Re: [IRCA] Cable connectors

[Chuck Hutton <charlesh3@xxxxxxx>]

Rick:
 
In my mind, the best design for usage of common mode chokes is found in the bible for 160 meter DXers called "Low Band DXing". In the 4th edition, figure 7-88 and the adjacent text nicely explain grounding with common chokes. An extract is at: http://wiki.contesting.com/index.php/Common_mode_chokes .  Another good reference is  http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf  which says:  " The low-pass T-network formed by two chokes with a “ground” between them is significantly more effective than the same chokes would be without the ground, and much more effective than the ground alone would be. "
 
When you insert a series high impedance into the path (as you do with a common mode choke), you make the current want to go somewhere else. The best thing is to get rid of it before any of it gets to the rest of the system. That's why you'll see a lot of people going to the trouble of adding a ground connection to the coax shield. Best practise is to insert a choke at both the antenna and receiver ends of the coax, with a ground in between and no closer than say 20 feet to the endpoints.

Chuck
 

> From: k9ao@xxxxxxxxxxx
> To: irca@xxxxxxxxxxxxxxxx
> Date: Mon, 6 Sep 2010 15:55:25 -0400
> Subject: Re: [IRCA] Cable connectors
> 
> (CLIP)

> Ground isn't a part of the issue with common-mode currents on the coaxial 
> cable's outer shield surface. This isn't a matching transformer, or other 
> circuit that may have a connection to the local ground. The choke impedance 
> places itself in series with the currents flowing on the outer surface of the 
> coaxial cable's shield surface. There is no ground connection.
> 
> Using #31 core material, if you stacked maybe 5 or 6 toroid cores, and 
> assuming that you wound the feedline through the cores so as to minimize 
> capacitance coupling between turns, i.e. very large loops not wound tight on 
> the cores surface and separating the turns radially from each other, you can 
> get in the neighborhood of 3k to 4k of series impedance to common mode 
> currents on the coax shield at 1 MHz with 7 or 8 turns or so. > 
> Rick Kunath
> 
 		 	   		  
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