Kurt:
With my planned antenna arrays, I won't have appreciable mast torque
caused by off-center mounting of the boom on the mast. I'll have a small
V/UHF vertical at the top, a D40 rotatable dipole (same length on either
side of the boom bracket) below that, and the Bencher Skyhawk (purportedly
torque balanced by the element and boom compensator pieces.) If and when I
install the 2M Yagi (KLM 144-148-13LBA), it will be mounted at the center of
its boom; there's not enough element aluminum to worry about finding the
balance center of the boom.)
I think what you're cautioning us about is the case where we have large
HF Yagi arrays in which the balance point (and mounting point) of the array
is not the center of the boom. In these cases, wind blowing broadside to
the boom will want to turn the longer side of the boom away from the wind,
creating torque on the mast/rotator/tower.
Dick's papers on aerodynamic balancing discuss how to compensate for
such imbalances, e.g., fixing a weight onto the boom so the balance point is
shifted to the center of the boom, or entending the shorter side of the boom
with a piece of aluminum tubing sleeved onto the main boom, or installing
compensators.
BOTTOM LINE: You have to do some "cypherin' " before you put up an
antenna/tower system. Apparently, there's more to ham radio than Ohm's Law.
GL es V=IR de
Gene Smar AD3F
-----Original Message-----
From: Kurt Andress <K7NV@contesting.com>
To: EUGENE SMAR <SPELUNK.SUENO@prodigy.net>
Cc: Tower Talk <towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Thursday, June 07, 2001 5:46 PM
Subject: Re: [TowerTalk] Wind Loading redux
>EUGENE SMAR wrote:
>>
>> TT:
>>
>> I won't feel offended if you hit "delete" at this point.
>>
>
>> SNIP
>
>
>> A final point: If you blindly point your mast to due north (please
>> don't start!) and attach all your long-boom Yagis from the same side of
the
>> tower, you're unbalancing the torque on your tower system. Dick wrote a
>> couple of papers on aerodynamic balancing of antennas to minimize tower
>> torque. I'm now going to mount my four antennas (Skyhawk, 2M Yagi (23
foot
>> boom), D40 rotatable dipole, and triband V/UHF vertical) on the mast on
>> alternating sides, depending on how the math shakes out. No point in
>> deliberately putting unnecessary stress in your tower system, if it can
be
>> avoided simply.
>>
>
>Don't forget to think about what's happening when the antennas are
>broadside to the wind.
>
>The common practice of attaching a mast at the center of gravity of the
>antenna, is quite convenient for handling the antenna. But, it is really
>quite an awful practice, for what happens after we get it installed!
>If the mast is not connected at the physical center of the boom, the boom
>will generate torque when exposed to the wind. A small offset could be used
>to balance the asymetry of the boom/element mounting hardware, but that is
>usually pretty small.
>
>A good example of this is the good old KLM 5 el 20, on a 42' boom. At a
>sustained wind speed of about 35 mph (if I remember it correctly), the
>antenna torque generated by the placement of the mast equals the stall
>torque of a T2X rotator. The "effective moment" type of rating says it
>should be ok.
>
>I think this is a significant factor in the number of antenna's eating
>rotators in our empirical database. There are some commercially sold
>antennas that generate some spectacular torque values.
>
>Now, if you're real clever you can figure out where to point the antenna to
>get this torque to cancel out the one caused by the offset mounting of the
>antenna to the mast :-)
>
>If you mount the antennas on the mast to mitigate the antenna/mast mount
>torque source and don't fix the other probelm, you will still have torque
>loads, they'll just happen at other points on the compass.
>
>
>> 73 de
>> Gene Smar AD3F
>> P.S. I'd advise you actually to calculate the area of your antennas, and
>> NOT rely on manufacturers' specs (Unless you know how they came up with
the
>> numbers. And I apologize for starting another re-thread.) If you're
>> contemplating the purchase of an antenna, maybe the manufacturer will
>> sell/give you a manual for this purpose. (But then again, maybe not.)
>>
>
>Force 12 ,Hygain ,and Cal-Av tell us enough to use their areas. We should
>commend them for doing that!
>We would be doing ourselves a favor by pestering the heck out of the others
>to get the info. They might figure out it is easier to just publish figures
>we can use. Then everyone will be happy!
>
>--
>73, Kurt, K7NV
>
>http://yagistress.freeyellow.com
>
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