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K7LXC@aol.com wrote in a private e-mail:
> Hi, Kurt --
>
> Can you shed some light on the 'effective area' and 'projected area'
> differences? What's wrong with 'effective area'? Is projected area the spec we
> should be using? How is it derived? MANY TNX for your wisdom.
>
> Cheers, Steve K7LXC
Hi Steve,You get the new award for asking a simple question that requires a
difficult answer!
My initial response to Steve was, that I needed to spend some time to collect my
thoughts on the subject and that I wished to respond publicly, with his approval
(which he agreed to). I wanted to make this discussion public because I think
that
there are many antenna parameters that are not adequately defined. This one is
just a very simple value that has somehow wandered into the gray zone.
Preface:
I don't think there is any significant measure of wisdom involved in the
comments
to follow! There is certainly no original thought, on my part.
All of the fundamental information resides in the public domain. I happened to
spend an inordinate amount of time (by my own estimation) researching this and
many other issues regarding antenna mechanical behavior, while writing my
antenna
design software. This could be classified as anti-wisdom!
The calculation of "effective" antenna area has experienced an evolution in
method, just as some of the electrical performance parameters.
We need to arrive at some kind of standard practice used by all to allow
everyone
to intelligently use the antenna area values.
Basics:
All antenna area calculations start with the simple determination of the
projected
areas of the antenna components.
The projected area is calculated by multiplying the length x width. A piece of
tubing that is 2" dia x 24" long has a projected area of 48 SqIn.
Next, all of the pieces in a specific component of the antenna (like an element
or
boom) are added up to get the total. Usually, the total is divided by 144 to get
the area in SqFt.
Then the element areas are added up to get the total elements area when the wind
is parallel to the boom.
The boom area applies to when the wind is parallel to the elements.
So, we have the flat projected area of the antenna at two azimuth angles, 0 & 90
degrees.
What happens after this is what can cause confusion.
Problems:
Effective area Methods:
Back in 1992, when I wrote the 1st version of YagiStress, there was a popular
concept that said the maximum antenna area could be found by solving the
Pythagorean equality (A^2 + B^2 = C^2) using the total element and total boom
areas.
Max area = (Boom area^2 + Element area^2)^.5 This always produced a value that
was
larger than either of the two areas and it occurred at azimuth angles near 45
Deg.
I'm pretty sure that Hygain and Force 12 were using this method to generate
their
spec's. Leeson (W6QHS now W6NL) and I were also using it.
I was never able to figure out what the others were doing.
Drag Coefficients:
All recognized standards, for analyzing structures subject to wind loading,
allow
for the application of a drag coefficient to account for the shape of the
structural members.
This is often referred to as a "shape factor". EIA-222-C (1976) used .666,
EIA-222-D (1986) used 1.2, UBC (1988) used .8.
All of the factors reduce the flat projected areas by some amount to arrive at
the
"Effective Area" for an antenna using round members.
I think that some Mfgr spec's used this reduction and others may have not. It is
very clear that the spec's did not describe what the value represented.
Some manufacturers, in other publications clarified their calculations.
Confusion:
It was never very clear in my mind what the numbers represented. In some cases
it
was clearer than others, but trying to make intelligent comparisons was
impossible.
Now, maybe I was the only one who was confused. I'm sure most people thought the
areas were derived in the same fashion and could be compared. I am convinced
that
this was not true.
New Methods:
In the Spring 1993 issue of Communications Quarterly, Dick Weber, K5IU,
published
a paper describing wind flow over cylinders at various wind attack angles.
The methods described resulted in very different values from what many of us
were
getting. Leeson and myself independently made some test antennas and separately
arrived at the conclusion that the Weber method was correct. I know that Roger
Cox
at Hygain, and Tom Schiller at Force 12 also picked up on it. I have no direct
knowledge about the others.
Leeson changed his spreadsheets, but couldn't change his book. I made the
changes
for YS 2.0
Here are the changes that come out of the new method, it's termed "The Cross
Flow
Principle" by Weber, or the "Sin^2 behavior of Cylinders in Yaw," by Leeson:
The wind flow over the cylinders results only in loads that are perpendicular to
the axis of the cylinder. This means that all element loads result in forces
along
the boom axis.
Asymmetric element placement along the boom does not result in a wind torque
imbalance. This makes the Leeson element torque compensator unnecessary and
ineffective.
The Max Projected Area of a Yagi is the largest value determined for the boom or
the elements. If the boom area is larger than the total for the elements, the
boom
area is the max area.
The minimum is somewhere in between 0-90 deg azimuth. The min area angle is
determined by the ratio of the elements to boom area. If the boom and elements
areas are equal the minimum area occurs at 45 deg.
What do users need from an antenna area Spec.?
I define a user as one who will use the information to evaluate it and make
decisions, or a designer who will use the info to determine loads on a
structure.
The first thing most recipients of a specification do, is make an attempt to
compare the area values to other spec's to determine which is "best" or which
best
suits their application.
The second thing a user might do with the area value is design, or have his
installation designed.
In the U.S., some municipalities require UBC compliance, others require EIA. I'd
guess that differences exist in Europe also.
If the antenna area values are " flat projected areas", it is clear what the
values means and the designer can proceed with applying the appropriate shape
factor and wind pressures according to the code.
If the areas have been already factored, and the spec doesn't tell what was
done,
the information is useless. It actually can be dangerous, if the designer is
forced to guess what the value means!
The third thing a user might attempt to do with the information is select a
rotator. Efforts to match antennas and rotators, based on area alone, are
useless.
That is another discussion for another day.
Suggestion for a Standard Antenna Area measurement:
Manufacturers should calculate the flat projected areas of the antenna at 0
Degrees & 90 Degrees azimuth, and present them as such. That's it!
The user can decide what shape factors and wind loads to apply for determining
loads on the mast and tower.
It is important to list both values. Some antennas have more area at 0 Deg,
others
more at 90 Deg.
Example: Most 20 meter yagi's with 4+ elements have more element area than boom
area. 10 & 15 meter yagi's tend to have more boom than element area. This
assumes
that the designer has tried to minimize area.
We need both 0 & 90 Deg areas to determine the loads on a mast or rotating
tower.
The max loads will usually occur at either of the two angles, unless we're lucky
enough to get them equal at both.
Another antenna property that we need, but has not been a consistent part of the
antenna spec's is antenna torque. There is only one generic value, for Mfgr's to
define here.
It is the torque developed when the boom is broadside to the wind. This is
caused
by either placing the mast connection away from the center of the boom. Or, coax
and balun loads that will cause an imbalance.
There is another (usually small) antenna torque developed by the connection to
the
mast (or tower), when the antenna is pointed into the wind. If we mount the
antenna to a 2" Dia or other common size mast, the Mfgr can provide this torque
value. Since, the Mfgr has no control over how we will mount the antenna to a
tower sidemount or TIC ring., he can't determine what this value is. That's our
job!
Just getting the torque value for the wind broadside to the boom case would be a
great improvement! It might make some Mfgr's stop trying to attach the mast to
an
antenna at the weight balance point, which is usually not at the zero wind
torque
location. At the very least, providing this value, would allow us to understand
why some antennas are "Wind Vanes!," and avoid them, unless we plan to overpower
the problem with more robust rotator!
Comments are welcome and invited, 73, Kurt
--
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress
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K7LXC@aol.com wrote in a private e-mail:
<BLOCKQUOTE TYPE=CITE>Hi, Kurt --
<P> Can you shed some light on the 'effective
area' and 'projected area'
<BR>differences? What's wrong with 'effective area'? Is projected area
the spec we
<BR>should be using? How is it derived? MANY TNX for your wisdom.
<P>Cheers, Steve K7LXC</BLOCKQUOTE>
Hi Steve,You get the new award for asking a simple question that requires
a difficult answer!
<BR>My initial response to Steve was, that I needed to spend some time
to collect my thoughts on the subject and that I wished to respond publicly,
with his approval (which he agreed to). I wanted to make this discussion
public because I think that there are many antenna parameters that are
not adequately defined. This one is just a very simple value that has somehow
wandered into the gray zone.
<P><B><U>Preface:</U></B>
<BR>I don't think there is any significant measure of wisdom involved in
the comments to follow! There is certainly no original thought, on my part.
<BR>All of the fundamental information resides in the public domain. I
happened to spend an inordinate amount of time (by my own estimation)
researching
this and many other issues regarding antenna mechanical behavior, while
writing my antenna design software. This could be classified as anti-wisdom!
<BR>The calculation of "effective" antenna area has experienced an
evolution in method, just as some of the electrical performance parameters.
<BR>We need to arrive at some kind of standard practice used by all to
allow everyone to intelligently use the antenna area values.
<P><B><U>Basics:</U></B>
<BR>All antenna area calculations start with the simple determination of
the projected areas of the antenna components.
<BR>The projected area is calculated by multiplying the length x width.
A piece of tubing that is 2" dia x 24" long has a projected area of 48
SqIn.
<BR>Next, all of the pieces in a specific component of the antenna (like
an element or boom) are added up to get the total. Usually, the total is
divided by 144 to get the area in SqFt.
<BR>Then the element areas are added up to get the total elements area
when the wind is parallel to the boom.
<BR>The boom area applies to when the wind is parallel to the elements.
<BR>So, we have the flat projected area of the antenna at two azimuth angles,
0 & 90 degrees.
<BR>What happens after this is what can cause confusion.
<P><B><U>Problems:</U></B>
<P><B>Effective area Methods:</B>
<BR>Back in 1992, when I wrote the 1st version of YagiStress, there was
a popular concept that said the maximum antenna area could be found by
solving the Pythagorean equality (A^2 + B^2 = C^2) using the total element
and total boom areas.
<BR>Max area = (Boom area^2 + Element area^2)^.5 This always produced a
value that was larger than either of the two areas and it occurred at azimuth
angles near 45 Deg.
<P>I'm pretty sure that Hygain and Force 12 were using this method to generate
their spec's. Leeson (W6QHS now W6NL) and I were also using it.
<BR>I was never able to figure out what the others were doing.
<P><B>Drag Coefficients:</B>
<BR>All recognized standards, for analyzing structures subject to wind
loading, allow for the application of a drag coefficient to account for
the shape of the structural members.
<BR>This is often referred to as a "shape factor". EIA-222-C (1976) used
.666, EIA-222-D (1986) used 1.2, UBC (1988) used .8.
<BR>All of the factors reduce the flat projected areas by some amount to
arrive at the "Effective Area" for an antenna using round members.
<BR>I think that some Mfgr spec's used this reduction and others may have
not. It is very clear that the spec's did not describe what the value
represented.
<BR>Some manufacturers, in other publications clarified their calculations.
<P><B>Confusion:</B>
<BR>It was never very clear in my mind what the numbers represented. In
some cases it was clearer than others, but trying to make intelligent
comparisons
was impossible.
<BR>Now, maybe I was the only one who was confused. I'm sure most people
thought the areas were derived in the same fashion and could be compared.
I am convinced that this was not true.
<BR>
<BR>
<BR><B><U>New Methods:</U></B>
<BR>
<BR>In the Spring 1993 issue of Communications Quarterly, Dick Weber, K5IU,
published a paper describing wind flow over cylinders at various wind attack
angles.
<BR>The methods described resulted in very different values from what many
of us were getting. Leeson and myself independently made some test antennas
and separately arrived at the conclusion that the Weber method was correct.
I know that Roger Cox at Hygain, and Tom Schiller at Force 12 also picked
up on it. I have no direct knowledge about the others.
<BR>Leeson changed his spreadsheets, but couldn't change his book.
I made the changes for YS 2.0
<P>Here are the changes that come out of the new method, it's termed "The
Cross Flow Principle" by Weber, or the "Sin^2 behavior of Cylinders in
Yaw," by Leeson:
<P>The wind flow over the cylinders results only in loads that are perpendicular
to the axis of the cylinder. This means that all element loads result in
forces along the boom axis.
<BR>Asymmetric element placement along the boom does not result in a wind
torque imbalance. This makes the Leeson element torque compensator unnecessary
and ineffective.
<BR>
<BR>The Max Projected Area of a Yagi is the largest value determined for
the boom or the elements. If the boom area is larger than the total for
the elements, the boom area is the max area.
<BR>The minimum is somewhere in between 0-90 deg azimuth. The min area
angle is determined by the ratio of the elements to boom area. If the boom
and elements areas are equal the minimum area occurs at 45 deg.
<BR>
<P><B><U>What do users need from an antenna area Spec.?</U></B>
<BR>
<BR>I define a user as one who will use the information to evaluate it
and make decisions, or a designer who will use the info to determine loads
on a structure.
<P>The first thing most recipients of a specification do, is make an attempt
to compare the area values to other spec's to determine which is "best"
or which best suits their application.
<P>The second thing a user might do with the area value is design, or have
his installation designed.
<BR>In the U.S., some municipalities require UBC compliance, others require
EIA. I'd guess that differences exist in Europe also.
<BR>If the antenna area values are " flat projected areas", it is clear
what the values means and the designer can proceed with applying the appropriate
shape factor and wind pressures according to the code.
<BR>If the areas have been already factored, and the spec doesn't tell
what was done, the information is useless. It actually can be dangerous,
if the designer is forced to guess what the value means!
<BR>
<BR>The third thing a user might attempt to do with the information is
select a rotator. Efforts to match antennas and rotators, based on area
alone, are useless. That is another discussion for another day.
<BR>
<P><B><U>Suggestion for a Standard Antenna Area measurement:</U></B>
<BR>
<BR>Manufacturers should calculate the flat projected areas of the antenna
at 0 Degrees & 90 Degrees azimuth, and present them as such. That's
it!
<P>The user can decide what shape factors and wind loads to apply for
determining
loads on the mast and tower.
<BR>It is important to list both values. Some antennas have more area at
0 Deg, others more at 90 Deg.
<BR>Example: Most 20 meter yagi's with 4+ elements have more element area
than boom area. 10 & 15 meter yagi's tend to have more boom than element
area. This assumes that the designer has tried to minimize area.
<BR>We need both 0 & 90 Deg areas to determine the loads on a mast
or rotating tower. The max loads will usually occur at either of the two
angles, unless we're lucky enough to get them equal at both.
<P>Another antenna property that we need, but has not been a consistent
part of the antenna spec's is antenna torque. There is only one generic
value, for Mfgr's to define here.
<BR>It is the torque developed when the boom is broadside to the wind.
This is caused by either placing the mast connection away from the center
of the boom. Or, coax and balun loads that will cause an imbalance.
<BR>There is another (usually small) antenna torque developed by the connection
to the mast (or tower), when the antenna is pointed into the wind. If we
mount the antenna to a 2" Dia or other common size mast, the Mfgr can provide
this torque value. Since, the Mfgr has no control over how we will mount
the antenna to a tower sidemount or TIC ring., he can't determine what
this value is. That's our job!
<BR>Just getting the torque value for the wind broadside to the boom case
would be a great improvement! It might make some Mfgr's stop trying to
attach the mast to an antenna at the weight balance point, which is usually
not at the zero wind torque location. At the very least, providing this
value, would allow us to understand why some antennas are "Wind Vanes!,"
and avoid them, unless we plan to overpower the problem with more robust
rotator!
<P>Comments are welcome and invited, 73, Kurt
<BR>
<BR>
<P>--
<BR>YagiStress - The Ultimate Software for Yagi Mechanical Design
<BR>Visit <A
HREF="http://www.freeyellow.com/members3/yagistress";>http://www.freeyellow.com/members3/yagistress</A>
<BR> </HTML>
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