For the best results put a choke at the feed point of each antenna. Maybe another one at the “shack” end of their feed lines. That’s he ‘quick-n-diry’ way to do it. It’s also probably the most common way.

A common mode choke is where both line and neutral windings are wound on a single core. When using a current compensated choke to decrease common mode noise, (the interference pattern or the unwanted noise) you want to have a high impedance at the unwanted frequencies to knock down that unwanted noise.

What is a Common Mode Choke? A common mode choke is an electrical filter that blocks high frequency noise common to two or more data or power lines while allowing the desired DC or low-frequency signal to pass.

Common mode choke coils are suited for noise suppression on lines with large current flows, such as AC/DC power supply lines. Since they do not affect signal waveform, they are also suited for common mode noise suppression on lines where signal waveform distortion causes a problem, such as video signal lines.

Common-mode rejection is the ability of the differential amplifier (which sits between the oscilloscope and probes as a signal-conditioning preamp) to eliminate the common-mode voltage from the output.

Depending on the specific design you might get away with choking right at the feed point or you could do as some, including Palomar Engineers, recommend and place the choke back near where your coax enters the shack.

Ideally, CMRR is infinite. A typical value for CMRR would be 100 dB. In other words, if an op amp had both desired (i.e., differential) and common-mode signals at its input that were the same size, the common-mode signal would be 100 dB smaller than the desired signal at the output.

The op amp common-mode rejection ratio (CMRR) is the ratio of the common-mode gain to differential-mode gain. For example, if a differential input change of Y volts produces a change of 1 V at the output, and a common-mode change of X volts produces a similar change of 1 V, then the CMRR is X/Y.

When there is an imbalance in the antenna (for what ever reason), current will flow on the outside of the coax shield. The current which flows on the outer surface of the shield is called common mode current. In other words, it is the unbalanced current not returned within the coaxial cable.

advantage differential mode a high CMRR is good because it defines the difference at the output of an amplified differential mode input to an amplifier common mode input. Unwanted signals that couple into the differential input, predominantly will result in an unwanted common mode signal at the input.

Common mode chokes are designed so that common mode current creates a magnetic flux (through mutual inductance) that adds to attenuate the CM noise signal. The normal (differential) mode currents create magnetic flux that ideally cancel each other out so the normal signal frequencies are not attenuated.

CMT toroid style common mode chokes are designed to provide the highest common mode impedance over the widest frequency range. These parts are ideal for any application requiring a high DC current bias and are well suited for use in switch-mode power supplies.

USB, high-speed and Super-Speed data line common mode chokes effectively reduce common mode noise in high-speed interfaces like USB 2.0, USB 3.1 Gen 1, HDMI, IEEE 1394, LVDS, HDBaseTTM, MOST® bus, etc. They maintain excellent signal integrity for high-speed communications with -3 dB differential mode cutoff frequencies up to 6.5 GHz.

However, nothing is perfect, and some common mode current may leak past the choke, especially if it is at a nodal point. If this is the case, placement of a second choke somewhat further down the line (say, 1/8 wavelength velocity factor corrected) may hit a maximum and block more current.