Common Mode Current  (CMC)

We speak of the transmission line as carrying the RF power to the antenna, but what it really does is enable RF Current to flow between the transmitter and the antenna.  In order for current to flow, we need a closed circuit; 2 wires.  Current flows to the antenna on one wire and returns to ground on the other wire.



Coax has two physical wires, its center conductor and its shield, but due to "Skin Effect", the shield itself is actually 2 wires: the inner surface of the shield and the outer surface of the shield. 

Effectively, at HF frequencies, the coax has 3 wires.



RF current flowing to the antenna flows completely inside of the coax because the unbalanced output of the transmitter feeds 100% of the current into [the inside of] the coax.



In a perfect world, it looks like the drawing below:



Perfect (Dream) World

The drawing on the right shows what we would like to have.  In order to have current flowing, we need two paths or streams; the second stream being the return path to ground.



At any point along the two inner conductors (center conductor, plus inner surface of the shield), the current flowing in the two conductors is equal in magnitude, but opposite in polarity.  For this reason, it is called Differential Current.



An inherent feature of coax is that what is flowing on the inside is not affected by whatever is going on the outside around it.



When the RF reaches the dipole antenna, ALL of the RF flows into the antenna and is radiated - except for the tiny bit of power lost in the d.c. resistance of the [wire] legs of the dipole.

Also, in a perfect world, every year is a sunspot maximum and there is no QRM.  Right!

Imperfect (Real) World

The drawing on the right shows what happens when we connect the coax transmission line directly to the dipole, without the use of a balun. 

RF from the transmitter flows inside of the coax until reaching the legs of the antenna.  At the point where the legs of the antenna connect to the coax, the RF from the transmitter sees two paths to ground:

1. The path inside of the coax.
2. The D.C. path along the outside surface of the shield.

At his point the RF splits, and then:

• part of the current flows through the antenna's arm and is radiated as  it should be;

• and part of the current flows along the outer surface of the shield, back to ground. In doing so, it does what RF Current always does when flowing along a wire: it radiates.                 



The coax has become part of the antenna.

When CMC flows on the coax:

Some hams "claim" that RF radiation from the coax is a good thing, that it even improves the DX capabilities of the antenna.  HUMBUG!  It is actually very bad for many reasons.

Here's what really happens:

When RF flows on the outer surface of the coax, the coax has effectively become part of the antenna.  This not only changes the overall length of the antenna, it also modifies its configuration.

As a result of this change in length, the antenna resonance also changes.  Most people confuse "resonance" with the frequency at which the minimum SWR occurs (call it "SWRmin").  This of course is wrong; resonance and SWRmin are two different things.   However, SWRmin also changes when CMC is flowing on the coax.

Other negative effects of CMC on the coax:

• The entire coax radiates RF power, which may cause all sorts of problems with the consumer products in your house and possibly the neighbor's house.

• CMC on the coax often increases the noise level on the Receiver.

• You get bogus readings on an antenna analyzer.

• On OCFD Antennas, the frequency of SWRmin skews up the band (on the fundamental band).

• Normally it only skews on the fundamental band, not on the higher harmonic bands.  This is because the level of CMC is typically a magnitude greater on the fundamental band than on the higher harmonic bands. (Source: DJ0IP's 2013 CMC Test)

• The shape of the SWR curve across the band begins to flatten with CMC, and its dip broadens out when the CMC increases. 

• This is one reason why many people believe the bandwidth of the OCFD is much broader than that of a common dipole!  Without a very good balun at its feedpoint, the OCFD induces CMC onto the coax, and the SWR flattens/broadens.   

• Worst case, with lots of CMC on the coax, it may cause a double-dip in SWR.  Though this may sound good, when this happens, you have a significant amount of RF power flowing along the outer surface of the coaxial shield.

• When measuring the SWR and other parameters, if you touch the coax connector with your hand, the reading on the analyzer changes.

• This is also why you often see a different reading on the analyzer than on the station SWR meter (when connected to the station, there is a path to ground).
• Inserting a line isolator between the coax and the analyzer and grounding the coax side of the shield will usually produce the correct measurement.

• The level of SWR and other characteristics change noticeably with small changes in the length of coax between the transmitter and the antenna, even when the antenna is showing a low SWR.

• This is why some OCFD Antenna manufacturers specify certain lengths of coax to use, or lengths to avoid.
• This can get confusing because on bands where the level of SWR is somewhat high to begin with, it produces the same effect as having CMC on the transmission line.

REMEDY:  Fix the BALUN



According to Roy Lawallen, W7EL, in his excellent paper on baluns, entitled  BALUNS: What They Do And How They Do It,  first published in the ARRL Antenna Compendium, Volumn 1, in 1985 - page 157, the primary purpose of the balun is to impede the flow of RF current along the outer surface of the shield, thus maintaining a balance in the Differential Current inside of the coax, and preventing Common Mode Current from flowing outside of the coax.



The ability of a balun to impede the flow of Common Mode Current along the outer surface of its shield is dependent on the amount of Common Mode Impedance (CMI) that the balun exhibits on the transmitted frequency.  Baluns built to the wrong design (i.e., a voltage balun), or with inadequate components (i.e., wrong choice of ferrite mix in the Toroid, or insufficient turns count of it transmission line(s) will have insufficient CMI and must either be replaced, or a good RF Choke must be attached directly between the coax and the balun.



The most common cause of a bad balun:  nearly all manufacturers of OCFD antennas use a "single-core" 4:1 Guanella (current) balun.  Many balun manufacturers also recommend this balun.  Unfortunately:

This single-core 4:1 Guanella Balun DOES NOT WORK AT ALL.

This has been well documented by:  G3TXQ, W8JI, VK1OD, and of course myself (DJ0IP).

Back to Common Mode Current Section