COMMON MODE REJECTION (CMRR)

The purpose of a Common Mode Choke is to reduce, or eliminate, common mode current flowing on the shield of coax feedline.  It does this primarily by presenting a high impedance to the common mode current, which "encourages" that current to flow into the antenna instead of back along the coax shield.  How well it does this is measured as the Common Mode Rejection Ratio (CMRR), stated in dB.

The CMMR of a choke is a function of frequency, as illustrated by the thumbnail image to the right.  Chokes are resonant inductive devices, so the frequency response can never be flat across any substantial frequency range.

For example:  suppose a choke has 40 dB CMRR on some particular band of interest. This means that the residual common mode current that makes it through the choke and onto the coax shield will be 40 dB down from the level of common mode current that would flow onto the coax shield in the absence of a choke.  That is just 0.01 %, or 1 part in 10,000.  If the original common mode current was 1 amp, the residual current that makes it through the choke would only be 100 microamps.  Such a small common mode current will not cause any problems.

One reason that common mode current flows on the coax shield is because the shield is connected to one "leg" of the antenna, whether directly or through an impedance transformer or balun.  Placing a Common Mode Choke at the feed point effectively "breaks" that connection and stops common mode current from flowing on the coax shield.  

Connecting the shield directly to one leg of the antenna also makes the shield a part of the antenna.  This affects the impedance, efficiency, and radiation pattern of the antenna.  A Common Mode Choke placed at the feed point preserves the antenna's "native" characteristics by isolating it from the coax shield.

Another reason common mode current flows on the coax shield is that some of your radiated signal is picked up by the coax shield, which is really just another antenna wire. This induced signal flows as common mode current on the shield between the feed point choke and the rig - essentially "going around" the feed point choke. This current will not be conducted to the antenna wire itself because the feed point choke blocks it.  But it does make its way in the other direction back to the rig.  This current causes RFI, especially if it gets all the way into the Ham Shack.  Placing a second Common Mode Choke in the coax just before it enters the radio shack will block this common mode current and further reduce RFI and received noise.

Radio Frequency Interference (RFI) occurs when your transmitted signal interferes with other electronic devices.  This generally happens because some portion of your signal is allowed to flow on the coax shield of the feedline. That common mode current causes your signal to be radiated in close proximity to other electronic devices and household wiring, interfering with them.

This radiated signal can easily be strong enough to interfere with a wide variety of electronic devices. The current that makes your antenna radiate should stay in the antenna. It should not be brought into the house where it will cause RFI.  

RFI can cause burns, RF feedback affecting your transmit audio, hum in loudspeakers, disruption of digital devices such as computers and digital interfaces, erroneous meter readings, disruption of proper transceiver operation, and interference with lights, alarms and other household devices.

The solution?  Keep your transmitted signal off the coax shield with Common Mode Chokes - one at the feed point and one just before the coax comes into the shack.

The coax shield is just another wire.  And, like any wire, it will pick up electromagnetic energy generated by any source.  Electronic devices often generate and broadcast electromagnetic energy even though they are not intended to do so.  This energy is referred to as Electromagnetic Interference, or EMI. (We generate RFI and we receive EMI). The worst offenders in today's world are switching power supplies and LED lighting systems.  But some electric motors, faulty connections, and faulty transformers also generate EMI.

Such EMI is picked up as noise by the coax shield as it runs from your rig to the antenna. This noise is conducted in both directions - to the antenna and to the rig.  If no Common Mode Choke is placed near the feed point, this noise will be conducted right onto the antenna itself and then conducted to your receiver, just like any other signal. However, placing a good Common Mode Choke near the feed point blocks that noise and keeps it from being conducted onto the antenna.  

Some EMI will be picked up by the antenna itself, rather than conducted to it on the coax shield, and there is nothing that a choke can do about that. The best approach is to keep the antenna as far away as possible from household wiring and other electrical equipment.

In general, the coax feedline runs through and/or near the house (the most common source of EMI) and picks up EMI noise along the way. It can be from your house or a neighboring one. The antenna is generally farther away from household wiring than the coax.  So, by the time the EMI noise signals get to the antenna they are much weaker than when they are picked up by the feedline coax shield.  As a result, most of the noise will be removed from your received signal by the Common Mode Choke because it blocks that noise from being conducted onto the antenna.

However, noise induced on the coax shield is also conducted back to the shack and the rig, where it will cause problems. In fact, it will be conducted right onto the chassis of your rig. The coax shield will also "broadcast" that noise (just like any other transmitting antenna) into the radio shack, where it will be picked up by your receiver and other equipment. 

Placing a good Common Mode Choke in the coax just before it enters the shack will block that noise current and your receive noise level will drop.

Received noise is reduced in these two ways: it is blocked from being conducted onto the antenna itself and induced noise is blocked from entry into the shack. Sometimes this noise reduction is very dramatic. 

Inserting a Common Mode Choke at the feed point and another just before the coax enters the shack will greatly reduce received noise in nearly all situations.