G-Zero - Second Cavity Results
The machined cavity was created in order to eliminate flexing and match the cavity much closer to the simulated one, both problems with the previous plastic cavity.
This was covered in the blog and repeated here for continuity:
I also decided to cross that PONR or Point Of No Return and contacted Roger Shawyer directly. We had an interesting conversation last week and I have point him to this blog. We talked about the following:
- Other universities have tried building cavities but ultimately failed where we have – their simulations don’t match actual results.
- Chinese Results:
- Have been one (of few?) to have reproduced results.
- Yang Jung group have built actual cavities and have gotten results of 750mN using 3Kw power.
- The work has gone black/military and moved to Beijing, no more papers are being published.
- Yang Jung was “disappeared” for six months with the implication of instilling dogma.
- No one outside of China has seen their cavity, although Shawyer did traveled there and given talks.
- They rebuilt commercial simulation software which took 19 months in order to deal with the discontinuities of asymmetrical cavities.
- A Big Obvious Enterpris-ING company is the US contractor Shawyer has been working with
- Sounds like the trick to powering the cavity at the resonant frequency, is to tune the cavity to the magnetron after it has warmed up, and the because of reflection, the cavity will pull the magnetron frequency and lock it in.
- A simply geometry will get the cavity into a Q of thousands, but some type of internal shaping is required to get Qs of tens of thousands and the reason is because of phase continuity or coherence has to be maintained.
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.
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.
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.