COMMON MODE CHAOS

Common Mode Current (CMC)

 

 

 

 

 

The BEAST

 

that Bit

 

My COAX


 

 

(This Monster is for REAL)

WHAT IS COMMON MODE CURRENT?

WHAT CAUSES COMMON CURRENT?

WHAT ILL EFFECTS DOES IT HAVE ON MY STATION AND MY SIGNAL? 

WHERE CAN I FIND MORE INFORMATION ON HOW TO DEAL WITH THIS?

 

In a Perfect World, all of the power from our transmitter would be radiated from the antenna into the ether and received loud and clear in some far off corner of the world.

 

In the Real World, we have some realities to contend with:

  • Transmission Line Loss
  • Antenna Match Loss (SWR) which adds to the Transmission Line Loss
  • Ground Loss (different from one QTH to the next)
  • Common Mode Current (CMC)

 

This section is about CMC

 

 

WHAT IS COMMON MODE CURRENT?

 

Common Mode Current is UNWANTED current flowing on the outer surface of the coax shield.

 

 

WHAT CAUSES COMMON MODE CURRENT?

 

Common Mode Current may be caused by differnet things:

  • An imbalance in the antenna system (antenna + feedline).
  • External signals being picked up on the shield of the coax
    • RFI from consumer devices
    • RF from other transmitters (broadcast, ham radio, anything)

 

OK, SO WHAT CAUSES AN IMBALANCE IN THE ANTENNA SYSTEM?

 

An imbalance in the antenna system can be caused by several things.  We can control some of these things but often, some of the things causing the imbalance are beyond our control.

 

Things within our control:

  • Using no balun with a coax-fed balanced antenna, such as a dipole.
  • Using the wrong type of balun.
    • This is a very common cause.
  • Using a poor quality balun.
    • There are MANY of these on the market.
    • This is a very common cause.

Things we may not be able to control:

  • Objects in close proximity of our antenna, and typically closer to one side than to the other.  These objects include buildings, trees, metal objects, and even other antennas.
  • Characteristics of the earth surrounding the antenna.  In some cases it may be different on one side of the antenna than on the other.

 

WHAT ILL EFFECTS DOES CMC HAVE ON MY ANTENNA, MY STATION AND MY SIGNAL?

 

On the Antenna:

  • It can skew the radiation pattern.
    • This is especially critical with a beam (Yagi) antenna.
  • It can affect the SWR of the antenna.
    • It can cause high SWR.
    • It can shift the "apparent" point of minimum SWR.
    • It often makes SWR dependent of the length of coax.

 

On the Station:

  • It can cause RFI in the shack.
    • Can causes burning of the fingers on the key.
    • Can causes burning of the lips on the mic (Hot Lips!)
    • Can cause equipment such as transceivers or amplifiers to fault.
    • Can cause false readings on watt meters and SWR bridges.
  • It can cause RFI to consumer products in the house.
    • TVI, BCI, and interference with other consumer products.

 

On the Signal:

  • It can cause oddities on your transmitted signal
    • Hum, whistles, feedback and other strange noises.
  • It can cause a weaker transmited signal if the transceiver folds back power in the presence of SWR.
  • It can cause the need for a matchbox, which often eats 10+% of the power.
  • It can pickup noise in receive mode.
    • The noise can propogate all the way into the reciever, bi-passing many filters.
    • This is especially a problem on the low bands.

 

 

WHERE CAN I FIND MORE INFORMATIN ON HOW TO DEAL WITH THIS?

 

My site's primary focus is on solutions, not the underlying technology.  There are several excellent sources of information dealing with this subject.  I will divide it into two categories. Unless you have advanced knowledge on this topic, I suggest you start witht he first link: