Good Simulation Match to Shawyers Cavity


Going to build a bent probe first and test.

Two weeks ago, based on simulations, I had decided that a probe 280mm from the short, 28mm in diameter and 15mm in length would work best.  One last thing I had to check was the exact probe length and to that end, did a bunch of simulations:

Which looks like the lowest insertion loss is when the probe distance from the short is 17mm.  I then reran my simulations but with a much tighter frequency spread and got this:

Looks promising, right?  The insertion loss is great at 0.23dB but unfortunately, the mode is messed up:


And the Q dropped to 41K.  Oops.

I also modeled bent probes in order to put the probe at 280mm from the short, but without moving the probe hole (which already exists in the actual cavity).  The probes looked like this and the results were:


Not bad at 0.45dB, but it, too, lead to an impure TE0,1 mode (impure in the sense it didn’t have the circular TE0,1 mode through out the cavity) like this:


And the Q dropped to 48K from 60K.  Although it took me a while to realize it, the optimization has to happen with two variables at the same time, a high quality factor (Q) and low insertion loss.  There are a lot of resonances that have either a high Q or a low insertion loss, but the trick is to find one that has both (i.e. the TE0,1 mode!).

It is interesting to note that Shawyer has a few things to say about quality factor:

  • “The engine <the one we are duplicating> was built with a design factor of 0.844 and has a measured Q of 45,000 for an overall diameter of 280 mm.” (demonstratorengine.html,
  • “Conventional microwave technology limits the maximum Q of resonators to around 50,000, giving a specific thrust of 200 mN/kW” (applications.html,

I had hoped that we could get a Q of 60K or 70K because early simulations suggested we could, but I made a newbie mistake.  Those high Q factors are only possible with high losses.  As I noted in a previous update, I could get a Q of 32K from our actual cavity, but the insertion loss went up dramatically (over 6dB), making that particular resonance unusable.   Now looking back at Shawyer’s work, it would seem that our simulations are lining up with his results.  The best Q we can hope for is in the 41 to 48K range with an insertion loss between 0.23 and 0.45dB, as compared to our previous 60K with a 0.7dB loss, and that our simulations are showing exactly what we should expect.  Now to get the real cavity to work as it should.

I have two options, the first is to create a bent probe, which is what I will do first.

The second is to move the probe to 280mm from the short, closer to the narrow end and drill a new hole.   The probe needs to be straight up and down though and needs a mount.  One option was to build a ring that could be fastened to the cavity (shown below).  I modeled one in 3D and got a quote from, which is easier than it sounds, because their software calculates costs including shipping right from the program.  Turns out it would cost roughly $500 (including shipping).

Instead (and if the bent probe doesn’t work) then I am going to build my own probe mount with an extra square piece of copper I have.  The hardest part will be sanding the mount to fit the curve of the cavity.  The probe mount will look like this:

Having done a lot of simulations, I am now going to build a bunch of different shaped probes and test them.  I should then get a good idea of how well the simulations match the real cavity and what to change to give the best results.  Wish me luck.

I also contacted an old friend from my engineering days at SFU and he has agreed to take a look at building a feedback mechanism to control the magnetron frequency.  The great news is that he supports open source hardware and if he takes the job, we can publish the entire design! (for all those others who may want to reproduce our results.)