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

Richard Harrison wrote:
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. . .

That's almost correct, but not quite. Except for loss, a triangular
loop, square loop, folded dipole, or round loop radiate equal amounts
*regardless of their enclosed areas* -- the amount of power that's
applied to them. The round loop doesn't radiate any more than any of the
others. None is one more "effective" than another, except that the
patterns will be different, so one might be more effective than another
at communicating in a particular direction -- but the round loop won't
necessarily always be the winner.

The statements you made earlier about a round loop radiating more, and
the continuing hangup about enclosed area, are based on the assumption
that the loop is small and is driven by a constant current source. For a
given amount of wire, the round loop has the highest radiation
resistance, and therefore if fed with a constant current, it consumes
and therefore radiates the most power of any loop made with the same
length of wire. This is a set of conditions often used by textbook
authors to illustrate some basic principles, but it isn't representative
of amateur (or commercial, for that matter) antenna use. It's necessary
to read and understand the qualifications given by the authors before
quoting their conclusions.

For a given length of wire, you'll get the most efficiency from a round
loop for a given length of wire. But unless the loop is electrically
very small, the efficiency will be high enough that this won't make any
noticeable difference. Making a large loop round -- or increasing its
enclosed area -- won't make it "radiate better" or be "more effective".

Roy Lewallen, W7EL

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

Ed wrote:
How would a three sided loop work?"

ON4UN`s "Low-Band DXing" says:
"---the delta loop can be called the poor man`s quad loop."

However the patterns and performance with various options are presented
because it requirea only one tall support and is easy to erect.

ON4UN`s book is published by ARRL.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
[...] Pythagoras who found the approximate value of pi by [...]
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.

He should have measured the diameter instead, which is easier than
measuring the radius (and would have given him the correct answer).

Just yanking your chain. Good discussion.

73,
kz1o

Thank you Roy - excellent as usual.

I recall a RADAR range equation where aperture (capture area) was one of the
terms

--
Caveat Lecter (a RADAR tech)



"Roy Lewallen" wrote in message
...
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 ??

Dave wrote:
"He should have measured the diameter instead, which is easier than
measuring the radius (and should have given him the correct answer)."

Dave is correct. The circumference is pi times the diameter. The radius
is only 1/2 the diameter. I miswrote.

Best regards, Richard Harrison, KB5WZI