Wimpie wrote:
. . .
There is an "however". When you make a single turn loop from flat
strip that has the same width as the length of your two-turn loop, you
will notice: 1. reduced AC resistance (because of the significantly
larger circumference of the flat strip with respect to a thin round
tube, 2. inductance will decrease (H field lines have to take a longer
path around the wide strip), 3. radiation resistance will not change
with respect to a single turn loop from wire/tube.
This results in higher efficiency and increased bandwidth. The
overall result will be better then for your two-turn loop. I think
that is the reason why most programs are for single turn loops.

So for the transmit case, given fixed diameter of your loop, the
larger the copper surface (=length*circumference), the better the
efficiency. Best thing to enhance conductor surface is to use very
wide flat strip (high wind load), or multiple wires (with some spacing
in between) in parallel (limited wind load).
. . .

Flat conductors aren't as attractive as they look at first glance. The
problem is the same proximity effect mentioned earlier in the posting.
Current is distributed evenly around a round conductor (assuming the
perimeter is a very small fraction of a wavelength), but not along a
flat strip. Because of proximity effect, the current is much more
concentrated near the edges than at the middle. The result is that the
resistance is considerably higher than for a wire with the same surface
area. In figuring an "equivalent diameter" of a thin flat strip in order
to get the same L and C properties, the rule is that a strip is
equivalent to a wire whose diameter is half the strip width. This means
that a strip of width w or total "circumference" 2 * w is equivalent to
a wire with a circumference of pi * w / 2 ~ 1.6 w, in so far as L and C
go. Since the same phenomenon affects the inductance and resistance,
this would also be a good working rule for estimating the relative R of
a strip or wire.

Roy Lewallen, W7EL