Richard Harrison wrote:

If you had more trees (octagonalish) would enclose more area with the
same wire. Radiation is a function of loop area.

Can you please explain that a little more? If you put 100 watts into a
400 foot circumference loop and it radiates 95 watts, will an 800 foot
loop radiate four times that, or 380 watts? Then can you feed back 100
to put back into the loop, and have 280 left over to run your
refrigerator to keep your beer cold?

Or if the 800 foot loop radiates 95 of your 100 watts, does the 400 foot
loop radiate only 23.75 watts? If that's what happens, where does the
rest of the power go?

Is the radiation pattern the same for a long skinny loop as for a round
one, as long as the enclosed area is the same?

. . .

Puzzled,

Roy Lewallen, W7EL

Roy Lewallen, W7EL wrote:
"Can you please explain that a little more? (Radiation is a function of
loop area.)"

Not being a typist, I`ll refer you to "TV And Other Receiving Antennas"
by Arnold Bailey. On pages 407 and 408 Bailey gives two formulas for
computing the antenna resistance for a loop antenna.

On page 408, Bailey has Fig. 8-14 which plots radiation resistance (the
stuff we build antennas for) versus the loop perimeter in wavelengths.
For a square closed loop of one wavelength perimeter, the graph
indicates about 50 ohms.

Bill Orr, W6SAI in "All About Cubical Quad Antennas" gives the full-wave
vertical loop antenna an impedance of 125 ohms on page 15.

On page 14, Orr writes:
"For purposes of illustration, the two wire folded dipole may be "pulled
open" to a diamond-shaped loop fed at the bottom point. If this
distortion of the loop is continued the antenna will become a shorted
transmission line."

A perfect circle is the geometric shape enclosing the most area for a
given perimeter. The more corners a closed figure has, the more closely
it usually approximates a circle. That is why I commented on an octagon
versus a square.

Best regards, Richard Harrison, KB5WZI

I'm missing something here. I don't see anywhere in the response which
explains how "radiation is a function of loop area" and why increasing
the loop circumference would be advantageous.

Richard Harrison wrote:
Roy Lewallen, W7EL wrote:
"Can you please explain that a little more? (Radiation is a function of
loop area.)"

Not being a typist, I`ll refer you to "TV And Other Receiving Antennas"
by Arnold Bailey. On pages 407 and 408 Bailey gives two formulas for
computing the antenna resistance for a loop antenna.

On page 408, Bailey has Fig. 8-14 which plots radiation resistance (the
stuff we build antennas for) versus the loop perimeter in wavelengths.
For a square closed loop of one wavelength perimeter, the graph
indicates about 50 ohms.

Perhaps you build antennas for radiation resistance, but I think most
people build them to maximize radiation in some direction. In any case,
please exclude me from the "we" in your statement about what "we" build
antennas for. You said "radiation is a function of loop area" and
recommended a larger circumference. Equations are useless unless you
understand how to apply them. So can you use those equations to show us
how much more a loop of 800 foot circumference will radiate than one
with a 400 foot circumference? You can assume 100 watts applied to each,
and either zero wire resistance or any reasonable value.

Bill Orr, W6SAI in "All About Cubical Quad Antennas" gives the full-wave
vertical loop antenna an impedance of 125 ohms on page 15.

On page 14, Orr writes:
"For purposes of illustration, the two wire folded dipole may be "pulled
open" to a diamond-shaped loop fed at the bottom point. If this
distortion of the loop is continued the antenna will become a shorted
transmission line."

A perfect circle is the geometric shape enclosing the most area for a
given perimeter. The more corners a closed figure has, the more closely
it usually approximates a circle. That is why I commented on an octagon
versus a square.

Which brings me back to the question I asked,

Is the radiation pattern the same for a long skinny loop as for a round
one, as long as the enclosed area is the same?

Roy Lewallen, W7EL

Roy Lewallen wrote:
I'm missing something here. I don't see anywhere in the response which
explains how "radiation is a function of loop area" and why increasing
the loop circumference would be advantageous.

What you seem to be missing is that Richard H. said
absolutely nothing about radiated *power*. The radiation
*pattern* is certainly a function of loop area.
--
73, Cecil http://www.w5dxp.com

Roy Lewallen wrote:
"I don`t see anywhere in the response which explains how "radiation is a
function of loop area" and why increasing the loop circumference would
be advantageous."

Didn`t want to bore with details. Terman knows all and tells all. His
loop antenna story begins on page 907 of his 1955 opus:
"The radiation resistance of a loop antenna is less the smaller the loop
area. For the radiation resistance to be large enough to give good
antenna efficiency, it is necessary that the loop perimeter be of the
order of a wavelength. This introduces a difficulty, since when the
perimeter approaches or exceeds a half wavelength, then the loop current
in Fig. 23-40 will not be constant, nor will its phase nnecessarily be
the same in different parts of the loop. The prectical solution is to
build up the loop in such a way that the perimeter consists of resonant
antennas so arranged that the current everywhere in the loop perimeter
flows in the same direction around the loop. A variety of practical
physical arrangements for achieving this result have been devised, three
examples of which are illustrated in Fig. 23-41."

Yet another elegant phase inverter from Kraus as applied to a colinear
dipole appears on Cecil`s webpages.

Best regards, Richard Harrison, KB5WZI

Roy Lewallen wrote:
"Perhaps you build antennas for radiation resistance, but I think most
people build them to maximize radiation in some particular direction.."

You must have some radiation resistance or you have no signal, but not
all antennas are highly directional.

Page 8-10 of the 20th edition of the ARRL Antenna Book has (Eq 5):
Efficiency = radiation resistance / radiation resistance + loss
resistance

Zero radiation resistance = zero efficiency.

Best regards, Richard Hsarrison, KB5WZI

(Richard Harrison) writes:

Roy Lewallen wrote:
"Perhaps you build antennas for radiation resistance, but I think most
people build them to maximize radiation in some particular direction.."

You must have some radiation resistance or you have no signal, but not
all antennas are highly directional.

Page 8-10 of the 20th edition of the ARRL Antenna Book has (Eq 5):
Efficiency = radiation resistance / radiation resistance + loss
resistance

Zero radiation resistance = zero efficiency.

Best regards, Richard Hsarrison, KB5WZI

You're right that low radiation resistance makes it hard to attain
high efficiency. You want radiation resistance to be significantly higher
than loss resistance.

But ohmic losses usually aren't that bad in large dipoles and
loops. EZNEC or other modeling progarms can tell you how much you gain
by going up in wire size. For common geometries and wire sizes, the
difference will be a small fraction of a decibel.

For small ("magnetic") loops on the other hand, efficiency is very
much an issue, and you want to push the radiation resistance as high
as you can, and ohmic losses as low as you can. (Increasing the size
of the loop is the easiest way to increase radiation resistance, but
presumably there's a reason that you'er using a small loop in the
first place.

73
LA4RT Jon, Trondheim, Norway

Jon, LA4RT wrote:
"---but presumably there`s a reason that you`re using a small loop in
the first place."

True. In Norway I specified large dishes for scatter communications with
North Sea platforms but the government sponsored the geosynchronous
satellite program and ignored scatter. In the many tunnels I specified
300-ohm twinlead which works about as well as Andrew`s leaky coax and
costs much less. I think Norway is too rich to care about cost.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) writes:

Jon, LA4RT wrote:
"---but presumably there`s a reason that you`re using a small loop in
the first place."

True. In Norway I specified large dishes for scatter communications with
North Sea platforms but the government sponsored the geosynchronous
satellite program and ignored scatter. In the many tunnels I specified
300-ohm twinlead which works about as well as Andrew`s leaky coax and
costs much less. I think Norway is too rich to care about cost.

That's probably true :-)

73
Jon