Topband: Common mode chokes

[Jim Brown]

On Mon, 13 Jul 2009 07:53:44 -0400, GEORGE WALLNER wrote:

>I have been experimenting with twisted 
>pair RX transmission lines. I have used a twsited pair 
>removed from a CAT5 cable. With a well balanced 
>transformer and a common mode chocke at each end, it seems 
>to have very high common mode rejection (i.e. I have not 
>been able to measure it). 

Yes, I would expect it to work quite well as an alternative to 
coax where you want light weight for travel and don't care about 
deterioration from UV. On the other hand, coax is probably more 
convenient and more robust, and thus a better choice for most 
permanent installations. As most of us know, this is 100 ohm 
cable. 

BTW -- those interested in shielded balanced cable for RX antennas 
might also want to look at cables designed to carry AES3 digital 
audio. The spec for AES3 cable calls for 110 ohm twisted pair with 
a shield. In practice, the shield is not needed. Belden and Gepco 
make the best AES3 cables I know of. The easiest way to buy is 
probably from a sound contractor or a broadcast vendor. Note that 
these cables are NOT UV rated. 

Loss data for two of these cables are shown in Fig 16b of  

http://audiosystemsgroup.com/AESPaperSplittersASGWeb.pdf

The cable labeled MP1201 is a Gepco product (Chicago), and 1800F 
is a Belden product (Indiana). Both are braid-shielded cables 
designed for use as portable mic cable. Capacitance between 
conductors is on the order of 13pF/ft. I don't recall the part 
number of the Proco product. The cables on the graph with much 
greater loss are designed for analog audio. 

73,

Jim Brown K9YC