Morrison Hoyle wrote:
> > Based on information that Roger Cox of HyGain has previously posted
> > to the Towertalk group, putting the boom broadside to the wind is
> > best of HF antennas.
>
> Not so fast!
>
> ON4UN's excellent book Low Band Dxing (an ARRL publication) is much
> clearer on the subject. In section 13 paragraph 3.3.4.1 he writes:-
> "If the area of the boom is smaller than the area of all the
> elements, then put the boom perpendicular to the wind. And vice
> versa......"
>
> He then works through a detailed example of calculations. He also
> offers software including a WIND AREA module which makes the
> calculations fairly easy.
>
> It is clear that if you have a short boom HF antenna, the elements
> will have a higher wind load, but for a multi-element and therefore
> long boom antenna, the boom has the higher wind load. You need to
> calculate to decide which applies to you.
>
> These things are NOT a matter of opinion as some contributors seem to
> think. Do some number crunching and work on the basis of FACT!
>
> .
> ,--_|\ Morrison Hoyle - vk3bcy@dcsi.net.au
> / Oz \ - Phone +61 (0)3 56891205
> \_,--\_/ - PO Box 248, FOSTER, Vic 3960 Australia
> v
>
Not so Fast!
Let's try to examine the subject in light of the best available
information, not the opinions and emotions of others, as has been
suggested.
The best and most recent treatise on this subject (that I am aware of)
is
the Spring 1993 paper by Dick Weber, K5IU, in Communications Quarterly.
This presentation completely describes how wind flows over the
components
of a yagi antenna. In the discussion, all of the load vector
developments
are described. The entire system load case may be calculated via simple
trigonometric calculations.
I have forwarded this document to Steve Morris, K7LXC, who hopefully is
figuring out how to make it available to everyone via the famous $1 a
copy, procedure we have all made great advantage of.
I thought I presented the concept that where one should point any
antenna
is completely dependent upon what one is trying to accomplish, in a
previous post. Apparently, this idea has not yet found a home.
So, we will try it again.
Where one should point any antenna relative to the wind is completely
dependent upon the antenna configuration and what one is trying to
accomplish with the exercise!
Is that clear? Probably not!
Ok, let's examine the possibilities.
We either orient the antenna to minimize the loads on the antenna to
make it survive, or we orient the antenna to minimize the loads on the
tower to make it survive. These are usually mutually exclusive
requirements. I.E. You can usually have one or the other, but not both!
If the object of the exercise is to "SAVE THE ANTENNA", we want to point
it in the direction that presents the lowest possible area to the wind.
This will be either, 0 degrees to the wind or 90 degrees to the wind,
depending on whether the elements or boom have the lowest area.
But, this can be severely distorted by the actual antenna design. If the
antenna boom has the lowest exposed area, we would expect to want to put
it broadside to the wind. This is correct if the antenna design is such
that the boom design meets our windspeed.
EXAMPLE:
Antenna elements area = 9.3 SqFT Boom area = 4.3 SqFT
The safe wind speed for the elements are 100 Mph. The safe wind speed
for the boom is 50 Mph. This is because the designer made a mistake and
didn't get the boom design correct.
So, if we follow the above mentioned solution, we put the weak boom
broadside to the wind and have it fail at 50 mph, instead of turning it
90 degrees and have the antenna survive 100 mph.
Without knowing complete information about the antenna design we are
only left to imagine what to do.
The above statement attributed to ON4UN assumes that the antenna was
properly designed for equal windspeed survival for both the elements and
the boom. Then the lowest antenna area is the way to go, if one is
trying to save the antenna.
ON4UN has all of the same reference material that are given as
references in his book. So, we have no controversy over this concept.
If the object is to "SAVE THE TOWER", we want to point the antenna in a
direction that produces the lowest possible area (hence windload).
The K5IU paper, plainly explains how this value is found.
It never exists at 0 degrees or 90 degrees, but is somewhere in between.
Where the lowest total antenna area and load on the tower exists is
dependent on the relationship between the elements and boom areas.
Let's take a look at the venerable 204BA.
According to Roger Cox, @ Hygain, the 204BA has projected areas of 9.3
SqFt for the elements, and 4.3 SqFt for the boom. I get identical values
with my software.
Ok, so we run these areas thru the formulas provided by K5IU, to see
what happens at all azimuth angles.
We find that the the total windload for the antenna is greatest with the
antenna pointed into the wind (because the elements area is greatest).
The antenna windload for the antenna at 90 degrees is less (because the
boom has less area than the elements). But the apparent surprise is that
the absolute minimum antenna area (hence load on the tower) occurs when
the boom is at 65 degrees to the wind.
Like somebody said, we should make these decisions based on fact,
instead of emotion.
This information resides in the public domain. I wish someone else would
go get it and contribute to the discussion.
What K5IU presented is sound and survives all research.
I've only added the aspect of the question about determining the purpose
for making a decision. The basic principles exist on their own merit.
We simply need to decide if we are designing for antenna or tower
survival. Weak antennas and strong towers require different solutions
than strong antennas and weak towers.
Now that we've touched on that aspect, let's suggest a another variable
into the process.
Some antenna manufacturers are very dilligent about maintaining antenna
torque balance, others are not.
Some put the mast connection at the center of the boom, others do not.
Masts connected to any other location than the boom center, will
generate antenna torque when the wind blows and the antenna is oriented
other than boom into the wind.
This will significantly change our decisions about where to point the
antenna, of course depending on what we are trying to save, antenna or
tower.
Lots of fun stuff eh?
73, Kurt
--
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