COMMON MODE REJECTION (CMRR)
Why Common Mode Chokes are so important
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 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 current is unlikely to cause 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 more or less "breaks" that connection and stops any substantial amount of common mode current from flowing on the coax shield. The choke isolates the feedline from the antenna.
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.
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 acting somewhat like any other antenna wire. This induces 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. However, it does make its way in the other direction back to the rig. This current can cause RFI, especially if it gets all the way into the Ham Shack. Placing a second Common Mode Choke near the rig, perhaps just before the coax enters the Ham Shack, will block this common mode current and further reduce RFI and received noise.
Radio frequency interference (rFI)
Common mode current on the coax causes RFI
Common mode current flowing on the coax shield makes the shield just another antenna, or just another part of your antenna system. It radiates a portion of your transmitted signal.
The field radiated by the common mode current flowing on the coax shield can easily be strong enough to interfere with electronic devices if it is located in close proximity to those devices. The current that makes your antenna radiate should stay in the antenna and no extension of the antenna should be brought into the Ham Shack where it will cause interference.
RFI occurs when the coax shield acts like an antenna (because of common mode current) and radiates a portion of your transmitted signal in close proximity to the equipment in your Ham Shack and the house's wiring. The house wiring will pickup and conduct that signal everywhere and cause interference with electronic devices throughout the house.
RFI can cause RF burns, RF feedback affecting your transmit audio, hum in loudspeakers, disruption of digital devices such as computers and Digital Interfaces, erroneous meter readings, and disruption of proper transceiver operation.
The solution? Keep your transmitted signal off the coax shield with Common Mode Chokes - one at the feed point and one before the coax gets into the Ham Shack.
Reduction of received noise
The coax shield is just another receiving antenna
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. 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.
In general, the coax feedline runs through and near the house (the most common source of EMI) and picks up EMI noise along the way. But the antenna is generally farther away from the house. 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 the noise from being conducted onto the antenna.
Noise picked up by the coax shield is also conducted back to the Shack and the rig, where it will cause problems. The coax shield will broadcast that noise into the Ham Shack, where it will be picked up by your receiver and, perhaps, other equipment.
Placing a good Common Mode Choke in the coax just before it enters the Ham Shack will block that noise current. The coax shield will then not have any noise energy to radiate into the equipment in the Ham Shack and your received noise level will drop.
Received noise is reduced in these two ways. Sometimes this noise reduction is dramatic. The greatest reduction tends to occur in situations where the coax runs parallel to the house (and, therefore, all of its wiring) before heading off to an antenna that is a fair distance from the house.
Inserting a Common Mode Choke at each end of the coax will greatly reduce received noise in nearly all situations.
