Gary "Joe" Mayfield wrote:
> 1 degree is tough - must be a huge antenna, or a really high band. Alpha
> Spid claims 1 degree
>
Another point on the off center loading of the rotator. Keeping a 1
degree resolution with a rotator capable of that will probably require
some loading, otherwise winds are likely to cause some slight movement
unless the rotator and all hardware are "really tight". I'd balance it
with about a 10# differential. unless it's a good size dish. Then you
might want to keep as much as a 20 to 50# preload.
However even with C-band and the satellites being around 2.5 degrees
apart we didn't have to maintain that kind of resolution in the
pointing. Just the capability of returning to the same place within a
couple of degrees.
73
Roger (K8RI)
> http://alfaradio.ca/alfaspid.html
>
> as does the monster from emotator
>
> http://www.marcucci.it/english/prodotti/schede/scheda.asp?ID=13939
>
> The Yaesu rotors will not meet you spec.
>
> 73,
> Joe kk0sd
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of jimlux
> Sent: Monday, January 25, 2010 9:09 AM
> To: Tower and HF antenna construction topics.
> Subject: Re: [TowerTalk] Side mounted Az-El rotors
>
> Mike & Becca Krzystyniak wrote:
>
>> I'm looking to install an Az-El rotor side mounted on my tower for
>> turning a small dish. The dish also has the transverters mounted on the
>> back of the dish so total weight will be less than 25 lbs but the package
>>
> is
>
>> far from balanced. I'm looking for an Az-El rotor that can accommodate the
>> unbalanced dead weight and also target to ~1 degree accuracy. Any
>>
> thoughts
>
>> and shared experiences with mounting something like this to a Rohn-45
>>
> tower
>
>> along with Az-El rotor types(or alternate solutions) would be appreciated.
>>
>>
>
> 1 degree? that's pretty tight especially on a tower which might move
> that much in the wind. 1 degree is about 1 foot deflection in 60 feet.
>
> You're definitely going to want to cobble up some sort of aerodynamic
> and mass balance scheme for your dish/electronics. 25 lb isn't huge, so
> if you hung a weight that matches off the back side of the pivot at the
> same distance, you're looking at 50 lb total.
>
> The Yaesu 5500 az-el rotor can take 200 lb vertical load, but I don't
> think it has the accuracy you need. They say 4%, which I'm not sure how
> to relate to degrees. The other interesting thing is that it says it
> has a maximum run time of 5 minutes, which I find odd for a device
> pitched as a satellite antenna mount, and the typical LEO satellite pass
> lasts 20 minutes. I guess the idea is that you run it in little bursts,
> so the overall duty cycle is <25%. move for 10 seconds, wait for 30,
> move for 10, wait for 30.
>
> If you're building something (or adapting something)...
>
> Ultimately, I think your positioning accuracy is going to depend on the
> feedback scheme. You cannot get 1 degree, out of the box, with a
> standard pot (that implies a linearity spec on the pot of about 0.25%),
> but you can calibrate it, and they're moderately stable, especially if
> you're moving over the full span periodically, so the resistive element
> doesn't get worn spots.
>
> I would look into optical encoders. You can get 512 pulse/rev
> quadrature encoders. Each time before you use the system, you'd sweep
> through until you get the index pulse from the encoder. If you can
> calibrate that against some outside reference, then you can take into
> account static misalignment that changes with time.
>
> Another intriguing approach is to use a 3 axis accelerometer, which can
> tell you where "down" is, and from that you might be able to recalibrate.
>
> Professional az-el mounts use a synchro to encode the position and then
> run that to a synchro to digital converter to get a 14 or 16 bit word.
> There are also precision optical encoders that use two gratings, and
> convert the light transmission (which looks like a triangle wave as a
> function of angle) of two quadrature sensors and calculate a precise
> position.. And, finally, some pedestals use a precision gear train and
> two encoders, one for coarse and the other for fine. You can get
> synchro or absolute digital encoders that have the geartrain built in.
> Way expensive new, but I've seen them surplus on occasion.
>
> You still have the calibration issue. When I built a satellite
> measurement station, we set up a remote controlled beacon on a hilltop a
> few miles away that we could point to, and peak up the response, then
> compare that to our previous reference values. More as a check than an
> adjustment; it was 90 degrees away from the normal look direction (up).
>
> Do you really need 1 degree? You might want to run a link budget and see.
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