[TowerTalk] Rotator Selection Question

[Michael Tope]

Thank you for the detailed reply on your rotator experience, Dave. I've 
changed the title of the thread because the OptiBeam thread got hijacked 
by the lengthy discussion of VSWR bandwidth vs performance bandwidth. 
Your QTH sounds a lot more unforgiving than the mesa location I am 
dealing with. I believe it has seen 100 mph gusts over the last 25 
years, but from what I know these occurrences are very rare. If it were 
just the OB2-40M, I wouldn't be as concerned, but I expect that log 
periodic antennas like the InnovAntennas DELPA-13 or the M2 10-30LP8 
that cover 30 meters have a much higher mass moment of inertia (i.e. 
lots of mass concentrated at large distances from the mast) than a 2 
element 40 meter beam. A higher mass moment of inertia typically means 
more stress on rotator components. This is why I want to get the rotator 
selection right.

Whatever rotator we choose will be controlled through 1600 ft of control 
cable. That sounds pretty daunting but the site is fed by 12 conductor x 
#12 AWG control cable, so multiple conductors can be connected in 
parallel to control voltage drop. I am however a little worried about 
using any rotator that tracks position via pulse counting rather than a 
potentiometer. The 12 conductor bundle has an over-shield, but the 
conductors aren't shielded from each other, so I am concerned about 
noise in the motors circuit getting coupled into the pulse counting 
circuit for rotators that use that kind of positioning scheme (e.g. M2 
OR2800 or Alpha-Spid Big RAK). We have been thinking about the Yaesu 
G-2800DXA or one of the large Prosistel rotators both of which use 
potentiometers as azimuth indicators.

Any others with experience or suggestions for rotator selection to turn 
a fairly large log periodic (e.g. InnovAntennas DELPA-13 or M2 10-30LP8) 
with an OB2-40M stacked above it would be appreciated.

73, Mike W4EF.....

On 5/21/2021 2:13 PM, David Gilbert wrote:
>
> 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.................
>>