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 [ – 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” ( 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


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.)


I Think I Know What To Fix!


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:


  • 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) & (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.