[TowerTalk] Beverage Layout Problem

[Jim Brown]

On 5/13/2014 9:18 AM, David Gilbert wrote:
> Can you come up with a scientific explanation for why lossy coax picks 
> up more noise than does a lower loss coax? 

Coax with a poor shield can convert common mode shield current to 
differential voltage between center and shield. The standard measure of 
this is the Transfer Impedance of the shield, and the lower limit on the 
transfer impedance is the resistance of the shield at the frequency of 
interest. Many so-called RG6 and RG8-X cables have thin, poorly 
manufactured shields, so their transfer impedance is high. The higher 
the transfer impedance, the greater the transfer of noise. Another 
factor increasing transfer impedance is poor uniformity of the shield.

That's why a good RG58 with a copper braid shield makes a reasonably 
good coax for Beverages. It's also why it's a good idea to add a common 
mode choke to that coax.

73, Jim K9YC