Topband: Re: Topband: Shielded, Balanced Flag

[Tom Rauch]

> The balance in a shielded loop is real because the turn inside the
> loop couples to the inside of the shield only.  On the other hand, the
> shielded loop suffers by distributed capacitance and extra losses
> whose effects aren't both desired (I'd say not tolerable) with
> antennas that are already inefficient. Incidentally, the shielded loop
> works when the signal to be notched is E-field dominant but its
> practical advantage is probably limited to this event.


I think the misconception is that we often mistakenly think the wire 
inside the "shield" remains the "antenna", and the "shield" magically 
makes the antenna isolated from surroundings.

What actually happens is the thing we put on the outside and call a 
"shield" actually becomes the real antenna. It is no more isolated 
from the surroundings than a bare wire would be. If we place another 
shield over the shield, the newest outside layer becomes the antenna 
and is once more on the outside exposed to the world.

As you say Mauri, the only thing we do is keep adding loss because 
each conductor must couple signal to the conductor under it, layer by 
layer, by the current spilling over the open "shield" ends and 
running along the inside of the "shield".

And as you say, the "shielded" loop antenna responds to the electric 
field....and as a matter of fact is electric field dominant just 
1/10th wl or less from the antenna...whether we have a shield or not. 
I never use shields on loops, because they never make any difference 
except to add loss or change the tuning generally in a deleterious 
way.



73, Tom W8JI
W8JI@contesting.com