Re: Att Chris - Dish elevation control
Chris van Lint
Hi Paul,toggle quoted messageShow quoted text
As far as I am aware there is no affordable commercial equipment for automatically controlling the Az / El for larger satellite dishes. Back in the 1980's when I was decoding NOAA HRPT images on 1691MHz I was using an 80cm dish and a self designed active helix antenna. These satellites are polar orbiting hence have to be tracked. They also are closer to earth compared to geosationary birds. I could be control the dish by using a Kenwood (now Yaesu) KR5500 azel rotator. These rotators are intended for controlling beam antennas - not dishes. However the dish being comparatively small and light weight (aluminium), could with some mechanical modifications be made to work quite well. However this is about the limit of the torque the rotator motors will handle, so I could not use it for my 2m dish, quite apart from the impossible mechanical challenges to fit brackets etc. The paid version of David Taylor's WXTrack program, includes a facility which will track and move the KW rotators to the exact coordinates the satellite is positioned at in real time, as long as the Kepplerian data is up to date.
In a commercial situation they use large rack and pinion arrangements for azimuth and worm gear or hydraulics for elevation. The satellite continuously transmits a VHF signal, which is interpreted by an auto tune arrangement on the ground, which delivers a signal to the pointing motors. Some form of hystereses adjustment is needed to stop the dishes moving continually with small position changes.
Obviously none of these schemes will work in my present situation and the only possible solution is to use dish actuators. These can be used quite easily to control azimuth. However to automate the movement is not simple. The KW rotator has an internal potentiometer which feeds a signal to the rotator controller in the shack and tells it where it is pointing to. The same scheme is used in the elevation control. Commercial dish actuators do not have build in sensors which would allow accurate positional data to be sent to the controller. I have thought about modifying one, but filed it in the too difficult box. The only solution for me was to tell the actuators where they should be pointing at any given time. I used an azimuth actuator for elevation control by installing it in a vertical position and have it push the bottom periphery of the dish. A 2m dish with a steel frame and in spite of having a perforated aluminium skin, is quite heavy and some form of counterweight for elevation was needed.
I now had to construct the electronics to control the 2 actuators. Using David's excellent program (seriously anybody messing around with satellites should have WXtrack installed on their PC), I used the manual time facility and plotted the satellites predicted positions over a period of 24 hours. I used two dual channel programmable timers (dual channel because the output to the actuators has to be reversable). I could only get find 30 step timers, but as it turns out, this is sufficient for 24 hours, because there is a 300 and 360 minute period during the 24 hour period where the satellite does not move significantly and the dish does not need to be moved. The timer simply tells the actuator something like: 10 seconds up - 60 minutes no movement, 12 seconds up - 60 minutes no movement and so on. The process for the elevation control is the same. The relationship between seconds of applied power and movement needs to be determined empirically. I found that at the rated operating voltages of these actuators (between 36-46 V), movement was so fast that it was difficult to get accurate results. I constructed an adjustable power supply and use 20V to drive the actuators. Being V adjustable, this has the added advantage that I can fine-tune the seconds:movement ratio. The movements of the satellite are not static and every few weeks I have to re-start the timers (usually 1 hour earlier) to get them to track reasonably accurately again.
If there is a power interruption I am in trouble, because I have not found a way to start the timers at any point but "step#1"! so I have to wait for that time to come around.
This image basically shows the set-up. The square multi-coloured display in the top left-hand selects voltage and monitors current and power.
The rectangular box on top of the PC above the SDR-RTL is an active 8 channel C-Band splitter, with the gain (amplifier on top) adjusted to unity as referred to the input strength. This gives 8 independent devices lossless access to the C-Band dish. The actual timers are in 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).
08:54 PM 24/12/2018, you wrote: