I knew that several amateur radio enthusiasts had managed to achieve this, so, in my usual spirit of attempting something quite difficult on the cheap, I decided I ought to have a go. I already have a good sturdy tripod, so the idea is to build something that can sit on the tripod, and thus be portable.
The first step was to design and build an adapter that could be bolted to the tripod, to which I would attach my altitude-azimuth (alt-az) mount.
It’s a relatively simple thing but is very sturdily attached to the tripod, which has an awkward circular metal plate to attach things to.
I had decided that the alt-az mount needed a good broad bearing at the base, so had thought about how to make a large roller bearing. It’s actually not difficult; two pieces of MDF are used as the bearing surfaces, with a thin aluminium sheet glued to each to add a bit of hardness. The ball bearings are held in a hardboard frame which is centrally located with a central bolt in the bearing. This is what it looks like with the top plate off:
The top plate is round, and will be the drive drum for the belt. Here’s what it looks like from the side, with the mount added to the top:
Around the time I was getting these parts made, the Lothians club had a video (instead of the usual talk) of a talk by Paul Marsh M0EYT which convinced me that this project ought to be possible. People are receiving signals from these probes with relatively modest equipment.
I already had a 60cm satellite dish, bought (along with the receiver, LNB, etc) quite a few years ago when B and Q were selling them off cheaply. It’s probably a bit marginal for probes at Mars, but should be OK out to the Moon. The alt-az drive is designed so that I can put a 2m dish on in its place.
Here’s the azimuth part of the mount ready to attach to the adapter plate:
The bearing for the altitude part of the mount is simply holes on MDF, with a large smooth steel bar (from a recycled HP bubblejet printer) as the bearing axle. Here’s the azimuth part bolted to the adapter:
I had the main part of the altitude section finished, with the dish attached, so it was time to get an idea of how it looked, and let me think carefully about how I was going to attach counterbalance weights.
It was now time to design the counterweight system. I had a good look at photos of large dishes on the web, and decided to go for the solution using two “arms” that go out diagonally behind the dish, far enough to avoid contact with the gubbins at the centre of the drive. This is especially suited to a mount fixed on top of a tripod. It’s fairly simple, too, so didn’t take long to make.
Here are various views with the dish in different positions. It is pretty well balanced, although I have at this stage got the counterweight boxes (plastic tubs with screw lids!) offset too far off centre for the relatively light feed / head amplifier assembly. This can easily be changed.
I’m pleased with the result, and it even looks quite elegant!
Now it’s time to work on the details of the motor / drive belt design …