Dish Antenna

May 31, 2021

The first steps in making a mesh dish antenna. I used the program Parabola Calculator to calculate a 90 centimeter dish with an f/D of 0.4, then figured out a suitable size for the base plate, which turned out to be 180 x 180 mm. With the corners cut off it became a nice looking octagon.

Linear Diam.   95.54  
Diameter   90    
Depth   14.06    
Focal Length   36    
Volume   44730.88    
FLength/Diam   0.4    
Area   6361.73    
         
x -45   y 14.06
x -39.38   y 10.77
x -33.75   y 7.91
x -28.13   y 5.49
x -22.5   y 3.52
x -16.88   y 1.98
x -11.25   y 0.88
x -5.63   y 0.22
x 0   y 0
x 5.63   y 0.22
x 11.25   y 0.88
x 16.88   y 1.98
x 22.5   y 3.52
x 28.13   y 5.49
x 33.75   y 7.91
x 39.38   y 10.77
x 45   y 14.06

 

Click on the image below for a pdf version of the base plate design.

June 1, 2021

I started off today by making two jigs. The first one was more of a template to speed up drilling holes in the end of the ribs. Just slide in the end and align the drill bit with the hole. Easy as pie.

You will have to cut the ribs first, so I made a jig to hold down the aluminium strips. By making sure the ends were square I had no trouble making near perfect right cuts.

So I cut two ribs and riveted them to the base plate. With some string I tensed them up and found out....that my calculations were wrong. I cut the two ribs too long because I didn't take the base plate into account. But the result was a dish with a diameter of 1 meter and a focal point of 0.3, so after putting new figures into the Parabola Calculator I decided to cut the other ribs to the same size.

I finished cutting, drilling and riveting all the other ribs and found out that the template for my base plate was also wrong: I made a design for 24 ribs, not 16. Yesterday wasn't clearly my best day for doing math and counting. This was easily remedied though: just put them in between two lines instead of right on the lines.

With some string I tensed everything up and then you could start to see things getting into shape. The measurements were all in accordance to the ones that Parabola Calculator spitted out, so all's well!

June 2, 2021

Spend a lot of time today as the work was more involved than I thought it would be. First off I cut connector strips (3x12.8mm) long and drilled a hole in the center. These were riveted to the end of each rib.

Next I cut 16 connecting ribs (183 mm long) and gave them a 10° angle.

The result: a trapezodial shape.

Next was riveting them to the ends of the ribs. Sometimes they didn't line up perfectly, but the margin was small enough.

After five hours of work the end result. Exactly one meter in diameter and 20.5 cm deep.

June 4, 2021

I bought stainless steel screen door mesh yesterday and today it was time to put it on the frame. First I cut a 9 cm circle out of 2 mm aluminium to put in the center to hold down the mesh. Then I cut 18 cm long alu-strips and started to rivet them down. I calculated that I didn't have enough alu-stock, so I alternated between the ribs.

That worked well, so I did a second batch, now towards the outer rim. After that the hard part: folded the mesh over the outer rim and cut 15 cm long alu-strips, drilled holes in them and then riveted them on the outer rim between the ribs. I messed up the first one, so the mesh got damaged, but once I got the hang of it the rest went smoothly, although it took me all morning and part of the afternoon to finish the job. The end result is shown on the photo below. There are a couple of places where the mesh is not in contact with the ribs, i.o.w. it's a bit too tight. Once I know how much alu-strip I have left I can start improving on it.

June 5, 2021

Last day of the build. I cut three 1.5 cm U-shaped holders for the legs, then cut three 30 cm aluminium tube (8mm), drilled holes and riveted everything to the ribs.

A piece of 15x15 cm 2mm alu-plate was cut into a triangle, a 40 mm hole was made in the middle, then three 5 mm holes in each corner. My home-made brake came in handy to give the corners a 60-something degree angle. This is going to be the antenna mount plate.

I put riv-nuts in the end of the alu-tube, then 13 cm of M5 sized threaded rod and on top of that the antenna mount plate.

I got some PVC stuff from the plumbers section of our local DIY store and mounted that on the plate. The white part is a permanent fixture in which you can screw the grey connector part. On this grey part you can mount any antenna you like. They are cheap (less than a dollar US), so I'll be able to use one for each antenna. With the threaded rods I can adjust the height of the antenna mount, so that each antenna is perfectly aligned to the focal point.

Since everything went very smoothly I had time and energy left to make a mounting bracket for my rotator. 50 cm of square 32mm alu-tubing, two pieces of round 1" tubing and four rivets later I had this.

I'm waiting for the rotator clamps to arrive, but for now the finished dish looks like this.

Okay, maybe not finished. I still need to find something to cover the rim, because that stainless steel mesh stings vehemently when you pick the dish up.

June 6, 2021

Testing day. Mounted the logper and it was almost in the focal point of the dish. Put the whole thing on my balcony wall, but nothing heard or seen. There were sections where the noise floor went up, e.g. around 1200 MHz, but we'll need to wait till the weather gets better and we can escape my RF sucking sheet metal roof.