Topband: Common-mode noise/long buried feedline runs

[Telegrapher9 at aol.com]

Mike,

Tom's, W8JI, experience with this shows that in hIs situation it is not 
difficult to deal with. 

Food for thought for those who have had issues or want to know the magnitude 
of the coupling.There will be coupling between the coaxial cables but it 
appears that it is small enough not to matter in Tom's case. Besides imperfect 
connector connections there are two other coupling mechanisms that I know of. 
Leakage through the shield (called transfer impedance I believe) and the resistive 
voltage drop along the outside of the cable shield. I have encountered both 
problems in the lab using single braided shield coax. The solution for me was 
to use double shielded coax. In very sensitive applications triax can be used. 
I have made triax in the field using copper tape over standard coax. The extra 
shield is gavanically isolated from the coax shield at one end and is 
connected to the coax shield at the other end. At the not connected end the extra 
shield connects to earth ground. The RF current that is induced onto the extra 
shield then has a path rather than flowing along the coax shield. 

   Dave WX7G