You’ll often hear radio amateurs discussing unwanted RF currents and the mysterious problems they can cause. But what exactly are these currents? Where do they come from, how do they find their way into our equipment, and why can they cause so much trouble?
To help shed some light on the subject, I’ve gathered some useful information about common mode currents. Hopefully, this will help explain what they are, why they occur, and what can be done to reduce or eliminate their unwanted effects.
Common mode currents are unwanted electrical currents that flow in the same direction along two or more conductors, or along the outside of a coaxial cable’s screening braid. They are important in radio communications, aerial systems and amateur radio because they can cause interference, unwanted radiation and problems with transmitter performance.
Common mode versus differential mode currents
In a normal radio aerial feeder, the wanted RF currents flow in opposite directions.
- Differential mode: Equal and opposite currents flow along the two conductors. This is how RF power is normally delivered to an aerial.
- Common mode: Currents flow in the same direction along conductors, or along the outside of the coaxial screen. These currents can turn the feeder itself into part of the radiating aerial.
How common mode currents occur
In a dipole aerial fed with coaxial cable, the RF current flows up the coaxial centre conductor and returns along the inside surface of the coaxial screen. However, if the aerial is unbalanced or its feed arrangement is asymmetrical, RF current can also flow along the outside of the coaxial braid. This is a common mode current. The coaxial cable then behaves as an additional, unintended part of the aerial.
Common causes include:
- Feeding a balanced dipole directly with unbalanced coaxial cable.
- Poorly designed or incorrectly installed baluns.
- Asymmetrical aerial arrangements.
- RF coupling from the aerial into nearby feeder cables.
- Inadequate RF isolation between the transmitter and aerial.
What problems do they cause?
- RF interference: Transmitted signals can enter nearby televisions, audio amplifiers and electronic equipment.
- RF in the shack: Microphones, metal cases and other equipment may carry unwanted RF.
- Altered radiation patterns: The coaxial feeder radiates as part of the aerial, changing its directional characteristics.
- Increased received noise: The outside of the feeder may pick up electrical interference from household equipment.
- Unpredictable SWR: Changing the feeder routing or length can alter the aerial system’s impedance.
How can they be prevented?
The most common solution is a common mode choke, often called a current balun or a 1:1 choke balun. A common mode choke presents a high impedance to RF current flowing along the outside of the coaxial cable while allowing the wanted differential RF signal to pass with little loss.
Two typical arrangements are:
- Ferrite choke: Coaxial cable is wound through or around a suitable ferrite toroid.
- Ferrite sleeves: Ferrite cores are fitted around the coaxial feeder to impede common mode currents.
The ferrite material and number of turns must be selected for the radio frequencies concerned.
An important point about coaxial cable
Although coaxial cable has only two physical conductors, RF currents can flow along three distinct paths:
- The centre conductor.
- The inside surface of the outer screen.
- The outside surface of the outer screen.
At radio frequencies, skin effect helps separate currents on the inside and outside surfaces of the screen. The current flowing on the outside of the screen is the one of particular concern in common mode interference. This is why a coaxial feeder can radiate RF energy even when the cable itself is correctly screened.