these links help in making satellite dish antenna. nitehawk/rasmit/offset.html VK4BKP25-03-2004, 07:43 PMOkay here we go. This thread deals primarily with the construction of a 75cm dish for Ku band. This type dishuses ribs to approximate the shape ofa parabolic curve and sheets of metal, or wood with foil glued on if you are crazy like me for the RF reflector.Anyone can join in and post pics of progress but keep them small in both pixel size and kb say 640x480 and 50k or less. If pics are too big Ill be editing them down.Reasons to build a dish would be to learn something and hopefully have fun doing it. If you just want a dish to watch sat TV on that is cheap and well made then I suggest you buy one!Even though this dish is small it will take a lot of time and effort to make itwork.Requirements for making a dish.1.0 You need too much time on your hands! (like me)2.0 You need a lot of patience.3.0 It helps if you are a handyman or have a good imagination of how to make things.4.0 Scrounging around at the local recycle yard will find lots of useful parts.5.0 Workshop or bench / garage / tools to cut metal or wood / glue / nails / PVC pipe for the LNB focuser / aluminium for the struts / screws bolts nuts etc.You will of course need a Ku band LNBF. I like the straight ones since they can be enclosed in the focusing pipe better. The ones where the cable comes out at 90 degrees are not so good for centre focus dishes.Something to mount it on. Im going to use a heavy duty camera tripod for portable use. You may want to do yours differently. I wont go into depthabout mounting it. Thats where the imagination comes into it. Theres also some threads here in the dishes forum by Bassett and others with good ideas.Ready?!VK4BKP25-03-2004, 08:16 PMIll keep everything in mm. Our dish will be 750mm diameter with an f/D ratio of 0.7. The focus point will be 0.7x750=525mm from the centre of the dish. The depth of the dish will be D/16(f/D) where D = dish diameter f/D is the focal ratio, in our case 0.7. So that breaks down to 750/16x0.7 or750/11.2=66.96mm. Our depth will be 66.96mm.So to review -dish diameter 750mmf/D ratio 0.7focus 525mmdepth of centre 66.96mm.With those details we now need to plot a profile paraboloid curve to make a rib template out of.The equation for this is X=Ysquared/4D(f/D) = Ysquared/4x750x0.7 = Ysquared/2100. Where X = height of plot point and Y = interval of plot.Lets make plot points every 20mm. From this then we get 20mmx20mm / 2100 = 0.19mm. The next plot point will be at 40mm so 40mmx40mm / 2100 = 0.76mm. The next plot point will be at 60mm the next 80mm etc etc.Remember this is for the radius of thedish diameter so continue to make plot points along a straight line out to750mm/2=375mm.Ill do a scan of mine when Im finished for an example. Heck, Ill even work the lot out and scan that incase the above math is confusing.VK4BKP25-03-2004, 10:33 PMHeres a scan of the plot points. Ive rounded them off a little so if you want to be perfect use above equation. Probably best to check my work anyway. Hope you can read my scribble. :)VK4BKP25-03-2004, 10:41 PMIve used a thick piece of cardboard and drawn a line 2cm from the bottom edge. Working from right to left Ive drawn marks every 20mm (the Y spacing). Not sure why I went from right to left. I guess its possible to go from left to right just as easily. Directly above every 20mm spacing is where Ive plotted X. This is what all the math is about. You can sort of see a curve starting to take place now.Sorry my scanner wont fit the whole thing in.VK4BKP25-03-2004, 10:43 PMAll I do next is join the dots! Not very straight am I!VK4BKP25-03-2004, 10:44 PMThis is the whole thing.VK4BKP25-03-2004, 10:51 PMNext I use a sharp hobby knife and cut on the parabolic curve only.Im going to use a 42cm diameter base plate for this dish. Reason for such a large size helps accuracy. So because of that I make a mark 21cm from the hub end of the rib so it will sit on the baseplate flat. It is very important that the bottom of the rib isflat otherwise the shape will be out. I make another mark at the high end on the rib about 2cm down. Draw a line from mark to mark and cut it off.Almost there!VK4BKP25-03-2004, 11:02 PMThe next step is also very important. It is working out how to but the ribs together at the hub.Im going to use 9 ribs on this dish. More would be better but Im feeling abit lazy. :pMy rib thickness is going to be 12.5mm since thats the thickness ply I will use.What I do now is multiply rib number by rib thickness and divide by Pi (3.14159265) 9x12.5/Pi=35.8mm. I divide that by 2 and get 17.9.At the hub end of the rib template I then cut off 17.9mm so all the ribs will fit on the baseplate in their correct position. This will leave a hole in the middle of the dish 35.8mm in diameter.This is the finished template. Ready to place on my 12.5mm ply to cut outwith a jigsaw. (Hate that part)VK4BKP26-03-2004, 06:24 PMThis is the base plate made from 9mm ply. 9-12mm would be okay but9 is what I had laying around. The base plate is 42cm in diameter. Overkill for this dish but I made it thisway in case I wanted to change to an old Austar mounting plate I have lying around. Plus it suits the rib template I just made.From the centre mark on the base plate I use a protractor to make 9 radials. Since 360 (degrees) divided by 9 = 40 I make each radial from the centre 40 degrees apart.Around the centre mark I also drew a circle 36mm in diameter. This is the mark at which the ribs but up against, leaving a 36mm hole. You can cut the hole out if you like. Lets the water out. :)Theres a few extra marks on the plate from a previous job. Just ignorethem.VK4BKP27-03-2004, 01:02 AMBase and ribs after being cut out.VK4BKP27-03-2004, 01:03 AMGlued and screwed together now.grasshopper27-03-2004, 12:07 PMgreat thread has inspired me and im sure will many others, galvo sheetingis cheap as and has got me thinking, well done vkVK4BKP27-03-2004, 02:47 PMYes, galv sheeting should be good on the rib structure.VK4BKP28-03-2004, 01:58 PMOr in my case foil glued onto the wood frame.One panel on.VK4BKP28-03-2004, 02:02 PMLooks like 9 ribs are not enough to keep a nice shape. 12 would have been better. Theres going to be a loss of gain. Hopefully not too much. Oh well. :(Wrinkly alfoil while cosmetically uglyhas no adverse effect on performance.Rizenzean29-03-2004, 01:02 AMOK, heres my dish ;)Basically we do have the same calculation data, from diameter to feed point. But I only used alluminummaterial and its only 8 ribs-- im a lotmore lazy than VK;)the base plate i used is a milk can cover 11 cm in diameter. The ribs were bolted in the base plate.I got hard time in bending my ribs along the pattern so I used my bare hands so that the ribs may conform with the parabolic curve.heres my pattern...Rizenzean29-03-2004, 01:08 AMand heres my dish.Im planning to use metal sheet to cover the dish. I hope it would conform to a parabola:rolleyes:Im just finished with stage 1. stage 2will be the metal sheet installation. Hopfully I can finish this next week end on my day off;)VK any hints for the LNB arm ? how to fix it in the proper place ?VK4BKP29-03-2004, 01:30 AMLooking good there Rizenzean.Next step for me is making the LNB arms and LNB holder. Ill be using 3 lengths of 12mm aluminium tube for the struts and 2 pieces of PVC pipe to make the lnb focuser. What Im going to do is slip the LNB into one piece of pipe and then slide the wholething into a larger piece of pipe. The larger piece of pipe will have the arms attached. It will work a bit like atelescope focuser, for fine adjustment. For course adjustment there will be a couple of drill holes a couple of cm apart in the struts to move the entire assembly in or out. Ill make the arms a little longer than they should be for starters and move in from there.This is probably the most tricky stage- getting the focus point spot on as there are small false peaks before and after the main point of focus.Rizenzean29-03-2004, 01:35 AMMaybe Ill just wait until your finishedwith that stage ..:Dpls attache more detailed pictures so that I ca
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