Purpose & Function of a Common Mode Choke

A common mode choke (CMC) is used to reduce or eliminate common mode current, which is an unwanted current that can flow on the shield of coaxial feedlines. This type of current can affect the performance of the antenna system and cause interference to electronic systems. The CMC achieves this by presenting a high impedance to common mode currents, effectively blocking them while allowing desired differential signals to pass with minimal attenuation.   

A CMC consists of multiple windings on a magnetic core. These windings are arranged so that they create high impedance for common mode currents by generating magnetic fields that add together, thus impeding the flow of these unwanted currents.

By presenting a high impedance to common mode currents, the CMC encourages these currents to flow into the antenna rather than back along the coax shield, thereby improving the performance of the antenna and reducing interference

The effectiveness of a CMC in blocking common mode currents is measured by its Common Mode Rejection Ratio (CMRR). A higher CMRR indicates better performance in rejecting unwanted signals.

For example:

A CMC with 48 dB rejection on a 40-meter band means it significantly attenuates common mode currents. This level of rejection corresponds to only about 0.003981% of the common mode current being allowed through the choke onto the shield, which is considered outstanding performance.

Placing a Common Mode Choke at the feed point of an antenna system "breaks" the connection that allows common mode current to flow on the coax shield. This isolation helps maintain the native characteristics of the antenna, such as its impedance, efficiency, and radiation pattern 

Common mode current flowing on the coax shield radiates energy that will interfere with other electronic devices, either in the Radio Shack itself or elsewhere in the local area.  The key to stopping this RFI is to isolate the coax from the antenna with a Common Mode Choke.  Other techniques that keep the coax away from the house, such as burying it, will also help reduce RFI and noise pickup.

Locally generated noise is picked up the coax shield, which acts much like any other antenna wire.  Common sources of such noise in today's world are switching power supplies and LED lighting systems.  If Common Mode Chokes are not employed, this noise hurts in several ways:  1) it is conducted directly onto the leg of the antenna that is connected to the coax shield and then directly to the antenna input of the rig like any other signal picked up by the antenna, and  2) it is conducted back into the Radio Shack where it radiates energy and  3) it is conducted directly to the "chassis" of the rig itself.  Placing one Common Mode Choke at (or very near) the feed point and a second choke near the rig greatly reduces the effect of local noise.  Receiver performance benefits, sometimes dramatically.