> Before discovering the phased endfire loops for reception
> on 160m, I tried most kinds of loops known to man. In
> space, shielded and non-shielded loops work about the
> same. Near the ground, coupling with other objects and
> non-homogenous ground unbalances the non-shielded loop,
> taking the sharpness and depth from the null, or even
> destroying it.
What you have with a shielded loop is a single turn antenna.
This antenna has two curved halves, and if we excite those
two halves with equal currents the loop is balanced. The
ground interaction or interaction to surrounding objects
would be no different at all if we simply fed the shield
with the same distribution and did away with the inner
conductors.
The problem is when people feed a single wire they often
mess the system up.
The shielding is definately a less accident-prone feed
method, but make no mistake about it the shield is the
actual antenna. Nothing penetrates the shield walls, all the
coupling from the outside to the inner link is at the gap.
> The loop shield is a shield of sorts, providing equal
> magnetic coupling to surrounding objects, provided the
> shield gap and ground wire connection are substantially
> opposite each other. The shield "is" the antenna, with
> the received signal's RF current flowing around the ends
> of the gap, inducting signal to the internal loop wire by
> transmission line action.
I agree. The shield, if properly constructed, makes the
system better balanced than simply having a wire loop with
all coupling at the feedpoint and one side grounded
(unbalanced feed). A properly constructed system with
balanced coupling directly to the shield (no inner loop)
would have identical performance to a properly constructed
shielded loop in any environment.
If I built a loop I'd be sure the loop was excited equally
on each half. In my opinion it is better to plan success
than to stumble upon it by accident or the Edisonian method
of dozens of cut and try experiments. It saves time when we
realize what the shield really does, and it even helps us
understand coaxial cables.
73 Tom
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