Hi Hal,
This is probably worth going into, because it goes to the heart of
some common misconceptions about shields.
> For the vertical drops, use coax, configured as a true transmission
> line. For the beverage's terminated end, at the ground end of the coax
> drop, tie the shield to the ground post, and the center conductor
> through a resistor to the ground post. For 50 ohm coax, use a 50 ohm
> resistor. The ground end is now properly terminated, and the coax
> will be a transmission line, not a radiating wire -- yes? At the top
No, it will radiate EXACTLY the same as a single wire would. There
is absolutely no way around that.
The skin depth of the conductor at RF isolates the inside of the
shield from the outside, except at the ends where the inside and
outside of the shield are "shorted". The isolating skin- depth of the
shield is current can spill over the ends and make the turn.
The shield, because it is parallel with the center conductor, has the
same exact current induced in it that is flowing into the center
conductor. That same current (and voltage) appears on the outside
of the shield.
The result is you have the same current flowing to ground on the
outside of the shield as you would have if the shield was not even
there, and the voltage is identical. Nothing changes, except how
thick the conductor is and how lossy the system is. You add a tiny
bit of extra loss, that's all.
The transformer at the top won't help because it needs a "ground"
for the antenna to push against. It can only get that ground by
common mode currents flowing to ground over the shield.
Even if you added a shield over that shield, the current would
simply move to the outside of the new shield! It would do that even
if the shield was unconnected at both ends. Add 100 shields, and
the current simply winds up outside the outermost shield while
losses increase.
There is absolutely no way possible to cure this through "tricks"!
It's just a fact of life. This is why a shielded loop still receives a
signal, and why a shielded loop responds to both electric and
magnetic fields just as an unshielded loop does.
73, Tom
(W8JI@akorn.net)
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