[TowerTalk] OptiBeam OB2-40M

Fri May 21 17:13:52 EDT 2021

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 at 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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