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Gruending Design and Test Setup

Summary

  • Neil is now setup to go, need better magnetron power supply

My trip to West Kelowna was a success, I survived the 24 hours of driving (there and back) and we have a better idea of the challenge ahead.

The first thing we did was set everything up including the water cooling and magnetron and ended up with the following

On the left is the water pump and radiator, then the magnetron connected to the circulator which was connected to a section with tuning pegs finally ending in a short (metal bolted to the end). In the foreground you can see a multimeter and a high voltage probe we used to sample the magnetron power supply. We also used a microwave detector to test for stray radiation and the thermal imager to make sure the magnetron didn’t overheat.

We then hooked up the variable step attenuator, programmed it for 70dB and connected the Signalhound to the isolation port on the circulator. After getting a few sparks when the high voltage for the magnetron arced through the wire shielding, we recorded a few results.

It was soon apparent that the magnetron was jumping frequency too much to even show up on the Signalhound which takes measurements every 150mseconds. (As a reference, the HP 8753D analyzer at the university samples at 50msec, and even there it was jumping around way too much).

We then hooked up a high voltage probe to record what the magnetron input voltage looked like and I was surprised to find this:

From Neil’s blog he writes:

  • The peak to peak AC voltage is about 4.4kV and is shown AC coupled. The current waveform is DC coupled and is changing directions which makes sense in an AC powered system. The peak current draw is about 12A with a RMS current of about 8A. These numbers don’t make sense though because the RMS power consumed using these numbers would be 16kW (2kV * 8A) which is quite a bit more than a 110V plug can supply. I will need to try and repeat these measurements once I go over the test setup.

I was under the incorrect assumption that domestic magnetron power supplies used some type of rectification in order to create a 4.2Kv DC cathode voltage, but apparently you can run the magnetron off AC as well. We are guessing that the reason the magnetron output jumps frequencies, is because of AC input. I plan to send Neil another newer microwave oven that uses a different kind of power supply to find out if it too drives the magnetron with AC, or a DC voltage and if it makes any difference in the magnetron output.

The conclusion of our trip is that we need to test more magnetrons and magnetron power supplies in order to find a setup that has a much steadier output. Once we have a steady output that can be measured by the Signalhound, we can then use a feedback loop to center it on the cavities resonant frequency.

Along that line, I found a paper that mentions the magnetron theory of operation which had some interesting points:

  • “For most magnetrons the temperature coefficient is negative (frequency decreases as temperature increases) and is essentially constant over the operating range of the magnetron.”
  • “The “automatic” synchronism between the electron spoke patter and the r.f. field polarity in a crossed field device allows a magnetron to maintain relatively stable operation over a wide range of applied input parameters. For example, a magnetron designed for an output power of 200kw peak will operate quite well at 100kw peak output by simply reducing the modulator drive level.”
  • “The pushing figure of a magnetron is defined as the change in magnetron frequency due to a change in the peak cathode current.”

Here are my takeaways from the paper:

  • Frequency control may be easier by controlling the magnetron cathode current with a constant temperature then controlling the magnets

Or, it may be a combination between controlling the cathode current and magnets that gives the best result.

Here is my todo list:

  • Send Neil the other microwave to see what waveform that microwave uses to power the magnetron (TODO)
  • Send an email to John Gerling to see if we can get the specs for the ASTeX magnetron head because his company repairs them.  and John responded with “Sorry, can’t help you any further”.  It turns out that John was one of the original designers for the ASTeX magnetron and Gerling spun out the ASTex company which means he considers it IP. Darn!  Have to find another source…
  • Get schematics and/or repair manual for the MDX-10 power supply in case we can use any of the parts. (TODO)

High Voltage High Power Fun

A friend stopped by this weekend and dropped off some fun toys.

He needs a frequency locked 1000W 2.45GHz signal based on a microwave magnetron. I like the magnetron solution because it’s a cheap way to generate such a high power RF signal but they wander between  2.42GHz and 2.48GHz which is a problem for his application. I have volunteered to help him figure out a method to lock the output frequency. I think it will make a great writeup for the blog since I expect it will take a significant amount of reverse engineering and experimentation to make it work.

I would like to point out at this time that the output from a microwave magnetron is extremely dangerous. Microwaves like to boil water and people are 75% water. All of my experiments will be done in carefully controlled conditions and have been checked with a microwave leakage detector. The microwave energy in the experiments will be dissipated using a water load and also the test setup itself, not me.

The first thing we did is setup all of the equipment for the tests. The magnetron and control circuit are from a microwave oven and all of the waveguides are recycled from old equipment. The water cooling setup is a pump and radiator from an old CPU water cooling setup. Unfortunately it takes a lot more than an old microwave for a test setup. Here’s a block diagram of the basic test setup (you might have to click on the image to see it clearly):

With this test setup, the magnetron can only be run for short periods of time. For example, after 20 seconds of operation the magnetron case temperature can reach 50C even with the cooling fan from the microwave blowing across it. Since the magnetron can run for many minutes inside of a microwave the test setup needs to be reviewed to see if the microwaves are being reflected back into the magnetron. Since the magnetron output is shorted, I would expect to see most of the power reflected back towards the magnetron. The isolator should be attenuating this energy by 20db or so but that means that 900W (1000W-1000W/10) would be absorbed by magnetron. Here is a good application note on how isolators work. Section 7.9 suggests a method of frequency locking a magnetron which might come in handy.

We first tried to observe the magnetron output using a Signal Hound spectrum analyzer connected to the isolator reflected power output with an external attenuator. When we tried a 50MHz span centered at 2.45GHz, the sweep rate was about 300 to 400ms. Occasional peaks were observed but it was pretty obvious that either the signal wasn’t there or it was hopping around faster than the Signal Hound could capture. Different attenuation settings didn’t help. My friend had previously tried the same test using a newer Agilent spectrum analyzer and even with a sweep rate of 50mS he observed that it was difficult to see the magnetron output. Even when the Signal Hound was set to a 5MHz span the sweep times were still 150ms to 250ms. I will have to investigate further to see if it’s possible to speed up the sample rates using either a faster PC or with different sample settings.

We also measured the input voltage to the magnetron using a Fluke 87III multimeter and a Fluke 80K-40 probe and got a DC voltage of about -2.2kV and a ripple of about 2.2kV. This surprised my friend since he thought that the magnetron was powered with a rectified DC voltage. I added a CT238 current probe and captured the following waveforms with my Tek 754D oscilloscope:

The oscilloscope attenuation factors were set so that the displayed voltage from the 80K-40 probe was approximately correct on channel 1 (the black trace). The oscilloscope was also set so that the voltage displayed from the CT238 current probe is actually in amps, ie 1V = 1A. Normally you can’t use the 80K-40 as an oscilloscope probe because it has a 3dB bandwidth of about 400Hz, but in this case the waveform is about 60Hz so it’s an acceptable approximation. The CT238 probe has a frequency response of 250kHz which is more than adequate for the signal measured here.

The peak to peak AC voltage is about 4.4kV and is shown AC coupled. The current waveform is DC coupled and is changing directions which makes sense in an AC powered system. The peak current draw is about 12A with a RMS current of about 8A. These numbers don’t make sense though because the RMS power consumed using these numbers would be 16kW (2kV * 8A) which is quite a bit more than a 110V plug can supply. I will need to try and repeat these measurements once I go over the test setup.

So far there are more questions than answers, but that’s what makes this project so interesting. After a few hours of playing here are the next steps:

  • Contact Signal Hound to see if sampling can be sped up. Right now it’s too slow too see the magnetron output if it’s constantly changing frequency.
  • Verify the power test configuration. The numbers measured aren’t making sense.
  • Try to measure how much power is being reflected back into the magnetron. The large reflected power could cause the magnetron to change frequency rapidly.
  • Can magnetrons operate on DC voltage instead of AC? Maybe a DC voltage will help stabilize the magnetron.

(Original Post here.)

Signalhound Comparison and Attenuator Test

Summary

  • Signalhound and 70dB step attenuator seem to work fine

I was up at the university yesterday in order to test the Signalhound and the 70dB step attenuator and both of them seem to work fine.  Here is the test setup for the attenuator:

To change the levels of attenuation, a 15v power supply was required and the attenuator was hooked up between the ports of the network analyzer.  The attenuation was as expected and it was neat to hear the attenuator make a solid click when changing between attenuation levels.

The network analyzer was then used as a super accurate signal generator and hooked to both the Agilent spectrum analyzer and Signalhound at the same time.  The spectrum analyzer was attached to channel two and the signal hound attached to channel one.

The results were then compared:

Looks pretty good, although the Agilent is cleaner and faster, the Signalhound is certainly usable.

Many thanks to Kevin who took a few minutes to configure the network analyzer and spectrum analyzer to make the measurements.

I then packed up all the waveguide components, power supplies, pump, radiator, and magnetrons and carted them out to the car in preparation for my drive to Kelowna on Friday, October 22nd.

Bent probes and cavity a no-go and CNC Machinist

Summary

  • Bent Probes and cavity a no-go and CNC Machinist

Last week I was up at the university and tested the bent probes.  The results were bad enough that I didn’t record any results.  The problem is that there doesn’t seem to be much of correlation between what is expected and what is measured.  Worse, I don’t know why.

To move forward, I have to step back to simpler cavities and make sure I can create cavities that match measured results to simulations.  Once there, I can then make and test incremental changes, until I arrive back at the original shape with all the improvements necessary.

Making the cavities by hand, i.e. folding copper into a tube, isn’t an option because I can’t get enough precision.  Using 2.45Ghz requires using cavities that are on the order of the wavelength at that frequency, which is roughly 10 to 12cm or larger (depending on propagation in free or enclosed spaces).  A crappy metal lathe starts at $600 (princessauto.ca – I like the word “Metal Worker” on the side 🙂 ) and a good one goes for a few multiples of that.

Simple small tube cavities are easy on a manual lathe, but eventually, making large asymmetrical ones, which are at the heart of the EMdrive, requires a CNC controlled lathe.  Second hand CNC lathes can be had for about $12K and up but require 220V or 480V and require knowledge about the controllers.  I have neither the knowledge, money, electrical power or space in my garage for such a beast.   Because all the cavities are one-offs and can be large, local machine shops won’t touch them, as I found out when trying to get the $2K cavity built (I think I contacted ten different machine shops).   The cavity is large enough, 280mm dia. at the large end, that it is outside the build envelope of a lot of lathes, and the larger lathes that are capable are all tied up doing more valuable work.   Ordering from emachineshops.com is too slow, as the lead time is months and I want to iterate through cavity designs weekly.

I have decided to kill a few birds with one stone by becoming a CNC machinist, my third career.  My first career was a design/marketing engineer at a small BC company, my third was Levitee research, which I haven’t given up, and now a machinist.  I have been working on research now for ten years, four of which was Levitee specific, slowly burning through my savings ($30K left!!).  I also need to again start saving for retirement.  Again?  Yes, I was fortunate enough to have enough money from my first job, and willing parents, to “retire” at the age of 28 when I started down this crazy path. I need to turn on the money faucet once again.

The three birds I will kill with one stone are – learn how to be a CNC machinist, make cavities and money.

The good news is that there is a solid demand for CNC machinists in Edmonton, in large part, thanks to the oil industry.  To that end, I have, through a friend, applied at a local company which has some CNC positions open and without much surprise, they are reluctant to hire me.  To overcome those fears and because I don’t know anything about machining, I have now enrolled in an evening/weekend CNC machinist training course at NAIT and should be done by May 12th, 2012 (costing roughly $2K and 200 hours in time).

On the Levitee front, a few things did get done, for example, I received the Signalhound and I bought cables for Neil.  I plan to be up at the university next week to test the Signalhound against a more expensive spectrum analyzer to give Neil a better idea of the differences.  Things are still moving forward, even if I have to take a side step at this point.  Besides working with Neil to get the magnetron feedback loop working and simulating test cavities, things will slow down.

New Spectrum Analyzer and Welcome Neil Gruending!

Wow, another status update in the same week means things are getting done!  Summary:

An engineer friend from my university days has joined the team and will be designing and building the feedback control for the magnetron.   A spectrum analyzer was purchased and the bent probes built.

You can read up about Neil Gruending in a recent EEweb article which mentions that he owns five multimetres – he is an engineer’s engineer.  Neil and I worked on a project together many many moons ago and it was only in the last year that I reconnected with him through Facebook.  After reading that article, I realized he was the perfect guy to build the feedback loop and the timing looks to be right as he has time and interest, yes!

Even better, Neil is a big supporter of open source hardware and has agreed to share his design, including software and any circuit layout.  Sharing the design is important if other groups want to duplicate the work.

For most of this week, he and I have been talking about what equipment he needs and one of the first pieces is a spectrum analyzer.  Which was something else I learned about this week, the difference between a network analyzer, which I use all the time in the lab, and a spectrum analyzer.  The network analyzer has both a source and an analyzer which allows for the characterization of a passive device under test, like the cavity.  A signal is injected into one port and the response recorded on the second and the network analyzer has special capabilities for calculating things like Q (screen shots of which I have sent around previously)  The spectrum analyzer is useful for looking at a signal and recording things like power, frequency and frequency-spread and typical spectrum analyzers have no output capability.

What took up the most time this week was looking at all the options available for getting a spectrum (or network analyzer).  The biggest problem is price.  The yearly demand for analyzer is low, quite likely on the order of tens of thousands of units a year and they are complicated devices because they deal with Ghz frequencies.  Up until a few years ago, analyzer have also always included a display, which adds to complexity and cost.  Add “complicated” to “low volume” and in a mature market, you get astronomical prices as spectrum analyzers start at $10K and go up.  The higher frequency, the higher the cost.

Even second hand analyzers tens of years old, hold their value and there is a huge business selling and repairing second hand analyzers.   For example, take a look at this ebay search which shows that if a spectrum analyzer is under $4K  it is either 25 years old, or more likely, doesn’t go up to 3Ghz.  Even if you do find one on the cheap, they weigh a ton, which means shipping is in the hundreds of dollars, not to mention the hundreds paid in GST and brokerage charges.  Meh.

It is only within the last few years that it has been possible to purchase an analyzer that uses digital technology, DSPs specifically, and a computer to display the results.

The options were short listed to :

  • $4K for a second-hand spectrum analyzer available here in Edmonton [eBay.com – an Anritsu MS2602A for when the link goes dead ]
  • Or $1k to $2k for a USB driven, display-less spectrum analyzer that uses a computer to display the results and the two considered were these:
    • Signalhound (.com) – a small US based company that also repairs analyzers and does LCD retrofits
    • Spectran (aka Aaronia) – a larger German based company and they sell both a USB based version and a version with a simple LCD display.

The winner was Signalhound and I purchased their spectrum analyzer for $1076CND today and it should be here next week.  Here ‘s a pic:

Signalhound won for a number of reasons:

  • Price – $919US (not including shipping, taxes, etc).
    • Aaronia has a better product that goes up to 6Ghz versus Signal hound’s 4.4Ghz, but costs $500 more
    • Aaronia doesn’t have an API or programmable interface (which is important and explained later)
    • Keeping the cost low will also help those who wish to duplicate our work.
  • Performance – up to 4.4Ghz
    • One important specification of a spectrum analyzer is how sensitive it is, however, in our case, because the magnetron signal has to be heavily attenuated before being injected into the spectrum analyzer, sensitivity isn’t an issue.
  • Custom programmable – This was the biggest reason Signalhound won because it can be used as part of the feedback loop!
    • I asked the designer to comment on using the Signalhound in our application and his response was “The Signal Hound API can stream 480K samples per second on a 240KHz bandwidth.  It will be more than fast enough for the feedback loop you have described.” and the feedback loop I described was:
      • “We are (quietly) duplicating the “demonstration engine” on the “EMDrive” (emdrive.com) in a university laboratory environment.  The EMDrive is essentially a magnetron, the heart of a domestic microwave oven, dumping upwards of a 1KW at 2.45Ghz into a finely tuned closed resonating cavity.  At the frequency the cavity will be tuned to, it will have a Quality factor in the tens of thousands, which means a bandwidth 2 to 6Khz wide.  By “bandwidth” I mean that the input frequency will need to be centered at the resonating frequency and not wander outside that range.  Commercial magnetrons are great at heating food, but not so great at delivering higher power in a narrow frequency range.  Our plan is to create a feedback loop by first, reading the magnetron output frequency, attenuating it, measuring it, then controlling the magnetron modified with electromagnets instead of permanent magnets.”

Once the spectrum analyzer arrives, then I will be bringing it up to the university to test it against the current magnetron setup.  I will then package everything up including the magnetrons, power supplies, circulator, tuning waveguide and coupler sections and get it all to Neil.  The intention is to use, as a guide, the paper written by William C Brown  “The Magnetron – A Low Noise, Long Life Amplifier” [PDF ]

Oh and I also got the bent probes finished today:

Whew!  This project might get completed after all!

I will be up at the university this coming week to test the bent probes.

Good Simulation Match to Shawyers Cavity

Summary:

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, emdrive.com)
  • “Conventional microwave technology limits the maximum Q of resonators to around 50,000, giving a specific thrust of 200 mN/kW” (applications.html, emdrive.com)

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 emachineshops.com, 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.)

 

I Think I Know What To Fix!

Progress!

Summary – From simulations, it looks like I can fix three things on the cavity, increase the probe length from 3mm to 15mm, move the probe backward to 280mm and get rid of the gap around the tuning plate.

For the test runs  below, I varied four variables

  • the probe loop diameter
  • the length between the probe mount and the probe loop (how far the loop stuck into the cavity)
  • the distance of both the measurement and power probe from the tuning plate
  • the gap around the tuning plate

Below is probably the most indecipherable graph ever but it shows all the test runs:

Analysis:

  • The best result seems to be a loop 28mm in diameter 15mm in length and 280mm from the short with no gap around the tuning plate.
  • The three variables that had the most dramatic affect, did so in two different ways:
    • Probe diameter and probe location affected the insertion loss dramatically but not the frequency, e.g.compare (E), (G) & (J) where the loop went from 20mm to 28mm and the insertion loss went from 1.5dB to 0.34dB.
    • Probe length seemed to affect the resonant frequency, starting at a high frequency of 2.4417Ghz with a probe length of 3mm, swinging as low as 2.435Ghz @ 20mm and then returning to 2.4414Ghz for a probe length of 30mm.
  • The current tuning plate has about a 1 to 2mm gap around the edge in order to avoid the tuning plate scratching the inside of the cavity.  However, as a previous status report showed, having a completely aluminum cavity has little affect on the TE0,1 mode and a few scratches from the tuning plate will have negligible effect.
    • The simulation results above suggest the tuning plate gap at 2mm, has a slight affect on both the insertion loss and the resonant frequency.  For example, for test run (G) without a gap and (H) with a 2mm gap, the frequency moved 3Mhz and the loss got better (?).
  • One of the surprises during this round of simulation was the 9dB loss for test (B).  This shows that not only is 292.5mm a bad match, but it is also more susceptible to other factors.  At 280mm, the probe length could vary from 3mm to 20mm in length and have little affect on the insertion loss, wheres as at 292.5mm, going from 3mm to 15mm, the loss went from 0.6dB to 9dB!!  The susceptibility to other factors at that probe location is likely the problem with the current cavity.

For the coming next week:

  • It is clear that having the probe at 280mm is much better then 292.2mm and I want to test, via simulation, if I can use the same probe location, but bend the probe backward such that the loop is in the right location (15mm above and 280mm from the short).
  • Once done the bent probe simulation, I will then decide if I need to drill a new hole or not.  I will then build a new probe and remove the gap around the tuning plate.

A Million Simulations – Some Results

 

I spent the rest of August running simulations to see if I could find the root cause of the difference between simulations and actual, without much luck.

Besides looking for the source of the problem, I am also trying to make changes that keep the shape of the cavity the same.  Changing the shape would mean ordering a new one, but it could very well produce the same results.  I need to be able to make sure that what I simulate makes a difference in the real world.

Here are the things I checked:

  • The location of the probes in the actual cavity are in the same position as simulated (99mm from the lip of the large end)
  • Whether a grounded tuning plate makes much difference –  it doesn’t – a couple dB insertion loss (it will be grounded when under high power)
  • Whether a large gap around the edge of the tuning plate makes much of a difference
    • 4mm

      The tuning plate is also ungrounded results in 1.2dB insertion loss, yuck.
    • 8mm (huge gap!)
    • Analysis – The plate needs to be grounded and the gap should not be more then 4mm.  With a gap at 4mm and no proper grounding of the tuning plate,  we could see up to 1.2dB of insertion loss.  The resonant frequency and the Q did not change.
  • I also moved the location of the probes forward and backward from the current location at 292.5mm from the tuning plate
    The probe here is in the zero point of the field and doesn't create a TE0,1 mode at all.
    Can start to see the resonance deform.
    This is the currently implemented distance, 292.5mm
    This has the lowest insertion loss
    The TE01 mode was really weak at this position even though the insertion loss was great

     

    • Analysis – As expected, the best match is when the probe is centered in the peak of the field, around 280mm, and the current location is slightly forward of that at 292.5mm.  The resonance at 280mm had the same Q (76K), the same frequency (2.4425Ghz) but had a better insertion loss (0.5dB).   It is possible to plug the current probe hole with round copper stock and move the power probe backwards and opposite the measurement probe.  However, I have to check if I can move the probe a centimeter backward and still stay on the probe mount ring (pointed to by the arrow)

      Note to self – Create a wider mounting ring for the next cavity.
  • I have also started to lengthen the probe and it does have an affect, for example, it moved the resonant frequency downward by 100Mhz.  Unfortunately, it seems to have messed up the TE0,1 resonance as shown below and dropped the Q to 60K.

    Probe is extended 15mm into the cavity with a 25mm diameter loop on the end. Q is lower, insertion loss is excellent. Frequency is 2.4411Ghz.

    The probe is 30mm deep with a 15mm diameter probe, the TE0,1 is impure but note it moved to 2.438Ghz
    • Background– One of the neat features of CST’s microwave studio is the ability to insert variable names instead of numbers, for example, the diameter of the probe can be “diameter_of_probe” which you (or an optimization routine) can then change.  In practice however, changing a variable by a large amount usually breaks the model, for example, using a large diameter probe loop means it is too close to the probe mount.  Thus when changing a variable, a few other variables have to be adjusted too.  Besides running simulations, I also spent time building a more robust model that can handle large changes without breaking.I am now on version 2.02 of the model and can change the depth of the probes and the probe loop diameter without breaking it.I also fixed another problem – With earlier model versions, the lofts I used to connect the probe loop to the probe had numerous “degenerate tetrahedrals” which are really thin tetrahedrals.  Degenerate cells mean convergence of the adaptive meshing takes a long time, up to 8 or more samples.  Each sample also uses a large number of mesh cells, for example, I have had counts as high as 750K.  Without degenerate mesh cells, the model uses ~450K cells, and takes 4 samples to converge mesh adaptation.  (The number of samples to simulate depend on the frequency range, for example, a range of 25Mhz takes nine samples, a 100Mhz range can take 16 samples.  Even then, if the TE0,1 mode moved, it requires another couple frequencies to be simulated in order to see which resonance is the TE0,1 mode and what the Q is at that frequency)
    • Method – One of the problems to fix is that the frequency of the actual cavity for the TE0,1mode is 18Mhz higher then simulated (2.467Ghz vs 2.449Ghz).  If a longer probe moves the resonant frequency lower, then it should be possible to simulate a probe length that is 18Mhz lower and then the actual cavity should be at the right frequency.  If I can also get a rock bottom insertion loss (0.3dB or 0.4dB) and the Q isn’t too bad, then it might work.  Probes are easy to fabricate, especially long ones and it will be a good test to see if the simulation can result in real-world outcomes.Making the probe longer moves the TE0,1 mode lower, but messed up the field configuration.  At first, I tried a larger loop but that lead to a worse field configuration, but going in the opposite direction with a smaller loop fixed it. The simulations suggest that the total length of the loop is important to create a clean TE0,1mode.  It doesn’t matter if the loop is near the probe, only that it is the same total length from probe mount to termination on the cavity wall.Which is my next task, to optimize the probe length and diameter.

Simulating is a big time sink, because the simulations never run under 400K tetrahedrals and take at least two hours, which means I can do about three to four a day.

  • In preparation for a future cavity, I tested to see if an aluminum cavity made much difference compared to copper.  I was a bit shocked to find that a completely aluminum cavity had an insertion loss that was almost the same as copper (0.8dB vs 0.72dB), the frequency of the TE0,1 mode didn’t change and the Q was almost as high (60K vs 76K)!!  After talking it over with Kevin, it would seem this is because the TE0,1 mode is zero along all the walls (but not at the end plates)  Aluminum has something like %60 less conductivity than copper!
Aluminum cavity

Here are the Q calculations between the two, first copper:

Frequency/Hz  2.44270000e+009
Energy/J      2.68158093e-007

Layer     Solid       Conductivity   Mue           Loss/W(peak)  Q
—————————————————————————-
Copper (hard-drawn)   5.9600e+007    1.0000e+000   1.0743e-001   7.6618e+004
Resonator:Cavity                         9.3921e-002   8.7641e+004
Measurement Probe:Probe                  3.5290e-003   2.3325e+006
Power Probe:Probe                        2.4124e-003   3.4121e+006
Resonator:Tuning Plate                   7.5663e-003   1.0879e+006
Resonator:Tuning Plug                    4.3164e-006   1.9070e+009
PEC                   5.8000e+007    1.0000e+000   3.2168e-004   2.5588e+007
Power Probe:Probe Backing                1.1306e-004   7.2807e+007
Measurement Probe:Probe Backing_1        2.0863e-004   3.9455e+007
—————————————————————————-
**Sum**                                            1.0775e-001   7.6390e+004

And then Aluminum

Frequency/Hz  2.44270000e+009
Energy/J      2.60714955e-007

Layer     Solid       Conductivity   Mue           Loss/W(peak)  Q
—————————————————————————-
Copper (hard-drawn)   5.9600e+007    1.0000e+000   1.3125e-002   6.0973e+005
Resonator:Tuning Plate                   7.3542e-003   1.0882e+006
Measurement Probe:Probe                  3.4224e-003   2.3384e+006
Power Probe:Probe                        2.3446e-003   3.4134e+006
Resonator:Tuning Plug                    4.1946e-006   1.9079e+009
Aluminum              3.5600e+007    1.0000e+000   1.1812e-001   6.7750e+004
Resonator:Cavity                         1.1812e-001   6.7750e+004
PEC                   5.8000e+007    1.0000e+000   3.1734e-004   2.5219e+007
Power Probe:Probe Backing                1.1232e-004   7.1248e+007
Measurement Probe:Probe Backing_1        2.0501e-004   3.9036e+007
—————————————————————————-
**Sum**                                            1.3157e-001   6.0828e+004

Note that this is version 2.00 of the cavity, built only in CST 2010 which uses the new static data for hard-drawn copper.

I have also been thinking about how to power the cavity and Kevin helped me nail down a few variables.  For example, because the cavity has a really high Q, we might be able to use a signal generator to power the cavity, even if it only outputs mW.   The benefit of the signal generator is that it has very fine frequency control and we can match it perfectly to the resonant frequency., thereby not reflecting unmatched frequencies.   Once resonance steady state has been reached, then very little would be reflected as well

As I understand resonance, if we have a high Q, it means that just a bit of input power at the right frequency can set it to resonant, whereas a low Q, means we need a lot more power to start and keep resonance alive.  From another perspective, if Q is high that also means we can use a very small input over a long period of time and end up with a very strong resonance.

Unfortunately, it will take much too long, on the order of days.  Most signal generators output 1mW and if the cavity can hold 100W at steady state, then it would take 100,000 seconds or 27 hours to reach.   27 hours is doable, however, Shawyer said that his cavity contained 17 MW of energy (17 megawatts!!, emdrive.com) and it would simply take too long with only 1mW of input!

On a related note, there is something I don’t understand about Shawyer’s cavity – Shawyer said his cavity contains 17MW of energy, but at 1KW of power, it would take almost two thousand seconds (or ~ 30 minutes) to reach 17MW!!?  His test runs never last for more then a few minutes and movement occurs between 10 and fifteen seconds after startup.  Shawyer says that 15 second start-up delay isn’t pumping resonance either, but “The engine only starts to accelerate when the magnetron frequency locks to the resonant frequency of the thruster, following an initial warm up period.” – Hmm, thoughts anyone?

This status update turned out longer then anticipated because I wanted to be able to report something substantial and results seemed to be just one more simulation away.

The Conundrum – Why the Simulation Does Not Match Actual?

Summary:

  • Simulations suggest rotated probes don’t make any difference
  • Upgraded computer to 16GB of faster memory and faster processor to handle larger simulations
  • Still trying to solve the conundrum – why the simulations don’t match real world results.

At the end of July, it was clear that simulated results were not matching the real world cavity, enough so that running a full-power test would not provide any useful information.  The problem is because of three things:

  • The resonant frequency of the cavity for the TE0,1 mode is 20Mhz higher then in the simulations (no matter the position of the tuning plate).
  • The insertion loss at the TE0,1 mode is 2.2dB versus an expected 0.5dB
  • The Q is 2500 versus an expected Q of 10,000+.

So far, here are all the things I have checked:

  • Size of cavity (length, diameter of narrow and large end) – cavity matches simulation
  • Size of probes (28mm, optimal size) – matches
  • Skin depth for microwaves – the electroplating thickness is more then enough (12.7 micrometers versus a skin depth of 2 micrometers)
  • Orientation of the probe –
    • In the simulation, I rotated the power probe by 5, 10, 15 and 25 degrees to see if it affected the resonant frequency, Q or insertion loss – it does not.  Below are the four examples which show the TE0,1 mode from 5 to 25 degrees (hold your mouse over to see the file name):
  • Tuning Plate at large end – I also ran a series of simulations with the tuning plate at the large end, but it didn’t lead to any conclusive results.  It did however, result in an interesting resonance in the center of the cavity:
  • Rebuilt the cavity model from scratch with better probes and a finer mesh for important parts.
    • Part of the problem with the old cavity model was that I had imported the probe from a previous model and the probe parameters (like diameter, height, etc.) could not be independently controlled (the power probe was a mirror of the measurement probe).  I also wanted to rebuild the model from scratch in the CST 2010 environment to double check all the variables and use any updated static data, for example, the conductivity of copper is now more accurate in CST 2010 because it differentiates between annealed and hard-drawn. (we use the later)
    • The two pictures below are good examples of the differences between v1 and v2 of the model:
      • The diagram below is from the first generation model which shows the sharp edges where the the probe loop connects and has a much rougher meshing.
      • The second diagram below is from v2.0 of the cavity model and has a finer mesh and probes that look much closer to the actual probe shape. The model still only takes about 350K+ tetrahedrals, but the meshing is now finer around important sections.

One thing I plan to check with the new version two is how the orientation and flatness of the tuning plate affects results.  My end goal is to see if I can duplicate what I see on the bench with a simulation. If I can, then I should be able to fix the problem.

I am currently running a simulation where I check to see how sensitive the TE0,1 mode is to the position of the probes from large end of the cavity.   That is one thing I haven’t check yet and from my preliminary results, it may be the cause of the problem.

In order to run higher accuracy simulations with more tetrahedrals (>500K), I upgraded my machine to 16GB of memory.  The CPU is also slightly faster now at 3.7Ghz instead of 3.2Ghz (stock) and the memory also runs at 1333Mhz versus 800Mhz ($350 in upgrades).  The upgrade also allows me to drop in the “Bulldozer” eight core AMD processor when it comes out in the fall.

No Great Resonances to be Found

Summary:

Resonances with quality factors of 1000 are common, the cavity is the right size and all that can be updated/fixed are the probes and tuning plate.

I verified that the cavity dimensions match the simulations and then spent another good couple of hours playing with the cavity yesterday.  Here is what I learned (pictures taken with my iPhone):

  • Resonances with insertion losses of under 1dB and with Q’s higher then 1000 are common and easy to find.  Here are pictures of two of them:
    • This resonance is the “Best Hope” one which I have shown before and is in the unknown mode (insertion loss 0.5, 1261Q, frequency of 2.44GHz):
    • The resonance below was chosen at random and shows an uncalibrated result, which means the insertion loss measures is really about 0.7dB and the Q about 1000 (f of 2.47GHz).  Again, some unknown mode.

I also wanted to see what a high Q resonance looked like and found one:

A few things to note:

  • The bandwidth (shown as “BW” above) is only 160KHz which would explain the Q of 16000
  • The loss, even after subtracting the amount calibration would adjust for, is 3dB!
    • I think I remember being told once that having a high insertion loss would help Q and I can reproduce that here if I tune the insertion loss even higher,  I can get a Q of 32000.
  • Note how sharp the spike is compared to the first “Best Hope” graph above, both of which have about the same width of 5MHz

The last diagram is what the TE0,1 mode should look like but with a much better insertion loss.

I have brought the cavity home to work on some more.  The first thing I am going to do is bring the probes closer to the simulations by making them rounder and more horizontal.  The probes are twisted slightly which may be the reason for the insertion loss.  I will then fix the tuning plate by putting some copper mesh around it’s outside edges to connect it properly to the cavity.  I will also make sure it is as flat as possible and doesn’t twist when moved in and out.  Kevin pointed out that it may be necessary to investigate another way of tuning and perhaps a movable plate from the large end??  I don’t think it should make any difference, but I will run that through a simulation to see and I will also think about other ramifications, for example, won’t a larger plate flex even more?