Machined Cavity Refined

My momentum slowed a bit this week as I worked to get a proper drawing off to Ryerson Metals for a quote.  The good news is the price for the aluminum cavity including metal and machining might be under a thousand dollars.  I got a quote back for just the aluminum rod 12″ in diameter and 16″ long for only $730 which was a surprise.  My hope is that if I buy the aluminum and get it machined at the same place, I can pay for just the %13 of aluminum remaining, which could save me $635!  I suspect the machining will cost me a lot though because the rod will probably have to spend literally hours on the mill.  I will then have to get the cavity electroplated with copper which is the big unknown – will the copper be thick enough?  Will it be even enough? Can it be made shiny and ultra-smooth?  Can I solder the probes to it?  I expect the quote for machining back early next week and if it is under $500, I might get two!

Most of this week was spend redesigning the cavity again because I wanted to reduce the amount of aluminum required and this is what I came up with, simple and beautiful:

This cavity was built to exactly the dimensions as determined by the cavity exported from the simulations. It was also designed to make mounting the large power probe much easier because it fits exactly:

It is recessed on purpose because to get it flush, the rounded surface will have to be sanded down.

What took the most time this week is that Ryerson wanted a proper 2D drawing including a “tolerance block” and I learned how to do that:

That’s a screenshot of a PDF that allows you to zoom in which is really neat.  I got the template as a DWG file and then combined it with a 2D drawing of the model.  I built the whole thing in Rhinoceros because making the dimensions is trivial and then printed it to a PDF.

I also received a few things I’ve been waiting for this week:

  • Over the many years I have been doing this research, I have been collecting waveguide components and because I was looking at 5.8Ghz, I got a WR-159 waveguide-to-N converter (second from the right):

    The rest of the waveguide components are circulators (typically used to dump unwanted reflected power into a dummy load) and I have a nice cross section of waveguides going from 3.30Ghz up to 8Ghz now.  The waveguides are cheap and plentiful these days, especially WR-90 components (8.20 to 12Ghz) but the sources are still expensive and rare.

  • I also got the copper mesh and the quality looks excellent:

Hopefully in early October I will have a machined cavity to test and then we shall see if the Q measured is anywhere near the Q simulated.  I will be coming up this week to try the copper mesh and get a new lab key.