Well. I started out with a Prosistel PST-61D, and in addition to the
OB2-40 I have a Optibeam OB16-3 on the mast. It worked fine for a few
years, but eventually the wind gusts here stripped a section of the
toothed gear (not the worm gear) on the PST-61D ... and that happened
while the rotator wasn't in use. Some people have questioned that, but
those that do don't understand the winds here. I live near the southern
end of the Huachuca Mountains in southern Arizona, and the mountain
range runs primarily SSE to NNW. The winds are nominally from the
south-southwest, and the mountain range tries to block them. I live on
the eastern slope of the mountain range, I live near the southern end of
the range, and the blockage creates a zone of lower pressure in my
area. The winds are forced to snake around and over the end of the
range and then backfill in my area. The result is twisting winds that
tear down the hillside with significant force ... and noise. As I said,
I've measured wind gusts as high as 100 mph on a clear day, and I've
seen branches being ripped off trees 200 feet away from me while I was
standing in dead calm air. When we were first building our house and
only had the slab but no walls, I watched one gust lift two 16 foot long
2x6's off the slab and throw them 25 feet away ... through the air the
entire path.
When those twirlers (mini-tornadoes, actually) happen to hit my tower
straight on, the force on the boom is no long balanced ... it's
literally a push-pull situation where one end of the boom is being
pushed and the other is being pulled. The forces add! The chromoly
mast is 20 feet of 2 inch diameter, 1/4 inch thick material and of
course it has some amount of torsional spring that can store some of
that force and then release it in conjunction with the wind. The stress
on the rotator gears that results from all of that must be outrageous.
Anyway, I ended up finding a used PST-71D and installed that a couple of
years ago (see anecdote below). Since then I have had no problem. Both
of the Optibeam antennas have held up fine as well, with one exception.
The OB2-40 has dacron (or some similar material) truss lines that
support the elements. So does the OB2-40M, I believe. Those lines are
fixed to the vertical truss supports (the ones at the boom) with simple
plugs that fit into the top end of the truss support. They are held
there only by the pull of the truss lines, at least unless Optibeam has
changed things since. In my case, the wind gusts have lifted the
antenna elements high enough to pop those plugs out of the truss
supports. That happened to both of them a few years ago and it's such a
pain in the butt to drop both antennas to fix it that I never bothered.
It's a testament to the sturdiness of Optibeam antennas in general that
the OB2-40 elements have held up all this time even without the trusses
... although of course they do sag just a tiny bit more. Early on I was
worried that the elements might fracture over time due to not being
supported properly, but every time I see the wind bend the elements into
a C-shape like they did even before the truss supports failed I realize
that the lateral bending action is far more severe than the up and down
action. In any case, I strongly recommend that you pin those plugs in
place before you hoist up the antenna.
At this point, if I end up having trouble with the PST-71D I'll probably
have to go with a prop pitch, but it's kind of weird to have to be
concerned more about the rotator than the antennas.
Anecdote: The PST-71D weighs something like 85 pounds, and I used a
hand winch driven by a battery-operated drill to raise the rotator up to
the 65 foot level on my 70 foot tower (winch at ground level, pulley at
the top). The drill didn't break a sweat and still had lots of charge
when I was done. I never bothered to calculated the stored energy
involved since I had no way of estimated things like friction, but it's
amazing what there is in a 1.5 Ah battery.
73,
Dave AB7E
On 5/21/2021 12:29 PM, Michael Tope wrote:
Thank you sharing that information, Dave. I am curious what rotator
you are using at your QTH? We are contemplating some sort of 10 to 30
MHz log periodic (like the InnovAntennas DELPA-13 or the M2 10-30LP8)
mounted on the same mast below the OB2-40M, so we need to make sure we
select a rotator that is up to the task.
73, Mike W4EF............
On 5/20/2021 1:25 AM, David Gilbert wrote:
I have owned an OB2-40 (the non-Moxon version) for ten years and it
has held up just fine to ridiculous wind gusts on my southern Arizona
hillside. The antenna is built like the proverbial tank and I've
measured wind gusts as high as 100 MPH. The wind gusts are brutal
here for about three months every spring, to the point that I usually
can't bring myself to even look at my antennas when I hear one of
those mini-tornadoes come roaring down the side of the mountain. I
even watched one lift my teenage son about two feet off the ground
while he was weedwacking part of the lot. It picked him up
vertically (with the weedwhacker still in his hands), moved him about
three feet laterally, and set him back down. This is no exaggeration
... I saw it with my own eyes.
The OB2-40M is listed as having the same overall weight as my OB2-40
and the construction looks identical, except of course for the Moxon
wingtips. The wind area of the OB2-40m is 5.9 sq ft versus 6.8 sq ft
for the OB2-40 and I would think that the Moxon configuration adds a
bit of additional stability, so I don't see why it wouldn't hold up
as well or better than my antenna.
73,
Dave AB7E
On Wed, May 19, 2021 at 7:34 PM Michael Tope <W4EF@dellroy.com> wrote:
One of the radio clubs I belong to is interested in buying an OptiBeam
OB2-40M two-element 40 meter Moxon Yagi. Optibeam is kind of vague on
wind survival specifications. Their website indicates "Windload at 130
km/h 430 N / 0,54 m² / 5,9 feet²". To me that implies the
antenna is designed to survive 130 km/h winds (80.7 MPH), but it
doesn't
actually say that. The antenna is going to be installed at a Southern
California QTH at a fairly low elevation, so icing is extremely
unlikely. However, the antenna will be installed on a high narrow
ridge
that lend itself to very high uneven (shearing) winds when we get the
occasional winter storm or Santa Ana wind condition.
Anyone out there using one of these antennas? I am curious what you
think of it in terms of performance and reliability?
Thanks and 73,
Mike W4EF.................
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