Single Core Vs. Dual-Core 4:1 Guanella
The results of this particular test were surprising, even shocking.
This is probably going to burst a lot of people's bubbles.
Please keep in mind that the amount of common mode current on a transmission line varies depending on the point you measure it. The best case (point of least CMC) is one quart wavelength (and odd multiples thereof) away from the feedpoint. the worst case is one half wavelength (and all multiples thereof) away from the feedpoint.
CAREFUL: although this is coax, we cannot use its normal velocity factor to determine the quarter and half wavelength points. That VF only applies to RF current flowing INSIDE the coax. CMC flowes only on the OUTSIDE of the shield of the coax. The VF here is similar to copper wire, or about 0.96 to 0.98.
In addition, CMC is minimized when the coax is run perpendicular away from the antenna, and increases as it is run diagonally towards one of the sides; the closer to the leg (smaller the angle between the leg and the coax) the higher the CMC on the line.
When conducting my tests, I measured:
- using the theoretical best case (1/4 wavelength) and worst case (1/2 wavelength) of coax
- with the coax first running 90 degrees (perpindicular) to the antenna (best),
- and then I used the worst case length and skewed the coax such that it was positioned at an angle of about 45 degrees to one side of the antenna - forcing Common Mode Current.
What I saw was even worse than I had imagined it might be. (NOTE: this is one of the test which W7EL* said needs to be conducted. I did it.)
*W7EL's excellent paper on baluns: Baluns: What They Do And How They Do It.
DOWNLOAD THE RESULTS BELOW:
This document compares a single core and dual core 4:1 Guanella balun when used in an OCFD antenna with various lengths of coax and two different methods of running the coax.
Comparison of dual vs single core balun.[...]
PDF-Dokument [404.8 KB]