I probably would not have done it myself, had I not been able to
>borrow the instruments.
>>Some good data from a large sampling might lead to some interesting and
>As you've probably guessed, I suspect that most hams have very little (in
>other words, very poor) common-mode isolation. Here's one example of why
>I think so:
>Recently I visited a ham who is Amateur Extra, has two degrees in EE from
>MIT, and is an active and accomplished HF DX contester. He lives in a
>more rural neighborhood than mine; his neighbors' houses are relatively
>far from his; and I saw no electric power line above ground near
>him. Yet, his received noise level on the lower HF bands, at mid-day when
>very little noise arrives via skywave, was nearly 50 dB greater than
>mine! He also had an RFI problem in his house, despite transmitting only
>IMO, much power-line noise was being coupled from his house wiring into
>his antenna, and that much of his transmitted power was being coupled into
>his house wiring, via common-mode current on his transmission line -- even
>though he was using a popular brand of common-mode choke, or "balun." [I
>don't claim ('cuz I don't know) that this is the whole story; I have not
>yet had an opportunity to return to his QTH with instrumentation to
>>I would be quite interested in seeing where this goes. One question. How
>>do you have your tuner 70 ft away from your shack? Is it remotely tuned?
>To minimize loss, I wanted to run open parallel-wire line all the way from
>the feedpoint of my doublet to my shack; but I calculated that there'd be
>too much coupling between such a line (even a four-wire line) and the
>various other cables that it would have to pass near. So I ran
>parallel-wire line as far as I could; put a balun and an antenna coupler
>there; and ran coax the rest of the way. I put the antenna coupler there
>to minimize loss in the coax, by reducing the SWR. This coupler was
>indeed remotely tuned.
>Then I was able to find one _fixed_ setting of the tuner for which the SWR
>on the coax (and therefore the loss) was tolerably low across _all_ HF
>bands 80 through 10 meters, except 30 meters where I don't transmit much
>power anyway. Since then I've left the tuner fixed. Now QSYing is much
>faster. I intend some day to write up and publish my method for finding
>such a magic setting. I've found them for various line lengths, different
>tuners, different baluns, etc.; but have not yet found a general
>(never-fail) solution. However, I believe that there's hope.
>73 de Chuck, W1HIS
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>any questions and ask for Sherman, W2FLA.
>TowerTalk mailing list
Scott Townley NX7U
Gilbert, AZ DM43di