Topband: Shielded drop wires

[Tom Rauch]

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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