Dish construction

Dish construction ("The template")

This chapter descibes the project for building a 3.5 meter mesh dish, suitable for frequencies up to at least 6cm band. The dish' shape should match the septum-feed best, which means an F/d=0.43 is best. I started fabricating a template, into which the rims could be assembled. This template was made of a wooden base, onto a large piece of paper was fixed. The needed shape for an F/d was then drawn onto the paper, after which the template could be completed with small wooden blocks into which the seperate rim-parts could be placed. All the seperate pieces then can be drilled, and by using 4mm rivets, everything is mounted together.

Dish construction ("The Rims")

The rims where prepared on a roller, this helped a lot. Especially the support-rims could perfectly rolled, as these have a circular shape. The rims themselves are of coarse not circular, and needed manually to be bend until they could be placed without any force into the fixture.

Dish construction ("Assembling rims in template")

Now the aluminium 20 x 20 tubing was bend, the rims can be completed inside the fixture. The parts are fixated by rivets. Once one side is done. the rim could be taken out from the fixture to mount the other side plates.

Dish construction ("Rims completed")

Below pictures show 7 of total 12 rims after all the aluminium bracing-plates have been mounted by using numerous rivets. The assembled rims already have a very sturby feel to them, and have little weight.

Dish construction ("Mounting Rims to Hub")

Once all the rims are build, it is time to make the central hub to which all the rims are mounted. hanks to Peter, PA1PS, I received a local dutch fabricator for aluminium parts. The two discs where CNC milled in such a way the rims can be placed in exactly milled 20mm wide grooves. The two discs have threads for fastening the rims. Both discs are held at 20cm distance by using INOX M6 rods and nuts. To screw these M6 rods through, a electric drill came in handy. Once all the rims where mounted to the hub, ropes are attached to all the rim-ends. This helps to make sure the dish will have the correct shape. The ropes should meet at the center of the dish, all at the same point. To achive this, the holes in the rims are drilled 6.5mm to fit the M5 bolts. This leave room to manouvre the rim.

Dish construction ("Mounting Rings through the Rims")

Now it's time to feed the 8mm aluminium tube through the rims. These 8mm tubes where bend by hand in a circular shape. The inner tubes are in one piece, however as the diamter increased towards the edge of the disc, these 6mtr long tubes needed to be connected together. A piece of 6x4mm tubing was slit in the inner of the 8x6mm tube at exactlty the place of a rim. This gave enough stability to remain the shape. The outer circle is made from 20x20 tubing, like the rims. This is needed for the support mounting the mesh.

Dish construction ("Mounting the Mesh")

Once the dish is ready like this, it is time for mounting the mesh. In my case I used 6x6mm mesh, which guarantees the dish is able to operate at 6cm at least, depending the accuracy of coarse... The size of a 3.5mtr dish, using 12 rims, gives the advantage of using mesh which can be sourced as 1mtr wide material (25mtr long). The distance between the rims at the edge is just below 100cm! The mesh is cut by using a electrical grinder. I admit there is some losses of mesh material, however I think it the best way to cut the slabes to the correct shape...

Dish construction ("Dish Arm Assembly")

To mount the dish to a slew-drive, a construction was drawn by a friend. He used the drawing available from SUN-Drive to create the arm onto which the dish could be mounted. At the rear of the arm, heavy counterweights have been integrated (3 of the 4 positions in use), to releave some weight to the 3" slew-drive. Once all the pieces of the arm where welded together, a testfit was performed. It turned out one of the motors of the slew-drive would hit the read-arm in case the dish was at 90 dgerees elevation. Luckely we found that out in time and a small portion was quickly cut away. The arm was then send out for powder-coating. At the bottom of the slew-drive, a small flexible joint is visible. This is meant for connecting the shaft towards AZ-encoder. All the parts to assemble the dish-arm have been laser-cut which makes it easy to build together. Mind attention to the dsic at the front of the dish-arm. The holes exactly match the positions of the bolts in the rear-hub plate. In this way, the dish can be mounted to the arm giving it a very stirdy and strong builtup. Without the help of modern CAD-design, this was very hard to accomplish. Thanks Leo, for this great job!

Dish construction ("Mounting Dish to Mast")

As already mentioned, the dish will be rotated in both AZ-, as well as in EL-direction by using a slew-drive. This is the 3" SVH3 dual axis slew drive supplied from Coresun Drive Equipment Changzhou Co., Ltd. (SVH3-62-RC-24H3033-REV.A). The slew-drive is mounted on a peddistal, on top of an existing tilt-tower. Inside the top of thilt-tower is the AZ-encoder mounted in a weatherproof housing. The encoder inside is the HH12 that has a longer shaft mounted to it. This longer shaft reached the flexible joint in the bottom of the slew-drive. I was greatly helped by Jan, PA0PLY, who is in some pictures lending a helping hand.