Dear Rambabu and Kevin,
Here is an a final email to summarize where things have ended. As usual, you can find the entire thing on the research blog.
I have moved all of my equipment, including the vacuum chamber out of the University and have stored it. Neil, my engineering friend, who was going to design the magnetron feedback loop has sent me back the network analyzer and the thermal camera. He still has the magnetron, waveguide assembly, power supply and an oscilloscope.
As things stand, there are three phases left in order to get a high Q before any testing can commence:
- A working power supply with feedback loop (more below)
- The cavity – The cavity needs a couple thousand dollars of alterations (both suggested by Shawyer) to make it resonate with a Q of 50K.
- The two then need to be combined and the setup tested for deficiencies (i.e. the coax cable connecting the supply to the cavity may be prone to overheating).
As it turns out, the complicated feedback loop I had dreamed up may not be necessary because, as I have mentioned previously, it sounds like the magnetron gets “pulled” into the right frequency because of the strong resonance of the cavity. How this works exactly, however, is not clear to me and I am not sure if some type of external feedback loop is necessary. From the pictures of Shawyer’s demonstration engine, it would seem to have a computer controlled tuning wall at the back of the cavity. If I had to guess – the feedback loop works like this:
- The magnetron is turned on and allowed to warm up, the cavity is then tuned to whatever the magnetron frequency settles on (with the screw actuated tuning plate at the back). The hard thing is that the resonant frequency bandwidth of a cavity with a Q of 50K is a couple Khz wide, but a commercial magnetron can wander up to 30Mhz on either side of a target frequency. Once the frequency of the Magnetron gets close to the resonant frequency of the cavity, it is “pulled” into the right frequency because of a reduction in reflections (??). I am pretty fuzzy on how it works.
Let me tie up a few loose ends mentioned in previous blog entries:
- I decided not to go through with becoming a CNC programmer/operator for personal reasons – after talking to my working classmates, I realized I don’t want shift work. Larger shops who can pay more usually run their machines in two shifts, one from 6am to 3 pm and one from 4pm to 12pm. I have too many activities in the evenings to take on that type of lifestyle.
- Although I simulated a bunch of simple “test cavities”, I never did get the OK from NAIT to use their CNC machines.
Here is a summary of where the cavity was left:
- The unmodified cavity has a TE0,1,4 mode with a Q of 2500, an insertion loss of 2.2dB and a resonant frequency at 2.449Ghz instead of the simulated 2.467Ghz and a Q in the tens of thousands.
- I have created a model (v4) with Shawyer’s proprietary modifications and have simulated it with the Eignemode solver which resulted in a Q of 62K at 2.43Ghz. The model doesn’t have probes or optimizations (i.e. moving the tuning plate) and needs to be simulated with the Frequency domain solver.
On the plus side, Shawyer asked me today if I wanted to talk to a “CEO of a Canadian space company” who wants to do a “demonstration test”. As excited as I am by potentially getting paid to do this work, I don’t hold out much hope.
While the project has been winding down over the past year, I have been thinking about what I would have done differently and, I would go the other way with a larger cavity at a lower frequency. It has a number of improvements:
- The Q will be higher – as you get into smaller and smaller cavities, the same current gets concentrated onto smaller and smaller cavity walls, making the conductivity of the walls more important. With a larger cavity, the same current gets spread out over a much larger area and even copper or aluminum can make for a fantastic Q.
- I would make the tapered part of the cavity out of aluminum sheeting, but the end caps out of copper. As I mention in my blog, simulation of even the 2.45Ghz cavity in Aluminum had little difference because the TE mode works such that circular fields are zero along the walls.
- High power HAM radio equipment is readily available and relatively inexpensive compared to high-power 2.45Ghz systems and can pump up to 5000Watts of power.
- HAM radio equipment is designed to output finely tuned frequencies with narrow bandwidths, unlike commercial magnetrons which have a large and wandering bandwidth.
- Because the Mhz frequency has a longer wave length, it means the tolerances for the cavity are not nearly as tight as the Ghz range, which makes for less expensive cavities.
One downside will be a cavity measured in feet making it difficult to handle.