Why You need of a Common Mode Choke

A Common Mode Choke is used to reduce or eliminate common mode current on the feedline and effectively isolate the feedline from the antenna. This preserves the tuning, the gain, and the radiation pattern of the antenna. It also prevents Radio Frequency Interference (RFI) and keeps locally generated noise (such as that from LED lights, switching power supplies, etc.) from raising the noise floor of your receiver.

If the answer is yes, then you need Common Mode Chokes. Why?  Because you have common mode current on the feedline. The coax feedline shield is connected to one side of the antenna and it is, therefore, part of the antenna. Some of the current that would normally flow into that side of the antenna actually flows back down the outside surface of the coax shield (due to the skin effect). This is common mode current. We do not want common mode current because it causes a number of problems.

Yes, it is very nice. Then the coax won't affect the tuning of the antenna and virtually all of your signal current will actually flow into the antenna, which preserves its gain and radiation pattern. Whatever signal current we let escape down the coax shield as common mode current compromises the antenna's performance, causes Radio Frequency Interference (RFI) to other electronic devices (including the ones in our radio shack), and increases our receiver noise floor.

A Common Mode Choke is the device that allows us to "isolate" the feedline from the antenna.  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, prevents Radio Frequency Interference (RFI), and reduces received noise.

A Common Mode Choke consists of multiple windings of transmission line around a magnetic core. These windings are arranged so that they present a high impedance to common mode current flowing on the shield of coax feedline but no effect on your signal flowing through the choke.

This high impedance "encourages" your signal to flow into the antenna rather than back along the coax shield, thereby improving the performance of the antenna while reducing interference and received noise.

The effectiveness of a Common Mode Choke is commonly measured by its Common Mode Rejection Ratio (CMRR). A higher CMRR indicates better suppression of unwanted common mode current.

For example:

A Common Mode Choke with 48 dB CMRR reduces common mode current to only 0.003981% of its former self.  This is a very high level of performance.

Some people, mostly antenna designers, prefer to think in terms of the impedance that the Common Mode Choke presents to common mode current. A higher choking impedance means less common mode current will be allowed onto the feedline coax.  Exactly how much choking impedance is required is determined by the antenna and feedline system and is very difficult to determine.  So, in general, more is better.

Common mode current that is allowed to flow on the coax shield radiates a portion of your signal in such a way that will interfere with other electronic devices, either in the radio shack or elsewhere in your house and your neighbors' houses.  The key to stopping this Radio Frequency Interference (RFI) is to insert a Common Mode Choke in the feedline right at the antenna feed point.

Locally generated noise is picked up the coax shield, which acts like any other antenna wire.  Common sources of such noise are switching power supplies, LED lighting, and electric motors. 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 on the shield back into the Radio Shack where it radiates energy that is picked up by our receivers, and

3) it is conducted on the coax shield directly to the "chassis" of the rig itself, where it will be picked up as noise by the rig.

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.  Receiving performance benefits, sometimes dramatically.

Every antenna system needs two Common Mode Chokes because there are two types of common mode current that we must eliminate.  They are:

1) Conducted common mode current.  This common mode current is conducted right onto the coax shield because the shield is electrically connected to one leg of the antenna.     

2) Induced common mode current.  This is current that is induced onto - or received by - the coax shield.  The feedline coax shield acts like any other wire antenna and picks up your transmitted signal as well as unwanted noise from the local environment. 

We kill off conducted common mode current with a Common Mode Choke places at the feed point.

We kill off Induced common mode current with an additional Common Mode Choke placed in the coax just before it enters the house.  This choke not only prevents RFI, it also reduces our receiver noise floor, allowing us to hear more weak signals.