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
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