That's interesting. In what way is the "effectiveness" of a circular
loop decreased by changing its shape?

Roy Lewallen, W7EL

Gary V. Deutschmann, Sr. wrote:
Hi Ed

The IDEAL loop skywire is a perfect circle!

Any deviation from that reduces it's affectiveness, but not very
appreciatively that you would notice enough for it to warrant worrying
about it.

I've had loop skywires that resembled the letters M, R and even close
to the letter V all closed loops of course, and I saw no difference in
their performance.

TTUL
Gary

Hi Roy

That's interesting. In what way is the "effectiveness" of a circular
loop decreased by changing its shape?

I ducked class that day!

TTUL
Gary

Roy, W7EL wrote:
"In what way is the effectiveness of a circular loop decreased by
changing its shape?"

There is an old story about the kid who tells his dad about learning in
school that pi r sguare. Dad replied that what school taught him was
dumb. All the world knew pie are round. Cornbread are square.

Maybe it was Pythagoras who found the approximate value of pi by
constructing ever more equilateral sided figures inside and outside of a
circle until there was no significant difference in the lengths making
up the sides of the interior and exterior figures. He could measure
straight lengths. He found the value to be 3.1416 for the approximate
value of pi which multiplied by the radius would equal the perimeter of
the circle. Also, pi times the radius squared gave the enclosed area.

The figure which encloses the most area for a given perimeter is a
perfect circle. Distorting a circle reduces the area it encloses.

Radiation from any loop depends on its enclosed area. This is intuitive
from transmission line behavior. It`s often observed that the wider the
spacing between the wires, the more the line radiates. As we increase
the area of a loop, the distance between the wires increases. Like the
transmission line, iits radiation increases.

An antenna of any configuration radiates. Efficiency is determined by
the ratio of radiation resistance to loss resistance. The antenna with
minimum perimeter for a particular radiation resistance will also have
minimum loss with other parameters being equal.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
. . .
Radiation from any loop depends on its enclosed area. This is intuitive
from transmission line behavior. It`s often observed that the wider the
spacing between the wires, the more the line radiates. As we increase
the area of a loop, the distance between the wires increases. Like the
transmission line, iits radiation increases.
. . .

Ok, let's start with a triangular loop with negligible loss. We feed 100
watts to it. Since it has negligible loss, 100 watts must be radiated.

You've said that the radiation must increase as we round out the
triangle. So how much more radiation can we expect from a round loop fed
with 100 watts? 120 watts? 150?

Roy Lewallen, W7EL

Roy, W7EL wrote:
'Ok, let`s start with a triangular loop with negligible loss."

"Negligible loss" eliminates the differences between loops of most
shapes with the same enclosed areas. Area of a triangle is 1/2 its base
times its altitude, if I remember.

I`d rather use 16 ft of wire to make a square loop with 4-ft sides. Side
squared makes an area of 16 sq ft.

A circle of 16 ft perimeter has a diameter of 6.09 ft. Radius is 3.049
ft. Squared, it`s 9,27. and times pi it`s 20.13 sq ft.

Clearly the circle has the greater area for the same wire. Loss is based
on the resistance of the wire which is the same in both cases.

For more enclosed area, you get more radiation for the same wire and
loss.

As a short cut, I`ll quote Terman on page 907 of his 1955 edition:
"The radiation resistance of a loop antenna is less the smaller the loop
area."

Best regards, Richard Harrison, KB5WZI

I wrote:
"A circle of 16 ft peroimeter has a diameter of 6.09 ft."

My eyesight needs correction. It should have been 5.09 ft.

The area of a 16-ft circumference circle is 30.37 sq ft, not 20.13 sq
ft.

30.37 sq ft is more than 16 sq ft, so the circle radiates more than the
square for the same length of wire.

Best regards, Richard Harrison, Kb5WZI

Just a question

In microwave we talk about aperature as a determing factor of antennas.

To what extent does this apply to HF ??

--
Caveat Lecter

Caveat Lecter wrote:
"To what extent does this (aperture) apply to HF?"

To the fullest extent of the concept. See the 3rd edition of Kraus`
"Antennas", Section 2-11, The Radio Communications Link, beginning on
page 336.

Radio antennas scale to wavelength. Microwave antennas may be
impracticably large when scaled to longer wavelengths, but if built work
exactly like their higher frequency models.

Best regards, Richard Harrison, KB5WZI

It applies just as well. However, while the aperture of a parabolic
reflector is about the area of the reflector, this isn't at all true of
simple wire antennas like a dipole. For example, a half wave dipole's
aperture is just slightly larger than a dipole of infinitesimally short
length, and about equal to that of a loop. The aperture of a loop stays
almost constant as the loop size is increased, until it gets big enough
for the pattern to appreciably change.

Aperture is the same as directional gain (not numerically, but when one
is greater so is the other), which is the same as gain when loss is
neglected. Since aperture has no direct or obvious connection to
physical size or dimension of most wire antennas, gain is usually used
at HF as a descriptive measure rather than aperture. Note that the gain
of all but an isotropic antenna is different in different directions,
and therefore so is the aperture.

People with a weak understanding of the principles involved often fall
into the trap of thinking that a larger antenna must have a larger
"aperture" or, as amateurs like to call it, "capture area". That
mistaken notion leads to all sorts of false conclusions. But the general
misunderstanding of the terms are a real boon to antenna charlatans.

Roy Lewallen, W7EL

Also, the aperture is different in different directions.

Caveat Lector wrote:
Just a question

In microwave we talk about aperature as a determing factor of antennas.

To what extent does this apply to HF ??

Richard Harrison wrote:
I wrote:
"A circle of 16 ft peroimeter has a diameter of 6.09 ft."

My eyesight needs correction. It should have been 5.09 ft.

The area of a 16-ft circumference circle is 30.37 sq ft, not 20.13 sq
ft.

30.37 sq ft is more than 16 sq ft, so the circle radiates more than the
square for the same length of wire.

With the same power input? If I apply 100 watts to the square and get
(very nearly) 100 watts radiated, how much do I get from the circle?
Let's see, 30.37/16 * 100 = 190 watts. If I could capture that in a
screen room with another antenna, I could feed 100 watts back to the
transmit antenna and have 90 watts left over to run the refrigerator to
cool my beer. . .

Roy Lewallen, W7EL