I was fortunate in being given a 2.2 metre dish when Steve Birkett moved from his previous location. Steve is a satellite receiver pioneer and anyone interested should visit his company's web-site (see Links at G3PHO site). One of the conditions of the gift was that it had to be moved within a few days. I spent about a day and a half welding up a framework onto my trailer in order to support the dish in such a way that it would be safe from damage. Being a keen DIY man I was also going to incorporate what I had always wanted - to move out the sides of the trailer out slightly in order to be able to carry sheets of ply up to four feet wide and eight feet long. All went well and the following day with the help my two sons we got the dish and its supporting structure back safely. The dish I got was the smallest of three dishes - 2.2 metres diameter, but when attached to the actuator gear it was much bigger than I had thought and clearly whilst my trailer would carry it - it would not be a suitable base from which to operate it.
Out came the CAD system and the dish and its control gear were drawn up as a full sized model. Then a suitable trailer was drawn beneath it. There were several considerations:
1.I wanted to operate in several microwave bands and the dish had a fairly small f/D, so a cassegrain sub- reflector seemed best with feed from the back of the dish. I had seen and been most impressed by the system Paul Widger (G0HNO) uses. Design for Cassegrain feed completed - see separate project. Then looking at methods of manufacture - possibly by glass fibre and copper plating - however whilst discussing the project at our local club, a retired machinist and railway enthusiast Alan (G4VVT) came to the rescue with an offer to machine the subreflector from the solid.
2.The present mounting was polar with a linear actuator drive. However the designer of the drive had actually provided holes to enable its use in azimuth. Because of the length of the actuator versus the width of the trailer - it will have to be mounted fore and aft along the trailer. There was a manual elevation adjustment provided but this would need the addition of a finer control. Use for EME would require a more sophisticated actuator.
3. I will need to provide a control system for the actuator which will be able to be read off in degrees- the actuator fitted had a linear pulse counter output. I had already designed a PIC/LCD based system for my motorised satellite dish (also a pulse counter) - so the basics were there. It just needed to have a look-up table to convert the linear motion to degrees of angle.
4. I wanted to have a reasonable swing horizontally - above the wheels of the trailer. But I didn't want to get too high in case of problems due to side-winds in transport. I decided to make the wheel-base as wide as possible and to move the dish up a bit. I had noticed that Peter, G3PHO seemed to be getting good results with his dish high above his van roof - possibly due to minimised ground effects.
5. I decided that I would use Indespension units as with my other trailer - they had given excellent service for many years. However Mini wheels are getting thin on the ground now and time to move on -(all is not lost - Renault Clio wheels seem to fit the same studs as the Mini). There has also been an increase in trailer legislation over the intervening years and the use of old car parts is apparently no longer considered acceptable. After a bit of thought it was decided that a modification of a second hand trailer of current road-legal design might be the most satisfactory means of achieving the end.
6. May 2002 -So after scanning the local Ad's I came across a very low cost caravan - the timber framing of the caravan had rotted and was almost impossible to economically repair. Underneath it looked good - with a sound painted steel chassis and excellent suspension system along with all the jacking, braking and towing gear - so for a price of £60 I bought it and got back £15 in scrap aluminium sheet. It took a couple of days of work to strip it all down to a flat deck......
7. July 2002 After a thorough scaping and painting with magnolia hammerite the chassis looked good.
Last updated 2002 Sep 1