CQK8DO@aol.com wrote:
> I have experimented with my 4 el M2 40 meter yagi... I have found that it
> seems to ride the quietest, i.e. the least amount of visible thrashing,
> bowing, jerking, torquing the tower, etc., when it is quartering to the
> wind... For non sailors, that means with the boom (the keel) at roughly a 45
> degree angle to the prevailing wind... It does not seem to matter which end
> {i.e. dir or ref} of the antenna is upwind... Now, I need to build a weather
> station and interface to the computer and the rotator to keep it that way as
> the low pressure system slides past and the wind shifts constantly....
>
> Cheers .... Denny
Hi Denny,
Your empirical experience jives with what we have come to learn about wind on
yagis.
The azimuth of lowest area is somewhere between 0 and 90 degrees. The exact
angle
depends on the ratio of elements to boom area. If the boom and element areas are
equal the lowest area of exposure occurs at 45 degrees.
There are some other potential things going on that might makes us want to point
an antenna differently. But, your solution is a good start to reduce antenna
thrashing and the load on the tower.
I have had the fortunate (or unfortunate) opportunity to have a TH7 on a
temporary setup for the last 6 months. The rotor attachment to the pole is not
up
to the task of holding the antenna in any good breeze. The TH7 always ends up
with the boom broadside to the wind, no matter what. The areas provided by
Hygain
show that the area of the elements is almost 3 times more than the boom area.So,
this unintentional test shows that the antenna naturally seeks the lower of the
two exposed areas.
I've built and tested some dummy antennas (mast at boom center and very
asymetric
element placement) and witnessed that they have two stable positions when left
to
rotate freely. They are 0 and 90 degrees and more often than not they seek the
one that has the least area.
For most antennas like the TH7 or a 204BA parking it at 90 degrees is fine
because the boom has a higher safe wind speed and less area than the elements.
Some of the long boom monobanders can go the other way.
73, Kurt
--
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|