Machined Cavity and Higher Frequency?

This week has been one of learning because I have been trying to solve two problems: The cavity is not within tolerances necessary to get the high Q required.  As we get a higher and higher Q cavity, it creates another problem – the frequency of that high-Q resonance is very exact and even a 6Mhz variation of the source frequency will take the cavity out of resonance, something that could happen just because the magnetron warms up.  The commercial magnetrons we are working with typically have a 20 to 30Mhz bandwidth, which means it could be at 2.42Ghz or 2.48Ghz!

The good news is that I might have solutions for both problems, a higher Q cavity and a frequency controlled magnetron.

I will deal with the higher Q cavity first and there are two options, both of which require a redesigned cavity.

I started with going to a higher frequency because everything shrinks in size substantially which means costs drop.  A good example is the current 2.45Ghz isolator which cost $250 second hand (which is thousands less then new) and is about the size of a toaster.  A circulator for 5.8Ghz cost $40 on eBay and is the size of a tape measure.  The biggest benefit of going to 5.8Ghz is that I can purchase a 6″ diameter, 8″ long solid rod of aluminum for only $150 and sourced locally!!  That leaves lots of money to have the aluminum machined and then copper plated.  Perfect!  With a computer controlled milling machine, the cavity can be produced almost directly from a file exported from the EM simulation program!

This past Sunday afternoon, Lin let me into the lab (I have to pick up a new key!) and I took the previous cavity designed for 2.45Ghz and rebuilt it for 5.8Ghz.  This was easy – I took the 2.45Ghz cavity and reduced it by 53%, which I figured out by finding the percentage difference between the diagonal dimension of WR-340 wave guide, built for 2.45Ghz and WR-159, built for 5.8Ghz.  I then redid the simulations and as expected, making the cavity smaller raises the frequency of the TE0,1 mode.

Because the large end of the 5.8Ghz cavity was under 6″ in diameter, I spent a few days converting the exported drawings from the simulation program into something that could be made on a CNC mill (ignore the probe bump inside and out):

Getting the cavity made is easy, however, the problem is finding a source for a high power 5.8Ghz signal.  After not getting a response back on a quote for a 5.8Ghz source,  today I phoned John Gerling, of Gerling Applied Engineering, a small California company and learned a lot of things – for example, back in 2001, Panasonic use to make 5.8Ghz magnetrons for a cost of $1.5K each (wooah) but did not find a market and stopped making them.  The 5.8Ghz magnetron is identical to the 2.45Ghz magnetron and takes the same inputs.  However, my hope for finding a high power (there’s the kicker) 5.8Ghz source for a couple thousand dollars was off by an order of magnitude.

John did tell me about TWT or traveling wave tubes and I found a TWT complete with power supply on eBay (link removed) that can vary the power up to 3.3KW, runs at 6Ghz and is only $2500!!  The problem is the sheer size – it is 1200lbs which means shipping alone would be another $2K and then it might not even work.

I came to a realization – by moving away from 2.45Ghz I can build a more accurate, smaller and cheaper cavity, but the source then becomes prohibitively expensive.

The attempts at a 5.8Ghz cavity may have lead to a solution anyway – To machine the 2.45Ghz cavity from a single round metal rod requires one 12″ in diameter and 16″ long.  Nobody I can find makes aluminum rods that large, but I can buy a type of round 4140 alloy steel that is 12″ in diameter and 16″ long for $830 from a local supplier!! (weighing in at 514lbs!!)  I will confirm availability and price on Monday, but first I will see if I can find anybody that is even willing to machine that monster.  It will take a fork lift just to get it into the mill.  If both of those pan out, then I will check to see if it can be electroplated with copper.

On to the second problem which is locking the magnetron to a certain frequency:  As I was talking to John, I mentioned the “Magnetron – A Low Noise, Long Life Amplifier” paper (linked to previously) and it turns out that you can buy magnetrons that use electromagnets instead of permanent magnets and he gave me the part number.  The magnetrons retail for $3K but one recently sold one eBay for $350 (link removed)!  What I am looking for is this:

Note the coils?

It can output 6KW, is water cooled and has electromagnets which can be used to control the frequency!  I am now tracking down what type of power source and controller is needed and then hopefully can find some on eBay.  Given our circulator is already water cooled, a water cooled magnetron is even better because it removes the fan.  John also mentioned that reflected power can change a magnetron’s frequency in a “magnetron pulling” method, but the good news about our setup is that we are using an isolator which means that the reflected power the magnetron see is nearly zero and won’t affect the tuning.  Oh, did I mention the isolator we have can handle 6KW?!

Until next week!