On 22 nov, 03:24, Roy Lewallen wrote:
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

Hello Roy,

You are right regarding non-uniformity, losses in the flat strip are
higher then based on the uniform current distribution (because of non-
uniformity). But this does not declassify loop antennas out of strip
material.

Based on a uniform current distribution (20cm wide strip versus two-
turn loop from tube with D=2cm) one would expect heat loss reduction
of 3.2. In my posting on Art's comment a mentioned heat loss reduction
w.r.t. the 2-turn loop of factor 2.5 (to account for non-uniformity).

A strip (not near to other constructions) has effective diameter of
half the width to have same characteristic impedance (as you
mentioned). So a strip with physical circumference of 40cm (width =
20cm) has an effective circumference of 40*0.785=31.4cm. You need to
have tube with D=10cm to have same effective circumference. I agree
with you that this effective circumference is also a good starting
point for calculation of AC loss resistance.

When Dloop is no longer large with respect to Dtube, current in the
tube tends to take the shortest path, hence reducing effective
diameter (and loop area) of the loop. In case of the strip, effective
diameter (hence area) does not reduce. Radiation resistance is
proportional to A^2 (for electrically small loops), hence Dloop^4.
10% reduction on loop diameter, gives 34% reduction of radiation
resistance.

In my opinion, advantage of a strip is still significant with respect
to a tube as long as you use a strip with width 2*(tube diameter).

Best regards,

Wim
PA3DJS
www.tetech.nl
you can use PM, but please remove abc.