COMMON MODE REJECTION (CMRR)

Common mode current flowing on the shield of a coax feedline is very undesirable for a number of reasons:

1)  It diverts some of your signal current away from the antenna, which reduces the efficiency of the antenna which reduces your signal strength, and

2) It causes the feedline to radiate, which alters the antenna's pattern, and

3) It causes Radio Frequency Interference (RFI) to other electronic devices, and

4) It picks up locally generated noise, which raises your receiver's noise floor.

The purpose of a Common Mode Choke is to reduce, or eliminate, common mode current flowing on the shield of your 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 outside surface of the coax shield.  How well it does this is measured as the Common Mode Rejection Ratio (CMRR), stated in dB.

CMRR is the best figure of merit to use when comparing chokes.  Regardless of the details of whatever antenna system it is used with, a choke with a higher CMRR will reduce common mode current more than a choke with less CMRR.

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 about 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 amount of common mode current will not cause problems.

Generally, the primary reason that common mode current flows on the coax shield is because (without a choke) the shield is connected to one "leg" of the antenna, whether directly or through an impedance transformer, a balun, or a UNUN.  Placing a Common Mode Choke at the feed point effectively "breaks" that connection and stops common mode current from flowing onto the coax shield.  

When the coax shield is directly connected to one leg of the antenna it becomes an unintended part of the antenna.  This affects the tuning, efficiency, and radiation pattern of the antenna.  A Common Mode Choke placed at the feed point preserves the antenna's "native" characteristics by isolating the coax shield.

Another way common mode current gets onto the coax shield is that some of your radiated signal is "picked up" by the coax shield, which is really just another antenna wire hanging out there collecting signals - the strongest of which is your own. 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 makes its way in the other direction back to the rig. This induced current can cause RFI, especially if it gets all the way into the Ham Shack.  

The coax shield also picks up local noise that is induced onto the shield. This happens for the same reason that your signal causes RFI - it's just in the other direction. Noise generated by LED lighting systems and switching power supplies etc. "broadcasts" out to your coax, where it induces a signal on the coax shield. This noise eventually finds its way into your receiver. 

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 as common mode current. That common mode current causes the coax to radiate like any other wire with RF current flowing through it.  Your signal is then radiated in close proximity to other electronic devices and household wiring, interfering with them.

RFI doesn't care if your signal gets onto the coax shield by conduction (due to being connected to one leg of the antenna) or by induction.  Either way, common mode current on the shield of the coax is likely to cause RFI to other equipment in the radio shack as well as other consumer electronic devices throughout your house and your neighbor's houses.  The most common devices that exhibit RFI appear to be audio equipment, other radio equipment, HVAC system control electronics, and remotely or touch controlled lighting.    

In the radio shack, RFI can cause RF burns, RF feedback that distorts your transmit audio, hum in loudspeakers, interference with other radios (such as  SDRs, scanners, and spectrum analyzers), disruption of digital devices such as computers and digital mode interfaces (FT8 etc.), interference with relay control signals, erroneous meter readings, disruption of proper transceiver operation, and interference with lights, alarms, monitors and other household devices.

The solution?  Keep your transmitted signal off the coax shield with Common Mode Chokes.  Placing one at the feed point is the first step. If RFI persists or you feel your receiver noise floor should be lower, than install a second one just before the coax comes into the house or the shack.

The coax shield is just another wire out there collecting radio signals.  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, light dimmers, variable speed motor controllers, and faulty transformers also generate EMI.

Such EMI is induced or "picked up" as noise by the coax shield as it runs from your rig to the antenna. This noise is conducted in both directions - toward the antenna and toward 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 directly to your receiver, just like any other signal. 

Inserting 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 in that case is to keep the antenna as far away as possible from household wiring and other electrical equipment.

In general, the coax feedline runs through 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 house or from nearby power lines. 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 generally 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 a Common Mode Choke that is inserted in the system at the feed point 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 also "broadcasts" that noise (just like any other transmitting device) into the radio shack, where it can be picked up by your receiver and other equipment. Unless the coax, relays, antenna tuners, and the rig itself are perfectly shielded, it is likely that some of that noise will find its way into your receiver.

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

So received noise is reduced in three ways by Common Mode Chokes: 1) it is blocked from being conducted onto the antenna itself by a choke at the feed point, and 2) it is blocked (by a choke at the entrance to the radio shack) from flowing to the chassis or your rig, antenna tuner, power meters etc., and 3) it can no longer by "radiated" inside the shack, close to all of your radio equipment.

Summary:  Inserting a Common Mode Choke at the feed point and another just before the coax enters the shack will greatly reduce received noise in many, if not all, situations.