"Richard Harrison" wrote in message
...
Mike, W4EF wrote:
"I would agree, Richard, but at HF frequencies, the current path around
the shield isn`t real;ly broken by the gap."

To best describe what broken means, a picture helps. There is a picture
on page 13.18 of the 2006 ARRL Handbook. Fig 13.26 has a legend which
says:
"To prevent shielding of the loop from magnetic fields, leave the shield
unconnected at one end."


I am a bit behind on ARRL Handbooks, Richard, but from
what you describe, this is the same figure that appears in my
1992 edition (chapter 38, figure 2). In any case, what is
shown in the figure agrees with my understanding of "broken",
although admittedly when I made my previous post, I was
thinking of the case where the shield is broken on the side of
the loop opposite the feedpoint. For the purposes of this
discussion, however, it doesn't matter whether the break is
at the top (opposite the feed) or at the bottom (adjacent
to the feed). In either case, current induced on the inside
of the shield by current flowing on the center conductor loop
has a continuous back to ground via the outside surface of the
shield. IOW, the gap doesn't suppress the eddy current, rather
it forces it to flow on the outside surface of the shield, thereby
causing the loop to radiate.


I think the handbook has it right.


Yes, I agree it does. If you connect the shield at both ends, the
loop can't radiate because the eddy current caused by current
flowing on the inner conductor loop will confined to the inside of
the shield. Likewise, eddy currents induced on the outside of the
shield by EM waves passing the antenna will be confined to the
outside of the shield if there is no gap (reciprocity holds - the
antenna won't receive with no gap).

73, Mike W4EF.............................................. ...

Best regards, Richard Harrison, KB5WZI