Why Common Mode Chokes make your life better

Common mode current is current that should be flowing through your antenna out into the "ether", bringing the magic of radio to you. Instead, it is flowing on the shield of your coax feedline.  Not only is it wasted, it also wreaks havoc:

• It reduces the effectiveness of your antenna. 
• The feedline radiates and becomes an unintended part of the antenna system. 
• It distorts your antenna pattern. 
• It causes RFI, interfering with other electronic devices. 
• It allows electrical noise from the local environment to get onto the coax shield and, eventually, all the way to your receiver.  

This puts you in a bit of a hole. But we empathize. We've been there. So we make devices that help you get yourself out of that hole so you can get the most out of your station. This will, in turn, help you get the most pleasure out of your hobby.

That's why Common Mode Chokes make your life better. Happiness. Confidence.

___________________________________________________________

Yes, it is very nice. Once isolated, the coax won't affect the tuning of your antenna and virtually all of your signal current will actually flow into the antenna, which preserves its gain and radiation pattern. You hear better, others hear you better.

___________________________________________________________

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

____________________________________________________________

Common Mode Chokes present a high impedance to common mode current flowing on the shield of your coax feedline. But it does NOT interfere with your signal, either on transmit or receive.

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 0.001585 % 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 conducted onto the feedline coax. The minimum amount of choking impedance required is a function of your specific 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 it will interfere with other electronic devices, either in your 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 your receiver, and

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

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.  Unless you operate in a noiseless environment, receiving performance benefits dramatically.

_____________________________________________________________

Antenna systems generally reach their optimum level of performance by using two Common Mode Chokes. This is because there are two types of common mode current that we must eliminate:

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. 

Common Mode Choke #1, at the feed point, kills off conducted common mode current.

Choke #2, just before the coax enters the shack, kills off Induced common mode current. 

This combination does it all: it preserves your antenna's gain, preserves your antenna's radiation patter, prevents RFI, and protects you from local noise. You hear more stations, more stations hear you, and you get more operating pleasure.