Magnetron Testing

As usual, I underestimated how much it would cost to get the cavity machined and I have three quotes back now, for $1600US, $1900US and $2150CND, all of which includes the aluminum.  None of these include copper plating which will run me another $300 or so and ironically, I haven’t gotten a quote back from Ryerson for whom I went to all the trouble of getting a 2D drawing made up.  Just for fun, I also got a quote through for a gold plated cavity and it came out to  be $6K, not bad!  I have been impressed with both their software which includes a mini-quoting system with many types of materials and finishes.  The design was easy to import because, first a 2D wire frame profile was loaded from a DWG file and then a revolve tool was used to make the cavity.  All told, I had my drawing imported and the quote generated in less than an hour, very cool.  I will probably end up going with them because their price was the most reasonable and I can include 0.005″ of copper plating too.  I have to confirm the quote with them yet though.

I was also up at the university for a day this week testing the magnetron output and in it’s current form, it is unusable.

First, here is the initial test setup:

We tested to see what the attenuation was on the coupling port on the circulator and it turns out to be -62.6dBm.  If the input power is 1000W or 60dBm, then the remaining power at the coupling port is roughly 0.6mW (-2.6dBm where -3 dBm = 0.5 mW)   We then hooked up the magnetron like this:

And attached the coupling port to another 70dB attenuator for safety and then hooked the output into a power meter which read between -58dBm and -52dBm depending on how long we left the microwave on.

We then hook the output up to the spectrum analyzer and got this (The picture should be an animated GIF):

The GIF contains about fifteen pictures taken over a thirty second test and here is a quick analysis:

  • The signal is intermittent – this is likely because the power supply is a voltage doubler which only has a %50 duty cycle, which are cheap to make but not useful for our tests.
  • The primary signal starts at roughly 2.466Ghz and moves downward – The cause is pretty simple, the magnetron is heating up, going from 33c to about 90c, most likely because of the short and reflected power. (Err forgot my thermal imaging camera at home, oops).  The center frequency is moving with temperature.

What we would have like to have seen was this:

A clean sharp spike at 2.45Ghz (The above was created by a frequency generator, and would be perfect for our needs, but look at the power!  mW!  Scotty, we must have more power!)
The magnetron still may be useful, but we will need a water cooled one to keep the frequency stable and a continuous wave power supply.  I will start searching eBay and other sources next week.  Meanwhile I will confirm the quote with and see if I can get the process started.

I also have upcoming programming work with TOF in October which will probably consume considerable time, but generate funds to finance this endeavor.