Re: Att Chris - Dish elevation control

Paul Gulliver
 

Hi Chris,
Thanks very much for a very comprehensive answer, certainly a lot to think about there. Interesting you are controlling the elevation by moving the bottom part of the dish, got me wondering if a threaded rod could be rotated by a motor with the rod passing through a fixed bracket and attached to the base of the dish. Something to think about in the new year.

I never got involved with NOAA HRTP, but I did receive the low res apt from the polar orbiters and also built a petal dish for the geostationary birds in the 1980's, all displayed on the RIG Framestore in the days before computers were common place - all good fun then :)

You seem to have a very well organised shack with the various pieces well built - even the switches labeled :)

Enjoy the rest of the Christmas and wishing you a great and productive New Year

Paul


On 25/12/2018 at 02:43, Chris van Lint <chrisvanlint@...> wrote:
Hi Paul,

As far as I am aware there is no affordable commercial equipment forautomatically controlling the Az / El for larger  satellitedishes.  Back in the 1980's when I was decoding NOAA HRPT images on1691MHz I was using an 80cm dish and a self designed active helixantenna.   These satellites are polar orbiting hence have to betracked.  They also are closer to earth compared to geosationarybirds. I could be control the dish by using a Kenwood (now Yaesu) KR5500azel rotator.  These rotators are intended for controlling beamantennas - not dishes.  However the dish being comparatively smalland light weight (aluminium), could with some mechanical modifications bemade to work quite well.  However this is about the limit of thetorque the rotator motors will handle, so I could not use it for my 2mdish, quite apart from the impossible mechanical challenges to fitbrackets etc.  The paid version of David Taylor's WXTrack program,includes a facility which will track and move the KW rotators to theexact coordinates the satellite is positioned at in real time, as long asthe Kepplerian data is up to date.

In a commercial situation they use large rack and pinion arrangements forazimuth and worm gear or hydraulics for elevation.  The satellitecontinuously transmits a VHF signal, which is interpreted by an auto tunearrangement on the ground, which delivers a signal to the pointingmotors.  Some form of hystereses adjustment is needed to stop thedishes moving continually with small position changes.

Obviously none of these schemes will work in my present situation and theonly possible solution is to use dish actuators.  These can be usedquite easily to control azimuth.  However to automate the movementis not simple.  The KW rotator has an internal potentiometer whichfeeds a signal to the rotator controller in the shack and tells it whereit is pointing to.  The same scheme is used in the elevationcontrol.  Commercial dish actuators do not have build in sensorswhich would allow accurate positional data to be sent to thecontroller.  I have thought about modifying one, but filed it in thetoo difficult box.  The only solution for me was to tell theactuators where they should be pointing at any given time.  I usedan azimuth actuator for elevation control by installing it in a verticalposition and have it push the bottom periphery of the dish.  A 2mdish with a steel frame and in spite of having a perforated aluminiumskin, is quite heavy and some form of counterweight for elevation wasneeded.

I now had to construct the electronics to control the 2 actuators. Using David's excellent program (seriously anybody messing around withsatellites should have WXtrack installed on their PC), I used the manualtime facility and plotted the satellites predicted positions over aperiod of 24 hours.  I used two dual channel programmable timers(dual channel because the output to the actuators has to bereversable).  I could only get find 30 step timers, but as it turnsout, this is sufficient for 24 hours, because there is a 300 and 360minute period during the 24 hour period where the satellite does not movesignificantly and the dish does not need to be moved.  The timersimply tells the actuator something like: 10 seconds up - 60 minutes nomovement, 12 seconds up - 60 minutes no movement and so on.  Theprocess for the elevation control is the same. The relationship betweenseconds of applied power and movement needs to be determinedempirically.  I found that at the rated operating voltages of theseactuators (between 36-46 V), movement was so fast that it was difficultto get accurate results.  I constructed an adjustable power supplyand use 20V to drive the actuators.  Being V adjustable, this hasthe added advantage that I can fine-tune the seconds:movementratio.  The movements of the satellite are not static and every fewweeks I have to re-start the timers (usually 1 hour earlier) to get themto track reasonably accurately again.
If there is a power interruption I am in trouble, because I have notfound a way to start the timers at any point but "step#1"! so Ihave to wait for that time to come around.


This image basically shows the set-up.  The square multi-coloureddisplay in the top left-hand selects voltage and monitors current andpower.
The rectangular box on top of the PC above the SDR-RTL  is an active8 channel C-Band splitter, with the gain (amplifier on top) adjusted tounity as referred to the input strength.  This gives 8 independentdevices lossless access to the C-Band dish.  The actual timers arein the two ABS boxes with clear lids.


That's all there is to it.  It is a bit crude, I know, but it works(most of the time).

Cheers,

Chris






08:54 PM 24/12/2018, you wrote:
Hi Chris
In another thread you mention you use automated control for the azimuthand elevation of your dish.
I'm using a 1.1m dish to receive 3F5 at 54W, the elevation of thesatellite varies between 11 and 17 degrees and I have to adjust the dish3 or 4 times per day to keep a usable signal so I was wondering what youuse for elevation control.
I've searched for elevation motors but haven't had any success in findinganything suitable - is yours homemade or purchased?
Any pointers would be appreciated

Wishing you a Merry Christmas

Cheers,

Paul

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