Re: 3D printed cassegrain reflector.


Neil Smith G4DBN
 

Ian's mention of gold leaf got me thinking.  Assuming I could find a suitable size or surface treatment that would release easily, I wonder if I could make tiny gold-lined horns by burnishing leaf on to a mandrel, then electroforming copper on to the outside, thickly enough to encase it and make it stiff enough for machining, or at least casting into resin?  If the size or whateve would melt at lowish temps, and dissolve in solvents, it might cut out the painful steps of electroforming onto an aluminium mandrel.  Dropping the "oxide-etch/zincate/nickel plate" steps and the "machine out the mandrel/dissolve in hot caustic" steps would make things a lot easier.  I guess though that the copper would leach through the gold, or that maintaining adhesion of the leaf to the mandrel might not be straightforward. Just think of the bling factor though!

It's marvellous to see all of the work going on with the Cassegrain designs.

A French team are trying out a simple CNC router/mill and getting decent results, and I see that lots of folks are trying 3D printing of various geometries and surface finishes.  I'm sticking to manual machining of a flat template and using a 10mm diameter follower and 10mm diameter tool, to get close to a mirror finish, but that's not much use for anyone without a convenient machine shop and I'm snowed under with the day job and a long backlog of machining work. 

The elephant in the room is how bad the surface accuracy of those deep Edmund dishes is.  I think we are going to hit the limit of the parabola errors soon, with all the machined and printed subreflectors getting good accuracies.  Modelling with a perfect surface shows about 54 dBi according to Michel's simulations. I'm having amusing issues in getting sufficient stiffness and adjustability in the mount to optimise all of the variables.  The subreflector needs to be very precisely positioned so its rear hyperbolic focus coincides with the focus of the parabola, but also its axis needs to coincide with the axist of the parabola.  That might not be at where we think it is, so I'm allowing for lateral positioning, canting and axial positioning adjustment.  The feedpoint position isn't quite as sensitive, as the worst that will happen if the axis of the horn isn't aligned with the axis of the dish/sub combination is just a pointing error. Getting the feed phase centre to coincide with the secdond focus of the hyperbola seems to matter only in the axial direction, so a decent focus slide matters more than sideways positioning. There doesn't seem to be much impact on gain for a lateral error of 6 mm.  Using a very long interfocal distance for the hyperbola should mean that axial positioning of the phase centre of the feed is less critical than if the feed is on a stalk and close to the subreflector.  I just picked "roughly in line with the dish surface" as my chosen feed position so I could make a simple axial focus slide to mount the PCB and horn assembly. I am making a wild assumption that the axis of the Edmund dishes is close to the centre of the hole in the dish.  That might not be true, but I think 5mm either way is proabbly unimportant, and making the subreflector position adjustable will allow that to be compensated for.

I'm using thin carbon fibre rods to support the delrin spider which carries the threaded subreflector. That thread is 0.5 mm pitch, so it's easy to get extremely fine control of the focus.  The support rods are also finely threaded to allow adjustment of the spider position in three orthogonal axes as well as it's axial alignment.

The other aspect of the Cassegrains that I'm worrying about is the edge taper for the feedhorns. If the feed was at the parabola focus, and you want to get an edge taper of around 11dB to balance gain against overspill, then you need to consider that the distance from the horn to the edge of an f/d 0.25 dish is twice that to the centre, so there's an automatic 6dB or so of space attenuation, so the horn would need have a wider pattern than you'd expect, with -5dB at the dish edge to get the illumination at -11dB.  That's a bit theoretical as a feed as wide as that is hard work.  With the Cassegrain, the feedhorn sees a virtual dish that subtends a half-angle ( in this case) of about arctan(24/105) or around 13 degrees. In this geometry, the distance to the edge of the dish is about 1.5 times the distance to the centre, so the space attenuation is rather lower. That leads to a feedhorn with a taper that's about -9dB at +- 13 degrees being roughly right for the job.  I'm expecting there are a lot of confounding variables though, so my plan is to make a range of different dual-mode horns of various gains to see where the sweet spot is.  W2IMUs get a bit unweidly at those gains, so I'm working on the assumption that a Pickett-Potter will be a decent choice.  Simple horns have some nasty sidelobes and aren't really any easier to machine.

All of this mucking about is unlikely to result in something that's much better than you could do with a card template and a lump of something spinning in an electric drill held in a vice and filed to shape, then coated with burnished foil, but I'm interested in that extra 1dB that means an extra kilometer, or those few extra dB that gets me further still. The important and interesting bit of ham radio for me isn't the end result, it's all the buggering about that you have to do to reach that point.

My next step for later in the year is using this Prusa 3D printer to make a honeycomb-back mould pattern for a dish around 13 inches diameter, then casting it in aluminium. The outline plan is to make a super stiff but light axially-displaced ellipse antenna, with an inside-out Gregorian feed and no subreflector blockage. I'll mill the back flat, then mount it on a big cast-iron faceplate on my lathe to turn the reflector surface. It will need sufficient stiffness and precision so it will work OK from 47 GHz up to > 275 GHz.  I'll need to heat-treat it, or find someone with liquid nitrogen to make sure it's suitably stress-relieved and doesn't relax and bend as it ages.  Then I need to think about surface finishes that will work OK.  I guess anodising will be OK at mmwave frequencies, and it would look rather spiffy with an electric-blue dyed surface, but I'll have to find out. Yet more buggering about. Huzzah!

Some days I wonder if I should just give in and string up an EFHW and 81:1 unununun and play on those funny HF bands?  Nah, where's the fun in that?

If anyone's interested, I just published a new video on my Youtube channel about how I machine the coupler bodies for the various antenna types on the VK3CV boards.

It contains Humour, because there isn't enough of that in the world these days.

https://www.youtube.com/watch?v=uAG7y3tl9s8

I'll publish updates on my Cassegrain experiments and feedhorn works soon, but the day job is intruding into life again. On my day off too.

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