Common Mode Currents
Common mode currents
If there's anything that you as an Amateur Radio Operator should be concerned about,
it's definitely Common Mode Currents.
A lot of what is found below comes from this excellent article by Bill Leonard (N0CU)
Did you know that coaxial cable supports 3 separate current paths? It does, and the image above illustrates those paths. We all know that RF power flows out of our transmitters as an AC signal along the center conductor and the INSIDE of the coax braid. This is due to the fact that at HF and above, AC signals flow on the surface of the conductor and not through the center of the conductor. So, how does current end up on the outside of the coax braid? The simple answer is that common mode currents are caused by imbalances in the antenna or through antenna to feed line coupling. An imbalance in currents on each half of an antenna can be caused by many different factors, but even perfectly balanced antennas (dipoles) can have common mode currents flowing on the feed line due to near field coupling or a favorable path to ground through the feed line. RF power will flow down the outside of the coax braid IF the impedance at the feed line / antenna junction is MORE than the impedance of the outside braid looking back toward the radio. Depending on feed line length and signal frequency, the path back down toward the rig (and ground) may have a lower impedance than the radiation resistance of the antenna itself. This often happens when feed line length is at or near 1/2 wave length for the current operating frequency. In a nutshell, common mode currents are complicated and there are many engineering papers that have been written on the subject.
How do we resolve this problem? Here's where the impedance transformation of the feed line comes into play. If the feed line is exactly 1/2 wave length long, the impedance of the line, as viewed by the transceiver, is exactly the same at that of the far end connected to the antenna. If our feed line is a 1/4 wave long, the impedance at the antenna end of the feed line would be significantly different and significantly higher than that of the transmitter end. Thus, energy from the center conductor and inside of the braid does not view the path back to the transceiver on the OUTSIDE of the coax as better than that of the impedance of the antenna itself. Great! But being that Radio Amateurs want to use more than one band on a single antenna and a single feed line, how do we abide by the 1/4 wave rule? Well....read on....
Just an additional note on this. Coax is not the only transmission line subject to common mode current issues. Common mode currents can run along ladder line just as easily. This is one reason why ladder or window line should be measured out in odd multiples of 1/4 wave lengths for a specific band. You can immediately see the problem with this if we're attempting to operate on multiple bands using the same antenna and feed line. What may be 1/4 wavelength for one frequency may very well be 1/2 wavelength for another..... Thus the plight of the Amateur Radio Operator.
What is Common Mode Current
Common mode current is current that flows unimpeded by an equal magnitude current flowing in the opposite direction. At any point in time, current flowing in one direction on one side of a ladder line is acted upon by an equal and opposite current flowing on the other line. That is, electro-magnetic radiation formed by one flow of current is cancelled out by the equal and opposite electro-magnetic radiation formed by the opposite flow of current. In an antenna, an opposite flow of current does not exist and thus the electro-magnetic field is free to radiate out into space. In fact, common mode currents cause your antenna to radiate!
What's so bad about Common Mode Current?
Common mode current can disrupt how your antenna tuner works, it can elevate voltages on the surfaces of your radio, and it can cause EMI that will affect your TV, radio and kitchen toaster. If you read my introduction about my first antenna and my dismay that the key, microphone, and set screws in the knobs of my radio would bite me every so often, this is why. My first antenna used coax to connect my 40 meter dipole to my transceiver. I didn't even THINK about how long the transmission line should have been and I had never even seen an antenna tuner up to that point. Because of the difference in impedance between my antenna and my coax outside braid, current would flow back down the coax and right onto the case of my Eico 720 transmitter. I learned to keep my fingers off of the exposed metal surfaces of my Radio Shack Japanese J-38 knock off key. I still have that key 44 years later....
Common Mode Current and Reality
So, as mentioned above, Radio Amateurs want to use one antenna and one feed line for more than one band. The reality of common mode current is that it will find a path back to your station if the conditions are right. This is especially true when operating a multi-band wire antenna on a good range of frequencies with the exact same feed line. Also, keep in mind that common mode currents can come from other places. For instance, if your feed line parallels your wire antenna for any length of run, the antenna can couple to the outside braid of the coax, or ladder line, which will result in CMC in your shack. This is why you should always run your coax perpendicular to your antenna as much as possible.
How to stop Common Mode Current
The best way to stop common mode current is to add a 1:1 current balun or RF choke (or both) to your transmission line. Typically a 1:1 current balun/transformer is used at the antenna end of the transmission line to link an unbalanced transmission line (coax) to a balanced load (dipole). In this arrangement, the balun guarantees that equal and opposite currents are present on each half of the antenna. Any common mode currents flowing back toward the feed line at the feed line to antenna interface see the 1:1 current balun as a high impedance and thus stop common mode currents from flowing. Since common mode currents can also flow due to near field coupling with the feed line, an RF choke is an appropriate device to place as close to the antenna tuner or transmitter itself. RF chokes are constructed by either wrapping the coaxial feed line through a large toroid, or by adding individual toroidal beads to the outside of the coax. This bead method is known as a W2DU style RF choke. To be clear, RF chokes on coaxial line ONLY affect currents running on the outside of the coaxial shield. The inner conductor and inside of the coaxial shield are completely invisible to the to toroidal core, coil or beads.
Note, that toroid selection is fairly important here in that saturation and heating of the toroid can become significant if the wrong material is chosen. Toroid manufactures provide data sheets for their toroids that suggest operating frequency ranges. Choose a toroid or set of beads based on the frequencies that you want to operate on.