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Re: [TowerTalk] KT34XA Torque Balance

To: "TowerTalk" <>
Subject: Re: [TowerTalk] KT34XA Torque Balance
From: W0UN -- John Brosnahan <>
Date: Fri, 07 Jan 2005 22:11:56 -0600
List-post: <>

I recall that this aspect of antenna design and the calculations are addressed in David Leeson's book, "Physical Design of Yagi Antennas."

Actually Dave's work in his book was not correct and has been replaced with the cross flow
work of Dick Weber, K5IU. I sent the following info off-line to W4EF but it is probably
useful to a number of others since time has passed and we all tend to forget some
of these postings when they are not relevant at the time.

Here is a (probably too long) collection of posts to this reflector in the past that address
the issue of torque balance. And the info references the original publications for further

73--John W0UN

Large Antenna Wind Loading

A N Thompson <>
Fri, 22 Nov 1996 23:04:08 PST


While Mr. Leeson's book ("Physical Design of Yagi Antennas") is very informative, he is unfortunately incorrect in his analysis and comments on torque compensation (chapter 6). There is no need for dummy elements. The correct way to torque balance an antenna is described by Dick Weber, K5IU, in several articles that appeared in Communications Quarterly ("Determination of Yagi Wind Loads Using the 'Cross-Flow Principle,' " Spring 1993; "Aerodynamic Balancing: Part 1," Summer 1994; and "Aerodynamic Balancing: Part 2," Winter 1995).

73, Arliss W7XU


[TowerTalk] RE: Rotator ratings

from [<>Dick Weber] [<>Permanent Link][<>Original]
To: <<>>
Subject: [TowerTalk] RE: Rotator ratings
From: <> (Dick Weber)
Date: Tue, 18 Mar 1997 18:35:22 -0600

Sent:   Tuesday, March 18, 1997 1:56 AM
Subject:        Rotator ratings

In a message dated 97-03-15 21:34:32 EST, (Dick Weber)

> Why are rotors rated in "square feet?" Well, no one ever  sat down to
> it out using principles used by structural, mechanical, and aeronautical
> engineers that have been known for over 100 years. My guess is someone
> starting using this method and everyone else followed suit. This has
> in a real mess and is somewhat comical. For example, Hygain rates its
> Tailtwister at 1000 in-lbs of torque and says its good for 20 sqft of
>  ( By the way they don't say anything about how well balanced the antennas
> should be.) In addition, Hygain rates its HDR300 at 5000 in-lbs of torque
> says it good for up to 25 sqft of antenna. It doesn't make much sense that
> rotor with 5X torque can only handle 25% more antenna. Hygain is not the
> company with meaningless ratings. These companies should only advertise
> turning torque ratings and their brake holding ratings.
>  Hopefully someone will have the ratings for your rotor. Knowing the rating

> and using the information in the above articles you can get maximum
> utilization of your rotor.
      For a long time the time honored way of rotator selection was by using
the 'square foot' wind loading of the antennas method described above; it is
imperfect.  The biggest deficiency was that two antennas with similar square
footage could put very different demands on a rotator (i.e. 3L 20M beam vs.
long boom 10M beam).

    Several years ago Hy-Gain came up with a figure they call Effective
Moment that more accurately took antenna forces into account.  The formula
for EM is antenna weight times turning radius.  Yaesu uses the same figure
but calls it a 'K-Factor'.  Emoto uses a GD2 formula that is slightly
different but is a similar measurement.  This, and much more info, was
covered in my 3-part series on rotators that recently appeared in CQ Contest
magazine (reprints are available if you missed it).

    BTW, the TOWER TECH MARC (Mast, Antenna and Rotator Calculator) software
program is almost ready and it calculates the Effective Moment of the
antennas under consideration and makes rotator recommendations as part of the

73, Steve K7LXC


One last note on this topic. If the wind loading or balancing programs use the
method where the force on a tube at an angle to the wind is in line with the
wind direction the method is in error. This misconception crept into ham radio
about 30 years ago. For the past 100 years structural engineers, mechanical
engineers, and aeronuatical engineers have used the "cross flow" principle to
find the wind loads on members at an angle to the wind. This concept was proven
experimentally as far back as 100 years ago. The cross flow principle says that
the force on a tube is perpendicular to the axis of the tube. The magnitude
varies with the angle of the tube relative to the wind. There will be a
component of this force in line with the wind which is called the "Drag Force."
There is another component that is at 90 degrees to the wind which is called
the "Cross Wnd Force." When a tube is perpendicular to the wind the value of
the "Cross Wind Force is zero. When the tube is at 45 degrees the force
component in line with the wind (Drag Force) has the same magnitude as the
force perpendiccular to the wind (CrossWind force).

For anyone intereseted in seeing the derivation of this concept and a
comaprison to test data taken in a wind tunnel look in "Fluid-Dynamica Drag"
(page 3-11) by Hoerner. You'll probably have to check it out of a university
that has an engineering section. Or you can look in "Mechanical Engineering in
Radar and Communications" (p 162-165) by C.J. Richards. If you don't want to
read this stuff there's a very easy experiment you can do. Get a 2 ft length
of tube or plastic pipe. With your arm out the window of a car going about
30-40 MPH, rotate the tube relative to the wind. When the tube is strainght up
and down you'll feel only a force in line with the wind. Now rotate the tube in
the wind. If you rotate the top into the wind you'll feel an upward force in
addition to the force in line with the wind. The upward force is perdedicular
to the wind and is the "Cross Wind Force" which is the vertical component of
the force that is perpendicular to the tube's axis. Or, rotate the top of the
tube reward. You'll now feel a downward force in addition to the in line force.
Again this is the "Cross Wind force."

If the cross flow principle was not true ships with rudders wouldn't
turn. (They would only slow down as the rudder is moved in he water.) Also you
wouldn't be able to stick your hand out a car window and make your hand soar up
and down as the palm is angled into the wind. In addition all structures,
buildings, towers, and bridges that have been designed to handle winds have
been designed incorrectly by professinal engineers for the past 100 years.

If anyone would like other references on this topic let me know.

Dick, K5IU


[TowerTalk] Antenna Area Spec's

from [<>Kurt Andress] [<>Permanent Link][<>Original]
To: <<>>
Subject: [TowerTalk] Antenna Area Spec's
From: <> (Kurt Andress)
Date: Sun, 16 Aug 1998 23:59:31 -0700 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
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.

I don't think there is any significant measure of wisdom involved in the
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
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
to intelligently use the antenna area values.

All antenna area calculations start with the simple determination of the
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
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
What happens after this is what can cause confusion.


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
Max area = (Boom area^2 + Element area^2)^.5 This always produced a value that
larger than either of the two areas and it occurred at azimuth angles near 45

I'm pretty sure that Hygain and Force 12 were using this method to generate
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,
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
"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.

It was never very clear in my mind what the numbers represented. In some cases
was clearer than others, but trying to make intelligent comparisons was
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
this was not true.

New Methods:

In the Spring 1993 issue of Communications Quarterly, Dick Weber, K5IU,
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
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
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
for YS 2.0

Here are the changes that come out of the new method, it's termed "The Cross
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
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

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
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

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
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
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
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,
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
that the designer has tried to minimize area.
We need both 0 & 90 Deg areas to determine the loads on a mast or rotating
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
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
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
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
antenna at the weight balance point, which is usually not at the zero wind
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


See: for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

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