[Elecraft] P3 Spurii and a cure.

[Jim Brown]

On 8/27/2013 1:51 PM, Tom wrote:
> The
> >full shield coax that used a foil were (are) especially prone to ground
> >loops by not having a good connection at both connectors.

While I agree with your analysis that the problem was a bad shield 
connection to a crummy cable, there's a bit more to understanding what's 
going on.

Coax (and other transmission lines) reject noise and crosstalk by 
providing a low impedance, low reactance path for return current. That 
is, the current in the center conductor returns in the shield, so the 
total field outside the coax is zero, and no external field can couple 
into the transmission line.  When that return connection is not there, 
the current returns by whatever path mother nature finds, a large 
magnetic field is generated that is proportional to the loop area, and 
any RF current near that loop is coupled into the circuit.

Yes, one solution is to buy a decent cable. Another is to take a short 
piece of RG58 (or any coax with a copper braid shield -- the cable is 
far too short for Zo to matter) and put  BNC connectors on each end.  If 
you have a decent crimper for BNCs and some connectors, that's easy. If 
you don't, it's a bit more work. :)

73, Jim K9YC