Re: Dish for 122GHz?


Neil Smith G4DBN
 

Hi Terry, I have several of the 12 and 18 inch versions from Edmund in York and I'm making mounting rings and cassegrains for them. The first one is looking OK after a few attempts to make them using a machined template.  It's a lot easier on a CNC dammit!

The material for the mounts is due to be delivered on Friday, so I might have something to show this weekend. Several UK folks have bought those dishes.  They are not quite optical quality, but my measurements so far on the 12 inch version show it to be pretty good and certainly better than lambda/10 at 122 GHz, although they are a bit thin and you need to take care to mount them without any stress, hence the machined support rings I'm making up (with rings cut from 6.5 inch diameter 3/4 inch wall ally pipe!)

I'm trying to get the feedpoint just about flush with the centre of the dish so we can avoid needing delicate waveguide extensions, and be able to mount an enclosure directly on the back of the dish support ring/alignment spigot. I'm aiming for about a 12.5 dB edge taper.

The deep dish complicates that as the edge is twice as far from the reflector than the centre, so you get an extra 6dB of space loss, so need less actual taper, but as always with Cassegrains, it is a big balancing act between feedhorn gain and blockage, feed distance, reflector diffraction loss versus blockage and all the other excitements of geometry.

Once I have the rings and refelctor supports sorted so that there is something to mount a VK3CV board to, I'll do some detailed checks on the gain and pattern. If the 18 inch job is as accurate as I hope, it should be a killer system, but it is going to be terribly sharp and probably not as useable as the 12 inch versions. I also have a few of those 12 inch deep black-painted spun steel dishes with a WG16-sized hole in the middle, so I'll get one measured up to see if it will work as a Cassegrain.

I have to machine up some custom feedhorns to get the geometry right, so probably that will mean a 3 or 4 wavelength Pickett-Potter or simple horn.   I'll sort that out once the subreflector is totally sorted.  I'm putting an M12 x 0.5 mm thread on the back of the subreflector to make fine adjustment a bit easier, and probably using blade-style supports for the subreflector mount, hopefully fitting  those via small holes drilled through the dish into the support ring.  I almost persuaded myself to mount the blades using magnets embedded in the ring behind the dish, and on the ends of the blades, but then I had a sensible moment.

Lots of Fusion360 models, bits of paper, spreadsheets and whiteboard scribbles going on here, but today I've been making another batch of 21 dB Pickett-Potters and part-making another dozen coupler bodies for VK3CV boards, so no additional progress on the dish project.  Plus I was at the day job from 9 to 5, then watched Bryan's excellent presentation, my productivity is dreadful.

Day off tomorrow, and I have a full day of machining heatspreaders and copper horn cores ahead, so unlikely to be any more progress on the Cassegrain project until the weekend.

Neil G4DBN

On 09/12/2020 23:26, Terry_G0EZY wrote:
Apologies if this is something that has been reported elsewhere on the Group/Scatterpoint but the January QST "Microwavelengths" article has an item on the 122 GHz transverter. It includes a suggested optical quality reflector. I thought it was going to be an expensive import from the USA but the web site re-directs to a UK distributor. The 12 inch reflector is here, with 18 and 24 inch versions.

https://www.edmundoptics.com/f/large-parabolic-reflectors/11483/ <https://www.edmundoptics.com/f/large-parabolic-reflectors/11483/>

At 44 GBP it seems worth a look but I'll leave it to experts to comment on it's suitability.

The article shows the 12 inch version with a dual-mode feed horn and Cassegrain sub-reflector turned on a CNC lathe by W1GHZ

73 Terry

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