[TowerTalk] Side mounted Az-El rotors

[Gary "Joe" Mayfield]

1 degree is tough - must be a huge antenna, or a really high band.  Alpha
Spid claims 1 degree

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 at contesting.com
[mailto:towertalk-bounces at 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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