Cavity Finally Finished!

This week, I finished up all the last little bits including the tuning plate (shot taken with my iPhone 3GS):

And I bolted on the back plate (taken with my Vixia HF100 HD video camera):

The back plate looks scratched because I haven’t removed the plastic covering yet.  The seams are pretty good around the bottom although I will end up using conductive copper tape in places to make sure it is completely closed (which may not work because of the high power used)

I did a lot of sanding by hand this week, mostly over my neighbour’s rain barrel as shown in a still from the documentary footage:

The 600 grit sand paper works best when wet and because the inside is round, I did all the final sanding by hand, first with 280 grit then 600.  The final sanding was necessary to to remove oxidation from finger prints and flux.

I will be up at the university tomorrow (Friday) to build and solder the probes on because I need the measurements from the simulation.  Then we will subject the cavity to the first tests with the low power  network analyzer to see how well it resonates, if at the right frequency and how sensitive Q (the quality factor) is to tuning.  Like most new things, it will probably work, just not as expected, :-] .

Cavity Almost Finished!

I got the probes mounted over the past few days and below are pictures of the event:

The picture above shows the small measurement probe with the copper sheeting.  I test mounted the probes on the extra pieces that Motive sent to make sure everything went as expected.  I then mounted both probes:

The probe hole turned out a bit rough and next time, I will use a dremel tool to enlarge the hold instead of a drill bit which left rough edges:

I tried cleaning it up as best I could, but I suspect if there are problems with resonance or more likely arcing, I will have to redo the entire thing. The good news is that the plastic slides off easy enough and I have enough extra copper for another cavity.  I also have the experience of making this one up which will help.

Ideally the probe holes should look like this:

Here is a shot of the almost completed cavity:

I used the shorter top piece in order to make sure the copper stuck above it.  The top piece is only anchored by friction at the moment and needs to be attached to the bottom piece.  The bare copper is useful because we can use an IR thermometer to see if the cavity is heating.

Copper Lining Fitted & Vacuum Parts

After a many month hiatus, I’m back!! The programming job is finished and I am back on Levitee research full time (besides a bit of TOF work).

After a good many hours hand sanding and polishing the inside of the cavity, I ended up with this:

The extra solder at the top of the narrow end is because I was unfortunately a bit overzealous with the high powered sander and had to refill the seams.  I did, however, use a solder that was %50 silver which has better conductivity then regular tin solder.  I then sanded everything down ending with a wet 600 grit sand paper to get all the surfaces as smooth as possible and to take off any copper oxide and/or flux that had built up.

I then mated the copper and plastic shell:


(The end pieces still have the plastic covering which I will remove just before testing and the circular copper tuning plate still has to be soldered to the square piece. )

The plastic shell has been an excellent idea because it has proved invaluable for numerous reasons:

  • The plastic shell is computer generated right from design to fabrication and it is as near to perfection as economically possible
  • It is an excellent tool to measure the copper before soldering it.
  • Once soldered and worked, the copper takes up an unsymmetrical shape but the plastic shell forces it back.

The copper sticks out the bottom of the plastic shell more then enough, but I am having difficulty getting it to stick above the top, where it needs to touch the top plate for grounding purposes.  It is only a few millimetres too low  The problem is that the irregularities of the bottom means I can’t slide the shell down far enough.  In the future, I would cut the copper at the narrow end another couple of centimetres higher in order to make fitting it back into the plastic shell easier.  More height at the top would also mean more travel for the tuning plate if necessary.  I will probably have to put a bead of solder around the edge of the top plate in order to make sure it touches the copper inside the cavity.

I have also moved back out into the garage and here is a shot from the footage as I was demonstrating something:

That’s a crokinole board face down on the wet cutting table!

I was also up at the university last week working on the vacuum chamber and I think I have the last bits figured out:

  • I can get the blank 1.33″ seals I need for $17 each.
  • To get the microwaves into the cavity I use one of these:

    Which can handle 5KV and has an N-type connector on one side.  I then have to figure out how to connect it to the microwave cavity inside the vacuum chamber.. Hmmm.. I might need a type with N connectors on both sides…  I am also not sure the flange size is right, but it was only $40 to order it (used) and I will know in a week or so.  The flange should fit because according to this MDC page, the Del-Seal CF metal seal flanges, which is what I have, are compatible with Varian.
  • Then to connect the roughing vacuum pump, I use one of these:

I also noticed that two vacuum chambers about the same size as the one I bought went through eBay for a paltry couple hundred each!  One here and one here (eBay links removed).  I paid $2600 for mine, although mine is aluminum, has a turning mechanism that might prove useful and struts on the side to mount it.