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Rotator for the Junk-Box Antenna
My interest in communication via satellites is new and was instigated by the Pennine Ham presentation by GW4VXE on a “Guide to using satellites in amateur radio”. Although I have been involved in amateur radio for many years and can remember the launch of OSCAR 1 the first amateur radio satellite, I have always believed that the activity of communicating through satellites required specialist and expensive equipment. GW4VXE’s presentation dispelled that belief. Also, being someone who started pursuing an interest in radio and electronics in the 1950’s I was schooled in a “make do and mend” philosophy. In those days, most of the equipment used by radio amateurs was “war surplus” equipment or “home-brew”.
So at the outset of this endeavour I determined that I would use as much of my existing equipment as possible and make any antennas needed. Hence the “Junk Box Antenna”. Having struggled to hold the Moxon/Yagi and operate a handheld transceiver when listening to the International Space Station it seemed sensible to look to employ a rotator. Consulting the usual sources of amateur equipment soon informed me that a suitable rotator would cost more money than I was prepared to spend, especially since I was not sure how far this interest would go. So, planning began to design and make a rotator.
The design brief is as follows:
a) The rotator must be self contained. No wires into the shack other than the coaxial cable.
b) It must be battery powered.
c) It must be simple to operate.
d) As far as possible it should be made from equipment and materials I already have.
For many years when my grandsons’ were growing up we engaged in many projects involving the Raspberry Pi, and the BBC Microbit. I have several of these which are left over from earlier projects and either of them would be able to control a stepper motor and run the necessary software. The Microbit had the advantage that it had a simple display and two buttons on the circuit board which could be used in the project. I also had a 5v NEMA 17 stepper motor and motor driver board for the BBC Microbit.
Since the satellite antenna is mounted on a tripod and will only be used when the weather is suitable, the electrics will not need to be protected by an enclosure. Anyway, this is a prototype! The big problem in the design is how to connect the stepper motor to the antenna so that it can be rotated. As with the “Raspberry Pi / BBC Microbit” my grandsons’ projects came to the rescue and I have used their Lego gears to rotate the antenna. These are plastic and hardly ideal! I knew I would have to protect the plastic LEGO gears otherwise they wouldn’t last very long, hence the water bottle!
The water bottle helps drive the antenna and protects the plastic gears from experiencing too much force. In the next iteration of the rotator I will use a better drive mechanism.
The control software, like the rest of the project is very basic. Since the rise-time and the set-time of the satellite is known, it is easy to determine the transit time in minutes. The azimuth for the rise and set of the satellite is also known, so the angle subtended can be determined. This information is flashed to the Microbit. The two buttons on the Microbit are used to set the rotator turning either clockwise or counter-clockwise. The software in the Microbit causes the stepper motor to turn the antenna through the desired angle in the transit time of the satellite. To start tracking, the antenna is pointed at the azimuth of the satellite’s rise and either clockwise or counter-clockwise button pressed. There is then a short delay to enable me to get back into the shack to start listening.
The Microbit was programmed using MicroPython.
There is no elevation control on the antenna but I have found that if the inclination of the antenna is about 30 to 40 degrees as suggested by ON8NT, for most transits the reception is continuous. If the elevation of the satellite is greater than 75 degrees there is a routine in the programming to rotate the antenna quickly when the satellite is near maximum elevation. This works most of the time.
As a proof of concept, this simple rotator has been successful but it does need to be redesigned and rebuilt. I have an uneasy feeling another project is coming on.
Rotator for the Junk-Box
Satellite Antenna version 2
Having tried the prototype rotator and successfully worked a few stations through the ISS I decided to design and build a better version. Essentially, the rotator would use the same controller but have an improved drive mechanism. The great weakness of the prototype was that it used plastic gears and these were bound fail over time, so I considered the options. A worm gear mechanism would be an excellent choice but would be difficult to implement. A belt drive would be easy to incorporate in the original design but would be liable to slip. The current interest in 3D printing has made “timing belts” readily available and comparatively inexpensive. In keeping with the junk-box theme I have endeavoured to keep to a minimum the items purchased and recycle old projects but the drive pinion for the NEMA 17 stepper motor and a 15mm belt to mesh with it was purchased new.
The gear on the mast which meshes with the timing-belt was created using a piece of the same timing-belt superglued to the plastic mast (see picture)
The guides either side of the timing-belt were cut from a PVC elbow the same size as the pipe and with the rubber seal removed slipped into position either side of the drive gear. When tested these will be glued into position.
As mentioned earlier, the BBC Microbit and driver board is the same as used in the prototype with the software adjusted to take into account the change in gear ratio.
The housing for the control electronics is an old project box which I have recycled. Notice the blue tape which hides holes drilled in the lid which were used for the previous project.
The drive mechanism works well and is very secure in it’s movement.
In Use:
Opportunities to test version 2 of the rotator have been few and far between in view of the preparations for Christmas and the inclement weather, however, I did try on a couple of days and tracked AO-73 and SO-50 satellites. The elevation of AO-73 on one occasion was 49 degrees which was about ideal for my location and I was able to copy stations throughout the transit. SO-50 had an elevation of 73 degrees which was good for testing the performance of the rotator when satellites are overhead. Again the rotator performed well and stations were copied at a good signal strength. Overall, the initial tests of the rotator have been very encouraging.
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