HOW COMMON MODE CURRENT AFFECTS AN ANTENNA
( Dark Magic )
THE SHAMEFUL REALITY
Although many hams have heard of Common Mode Current, most hams have no idea what it really is and I have never met anyone who knew what all it does to our antenna system. Oh, I'm sure there are many who do know (i.e., K9YC, VK1OD, W7EL, W8JI, etc.), but in general, Common Mode Current remains a big mystery to most hams.
For the first 50 years of my ham radio career, I didn't give Common Mode Current much thought. Then in 2011, I found an Off-Center-Fed Dipole to be my best choice of antenna for this QTH. Within a very short time I awakened to the various problems Common Mode Current can cause us.
I began researching CM Current in books and on the Internet, but was very disappointed in what I read. Although it was described as a bad thing causing RFI issues, it seemed nobody went into any further detail describing what all it did and how much it can affect the antenna system.
It seemed it I wanted to find out about Common Mode Current, I would have to do it myself.
By chance I had met Steve Hunt, G3TXQ (SK) at a Hamfest (Rally) in England and become good friends with him. I also had a couple of friends who were engineers (David Cutter, G3UNA and Dr. Hartmut Hess, DJ1AT). These three helped me design and prepare for a Common Mode Current Field Test.
I spent the winter of 2012/2013 preparing for the Field Test, and then 3 full weeks working 12 hours per day executing the test. I learned things I had never heard of nor even dreamed of; and most of them were not pretty.
I will share my results with you on the next few pages.
First, let's take a look at my basic setup:
Although my concept was to test a dipole and OCFD antenna, I had to erect the antennas in the form of an INV-V antenna.
The setup on the left was always used. For many of the tests I inserted an additional length of coax at the position of the Barrel Connector to extend the coax to a half-wavelength.
All tests were run twice, first without the ground, then with the ground connected.
RF-wise, the radio was 'floating', ground-independent.
The AA-54 was also floating, unless I intentionally connected the ground strap.
"To Ground Or Not To Ground", THAT is the Question.
Although the test was conducted in the field, I ran the transceiver off of mains 234v a.c. It was important to isolate the radio RF-Wise from ground. This was accomplished in two ways:
- I used a switching power supply with the 13.8 v.d.c. output "floating" (both leads not attached to the chassis or ground.
- I followed the advice of Ian White, G3SEK and used RF Chokes in the a.c. mains lead and in the 13.8 v.d.c. lead. See photos in this section's Photo Gallery.
As you can see in the drawing above, the Ground Connection is shown as a dotted line. The Ground could be easily connected or disconnected by use of a large crocodile clamp.
Best practice dictates to always use an RF ground, but what effect does that have on Common Mode Current. Is it less or perhaps more; and why? WE SHALL SEE.