Topband: Beverage feed lines

Tom Rauch
Mon, 21 Oct 2002 09:58:46 -0400

> Has anyone in "Top-Band Land" had experience of what feedline lengths
> to avoid for signal pick-up on the coax shield??  I am making up new
> feedlines and my European feedline is 1/2 wave multiple at around 1960
> gary, kd9sv

Hi Gary,

Feedline length only affects signal pickup to a noticeable extent if 
the shield of the cable is somehow coupled into the system through 
common mode coupling.

If we use transformers without a direct connection between the shield 
and the Beverage ground it helps prevent common mode coupling between 
the feedline and antenna. With really long cables with splices or 
junctions, it can sometimes help to ground the cable and use common 
mode chokes periodically along the cable...but even this is helpful 
only to reduce effects of what really are shield connection problems.

We should also use good electrical layouts make good shield 
connections to preamps or receiving gear, to prevent shield common 
mode problems at the receiver.

Unless there is a system problem such as poor layout or poor 
isolation between the shield and antenna system, cable lengths will 
make no noticeable difference at all.

73, Tom W8JI