Common Mode (CM) Currents

As is the case with a common light bulb, an antenna system has to have two terminals in order for current to flow.  In the case of an unbalanced antenna such as a vertical or end-fed antenna system, the first terminal goes to the main radiator and the second terminal goes to a counterpoise or radial.   A difference in the current flow between the two will cause Common-Mode (CM) differential current to flow, usually along the outside of the feedline coax.  Once conducted on the outside of the coax feedline shield, the CM current can conduct to your rig, you, and everything attached to the rig including the power supply.

All AC mains operated power supplies have capacitive coupling between the AC mains input and DC output.  The CM current can couple through this path and effectively couple the noisy AC mains into the antenna system.  One of the prime design goals of the PAE-Kx33 Low-RFI Power Supply was as low AC input to DC output coupling capacitance as possible.  This was achieved, and at less than 70 pF, it is 14 times lower than our nearest competitor at ~1000 pF, making it much easier to choke this CM current when using the Kx33.

This CM current is present in both receive and transmit; in receive it allows the coax shield to pick up noise along it’s length and introduce it into the receiver, as well as disturbing the antenna’s intended pattern.  In transmit it will make the feedline coax radiate disturbing the pattern as well as radiating RF in unintended places and setting off alarms, ruining TV and or FM radio reception…

The best, but sometimes difficult to implement way to deal with CM currents is to design an antenna system to have as little CM as possible.  With many portable antennas such as some end-feds, it can be extremely difficult to balance the radiator and counterpoise currents.  In all cases it is best practice to have a good CM choke at the antenna feedpoint to block CM currents from the outside of the coax shield.  A good CM choke will effectively insert a high impedance in series with the CM currents, blocking them from the outside of the coax shield.

Perhaps the easiest way to demonstrate how a CM choke places a high-impedance in series with the CM potential is to see and hear it.  In this video linked below an FT-817 is being powered by a Pro Audio Engineering Kx33 Power supply, with a 18″ whip as an antenna ~3 feet away from the Kx33.  There is no counterpoise so by definition the chassis and anything attached to it like a ground connection or power supply will be used by the antenna system in an attempt to balance the counterpoise currents.  This is worst-case for developing a CM potential on the chassis of the rig.

In the video I repeatedly open and close a mix 31 Fair-Rite Snap-It core over 4 turns of the DC power cable.  If the noise heard was radiated, then choking the DC cable would not affect it.  If the noise was transverse (superimposed on the DC) then this choke would hardly affect it, but since the RF noise was CM, the ferrite knocked the noise down from an S3 to S1, the same level as when the battery was powering the rig every time the choke was snapped closed:

FT-817 + Kx33 CM noise web

We offer Fair-Rite mix 31 ferrite cores in many sizes and two styles: split cores and toroids.  You can see our selection here:

Fair-Rite Mix 31 Ferrites