Tuesday, 21 January 2014

Satellite tracking: hardware

Satellite tracking: hardware

I have made progress since the first post. I now have the main hardware assembled and am in the process of wiring everything together.

The original plan to use a pair of satellite dish positioner wasn't viable. Luckily I discovered that before my order for the second positioner was sent. It is possible to do it that way but there are two problems.

First, the positioners are driven by diseqc control. The controller I had was for linear actuators and simply passed the diseqc controls from the satellite receiver through. So I would have needed two satellite receivers with diseqc control for a positioner. Not a big deal except...

Second, all the amateur satellite controllers uses conventional rotators, with a 24 VAC motor and 500 Ohm potentiometer. A further consequence of this was that I couldn't use the terrestrial TV rotator as it used a pair of synchronous motors for turning and indicating. Neat system, but couldn't be used.

So I had to try and buy some cheap rotators; not easy, and I didn't want to pay $1200 for the Yaesu G-5500, which are a pair of coupled rotators, the G550 for elevation and G450 for azimuth, and a combined control box. I managed to get a cheaper G550 as old stock on eBay (Aust), only to have the guy list an older model G500 for even less, sigh...

As I investigated controllers, I realised I didn't need the rotator control boxes, just a bare rotator was ok. By chance, I had been keeping (for 20 years) a new toroid transformer that was 25 VAC, close enough. So I cleaned out an old device for its case, switch and fuse to mount the transformer.

Then the hunt was on for a rotator. I found Ken Brownson KT4KI was selling reconditioned CD-44 (Ham-III?) rotors/rotators with controllers on eBay (USA). I emailed him to see if he could supply just the rotator, which he could for US$250 delivered to Australia (turned out ~US$90 freight). He said he had reconditioned bare G500 elevation rotors as well, but I had already bought the other one. Anyone after a rotator/rotor, see the photos for details.

I had already built a mount for the TV rotator using a 600 mm length of 250 mm plastic pipe, with end-caps, mounted on some 19 mm form ply (not recommended though, I had the pipe, which is expensive, as are the end caps at $30 each). My (yacht) mast has a flat flange for attaching the second section of mast, but I can attach the form-ply base easily with screws and bolts. The pipe is to transfer horizontal wind load to the base, rather than through the rotator. This is standard practice for rotators as it doubles their wind load capability; its just easier to do in a open-frame tower.

The following photos show the hardware and how it fits together. I end up with a reasonably aesthetic cylinder, with a pipe out the top, for mounting on the mast and for mounting antenna.

The last photo shows the two rotators mounted and an old TV antenna attached for illustration purpose.
What is not so obvious is the black pipe on the right-hand side. It is a 2m length of fibreglass pipe I had made ($80 each at local fibreglass fabricator, I couldn't buy the off-the self, despite much looking). It is to mount the crossed antenna I originally needed for 70cm, horizontal for ATV and vertical polarisation for FM repeaters. The fibreglass pipe is too big to fit though the G550 rotor, but all is not was not lost. I will cut it in half and mount on the ends of the gal steel pipe, so I still get my non-metallic mounts for the antenna.
For satellite tracking, circular polarisation is best, as the satellite tumbles. As such, both the 2m and 70 cm each need to be crossed and, using phasing coax, made into circular polarisation. I think the circular polarisation should still allow the vertical and horizontal polarisation I need for other purposes. I shall see. At worst, I can use coax switches to move between the modes. 
One problem I have encountered using the older CD-44 is that its start or run capacitor is mounted in the control box, not inside the rotator, as is modern practice. This presented two problems. First was getting a 130 uF 50 VAC capacitor, most are much smaller for 240 VAC motors. I was able to get two 60 uF 450 VAC capacitors locally, but they are huge! So the second problem was where to mount them. Fortunately, the rotator has terminals for the capacitor, so it can be mounted with the rotator. With a fairly weather-proof enclosure, I can mount them inside it; so the PVC pipe enclosure wasn't such a silly idea!
The controller I decided on is the ARS-USB http://www.ea4tx.com/wp/?page_id=6. It can control both rotators and just needs 12 VDC (for itself and the two positioning potentiometers inside each rotator) and 24 VAC (to turn rotators). It can be used with existing rotator controllers, but for me it is easier not to use them (plus I don't have one for the CD-44). The ARS-USB is quite a sophisticated device and very reasonably priced at 200 Euro from Spain. It provides a standard Yaesu GS232A interface. I will discuss the controller more in another post on the control software.
The current job is to do all the wiring, again not simple.

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