CMC TEST GOALS
THE MAIN TASK of this test is to determine the impact of Common Mode Current on typical wire antennas such as the DIPOLE and OCFD, and to learn more about how to address the issues it causes.
THE FOCUS will be on "what is required" and "how much is required." to cure the problem.
THE GOAL is to identify cost-effective solutions to CMC problems when running 100W or less, rather than suggesting to "always assume "worst case" and buy or build the best solution available", which is then shown for a 1500W application.
ASSUMPTIONS TO BE INVESTIGATED:
1. The farther away the feedpoint is located from the center of the antenna, the more CMC trouble it will have.
2. In cases of severe CMC, not only is the SWR reading affected, the “apparent” resonant frequency can be affected considerably. My hunch is that CMC is causing skewing of the measurements and must be dealt with before we can achieve an accurate reading on the analyzer.
3. CMC trouble on an OCFD is by far the worst on its fundamental band and hardly present on its harmonic bands.
4. A major reason for the bad reputation of the OCFD is that initially people were using a voltage balun rather than a good current balun.
5. The second major reason for the bad reputation is, many that do use a current balun are using a single-core 4:1 Guanella balun (which does not work), rather than using a dual-core balun with each transmission line wrapped on its own dedicated core(s).
6. Incorrect routing of the coax away from an OCFD or Dipole will induce more CMC into the feedline than is being generated at the feedpoint of a well designed OCFD. As a result, the problem cannot be fixed at the feedpoint, but must be addressed farther down the feedline.
7. The W2DU (Maxwell) choke or choke-balun is insufficient for blocking CMC. The Guanella balun (or choke) wrapped on a
toroid is much more effective. (true or false?)
......a) Even if this is true, how much difference is there?
......b) Are there cases where the W2DU is sufficient?
......c) If so, when do we need to switch to a toroid solution?
8. Does the set
of 3 low cost RF Chokes (Low Bands, Mid Bands, High Bands) suggested by GM3SEK in RSGB's 'Radcom" magazine (based on the ideas put forth by W2JVN) work as well as K9YC’s [more expensive] RF
Choke solutions as printed in “THE ARRL HANDBOOK”.
......a) If not, where is it useful and when must we switch to the more expensive solution K9YC suggests?
9. Using twisted pair wire to wind baluns simpler and is good enough for hf bands; there is no huge benefit in using bifilar windings, which are much harder to wind. In practice, across the hf bands, you won’t see any difference between the two methods in: a) the SWR curve, b) CMC suppression, or c) the signal reports received on the air.
10. I have found that using matched cores (close tolerance permeability) in a dual-core Guanella balun improves the balun's SWR curve across the hf bands, particularly above 25 MHz. Since I know of no way to easily identify matched cores, I wind several, measure the impedance of each, and select two with the same or nearly the same inductance. I assume this will improve the CMC supression.