> case, the idea is to shield that 10 foot vertical section of the beverage
> as it drops down to the ground. Grounding the shield of the coax will
> protect the center conductor (part of the beverage system) from picking up
> the vertical component of noise.
Unfortunately that commonly held perception isn't true. The shield
becomes a perfectly coupled secondary winding over the center,
and has all the current the primary (the center conductor) has.
Radiation remains unchanged.
Think of the Beverage drop wire shield in a direct comparison to the
shield in a loop antenna. The shield in a small loop behaves
exactly the same way. If the shield actually shielded the center
conductor, the loop would be totally dead. Instead, the signal and
noise response does not change when the shield is added.
The shield might redistribute the voltage and current peaks on the
drop line (or in a loop antenna) but the overall system response to
time-varying electric and magnetic fields remains just as good as it
ever was. Nothing changes by adding a shield once you are a few
feet from the loop antenna or drop wire, the shield is simply the
"new antenna".
73, Tom W8JI
W8JI@contesting.com
|