S Parameter Plot from Simulations

The S parameter plot for the two loop probes is attached and the match is not as great as it would seem.

I spent most of today pricing out vacuum pumps and looking at vacuum enclosure designs. I have attached some of the designs I have been looking at and the round one is where I am starting. If I can get some free welding help from friends, I might try for the square one. Whatever the design, it is going to have a clear cast acrylic window to see inside.

Do you know of anyone in the university that has a large vacuum chamber? It would have to fit the 15″ (390mm) long by 11″ (280mm) diameter cavity and include a N to N-type connector in wall for the input.


Ol’Sparky & Wood Lathe

The spending spree continues:

I bought a grinder to sharpen the lathe tools and clean up edges of the copper after plasma cutting, among other things:

Besides “ol’ sparky”, I also bought “outside” calipers to measure the diameter of the wood and a face shield, both for working on the lathe.

I spent considerable time yesterday and this morning at the customs office near the International airport, getting the vacuum chamber through.  After a ten hours cumulative including driving time, waiting in line, and fixing my mistakes, the chamber should have cleared inspection today!  I can’t wait to see it at the university and see how it survived the trip and custom’s inspections.  I have some anxiety that customs in their desire to see inside the thing, will have dropped it or worse.  The good news is that I used the wrong tariff region code the first time through and I should get the $121 in duties refunded!  With much instruction from a helpful customs officer (Alan), I have also learned exactly how to do the paper work for the upcoming shipment of vacuum parts (which, did you know has an import code for exactly that – 9617.00.00.20, parts for vacuum chambers)  The vacuum parts got shipped on Monday and should be here by next week if things work as usual.

I borrowed the chain saw from a friend yesterday, but after making a few cuts, it was clear the blades were too dull.  I have taken the blades in to get sharpened which should be done by Friday.  I am busy all weekend starting early Friday afternoon with Salsa related duties, but should have wood on the lathe by Monday afternoon.

I finished roughing in my first practice log on the lathe today!

It gets hot in the garage on 30 degree days!

Wood Lathe & Vacuum Chamber Parts

This past week, I have been on a spending spree:

I bought the wood lathe for $325CND and it has now taken up refuge in my garage, along with a new shop vac to clean up the prolific shavings generated when turning:  I will have to get a video of the lathe in action because it really makes a great mess in short order.

I also contacted a local log house builder and got some large wood blanks (shown below) for free.  On Tuesday next week I will be going to a friend’s place to borrow his chain saw to cut the blanks to fit on the lathe.  The one with the bark still on is probably my best bet for the mold, given it has the least number of cracks.

I also purchased plastic sheeting for the model to make sure my idea for using the wood forming mold will work.  Ironically, the plastic sheets (shown below) cost me $34, about what it would have cost me for the copper sheeting.  One of the plastic sheets is transparent though and I can them make sure the tuning plate inside is working correctly.

I also purchased a whole pallet of vacuum parts ($250US eBay) and they will start their journey to the university on Monday ($248US).


I figured I am going to need vacuum ports, windows, fasteners and the like and instead of paying for everything in piece meal fashion, I would go with a bit lot.  I will probably resell some of the parts onto eBay to recoup my cost.

I also have a bid in on an 8″ flange which gives you an idea of how much single ports can go for, even second hand.

Updating Wiki & Wood Lathe Update

I spent half the day updating the wiki including a new page on the vacuum chamber with sections on the setup and acrylic window.

The rest of the time I spent learning about wood lathes and getting quotes.  The wood worker I talked with today said that 11″ in diameter and 15″ long wood is hard to come by because the wood needs to dry for up to five years. The other option was that he could glue two pieces together and he suggested a price of $500.  I can buy a second hand wood lathe complete with chisels for $325 (Removed link to Edmonton Kijiji)!  I then called a log home manufacturer based near St. Albert and from the sounds of it, I can get scrap ends from logs that have been sitting around for years, made of Alberta Spruce for nothing and large diameter ones too.  Part of the problem is that the lathe I have in mind only has a clearance of 12″ which will work for the 11″ diameter cavity, but not for a 14″ standard sized log.  Anyway, I’m sure I can figure it out.

JB Vacuum Pump & Wood Lathe

I took delivery of the JB vacuum pump today and it looks to be in decent condition, although I have to get oil and hoses for it yet.  I haven’t heard if the vacuum chamber got shipped but it should have left this afternoon.  I am also starting to bid on CF flanges for the chamber and a nice one popped up today.

I contacted a wood lathe shop about getting a wood mold that looks like the pictures below.  The idea is to wrap a single precut section of copper around the mold, strap it in place and then braze the seam closed.  The benefit of using a mold is that one complete piece of copper can be used, it will eliminate the circular seams and I should be able to get it within the 1mm tolerances required.  The diagram below was generated from a 3D CAD drawing which will also be used to make sure the mounting for the tuning mechanism works.

Acrylic Stress and Prices

Everything is paid for and ready for the vacuum chamber to start its journey up here on Monday next week with an ETA of October 8th.

I spent today talking to acrylic vendors about pricing for a large acrylic sheet to cover the 25″ dia. opening at one end the of vacuum chamber and got prices listed below:

The sheets come 48″ wide, and for a 28″ length of, it would cost:

  • 2″ thick – $1200
  • 1.5″ thick – $900
  • 1″ thick – $265

Clearly the 1″ Acrylic sheet is the best price, but the question is, does it have enough strength to hold back the air pressure vacuum over a diameter of 25″?

I found a handy power point slide (attached and zipped), which shows how to calculate the safety factor for a given strength versus diameter for a vacuum chamber window.  It turns out that for an acrylic window with 25″ diameter of unsupported space and 1″ thick, it has a safety margin of 6.8x.  In other words, the window can handle over six times more air pressure before breaking.  For a 1.5″ thick window, it has a safety factor of 15 times and for 2″ thick piece, 27 times.

Here are the calculations I carried out:

First, calculate the stress on the window for a known thickness versus radius (from the power point slide):

Sm=k(wR^2/t^2) where
Sm – the stress on the window in PSI
k – is the coefficient for circular plate, I used 1.1 which is a conservative estimate.  This constant is described in the power point presentation.
w – the uniform pressure across the window, which because Edmonton is ~2300ft above sea level, means our air pressure is 13.66psi.
R – is the radius of the unsupported part of the window, in our case 12.5″ or half of the 25″ diameter (this was reported by the vendor)
t – is the thickness of the window which in our case is 1″, 1.5″ or 2″

The stress for a 1″ thick acrylic window is 2300psi, for 1.5″ thick window it is 1043psi and for 2″, 587psi.

The safety factor of cast acrylic is then calculated by:

S.F. = Modulus of Rupture / Max Stress

where the Modulus of Rupture for Acrylic is typically 16,000psi.

Clearly the 1″ thick acrylic is thick enough to handle the air pressure.  (Please check my assumptions and calculations!)  I don’t plan to buy the acrylic until I can measure the inside dimensions of the vacuum chamber.  The good news is that it cutting the acrylic with a water jet shouldn’t cost me more then $100 and I can use the extra acrylic to cut 8″ windows for the ports if necessary.

My next step is to model the cavity with a 3D modeling program and then build a plastic replica before cutting any copper.  The replica is not really important, but the process by which the tolerances are assured is and may include jigs if necessary.

I also plan to add the following to the wiki:

There is a method used to make sure the cavity is resonating in the right mode, TE0,1 at the right frequency, 2.45Ghz.  Typically when the cavity is out of tune, it means that the TE0,1,n mode is resonating at a frequency other then the desired 2.45Ghz.  The problem is that there are a lot of resonant frequencies, as shown by each dip in figure XX (to be attached) and it is first necessary to determine which one is the TE0,1 mode before moving the tuning plate. The method is simple.  First, with an S-parameter plot, locate the frequency of the nearby dips and add a “field monitor” in the simulator, then rerun the simulator which will calculate fields for those frequencies.  After looking at the field configuration and determining which one is the TE0,1,n mode, then determine if it higher or lower then the desired 2.45Ghz.  If it is higher, then make the chamber longer, if it is lower then make the chamber shorter (check this!).  It make take a few iterations before the appropriate mode is resonating at 2.45Ghz.

On the lab bench, it will be critical to setup the cavity tuning as closely as possible to the simulated cavity, and then move it just enough to bring the resonance into focus.  The problem with a real cavity is that there is no method to tell which mode the cavity is resonating in.  It might be possible to use the expected S1,1 value… (hmm)

Vacuum Chamber Bought and MythBusters

I am now the proud owner of this:


Inside Dimensions – 30″ high, 25″ in diameter! I paid $1660US and more details are here!  It is located in Santa Fe, NM and I am getting shipping quotes now.

It can handle high vacuum too(!), i.e. 10-4 Torr or lower, although that requires two pumps, one called a “rough” in pump, one of which I have, to get it to 10-3 Torr and then a “turbo” or “oil diffusion” pump to get it to hard vacuum.  It helps that it has a water jacket that can be used to heat the chamber which makes getting it to hard vacuum easier.  When the molecules hit the heated chamber wall, they rebound with more energy and find their way out of the chamber faster.

It is made by a well know vacuum chamber manufacturer too, MDC who sells all sorts of flanges and adapters.

It is very similar to the one used by Mythbusters for their “antigravity” experiments!  Perfect!

3D HDPE Vacuum Chamber

The vacuum chamber on eBay sold for $491, not much over my max bid of $407, which is an excellent price considering buying it brand new would have cost at least $3KUS not including the $500US pump nor the $150US chrome wire cart.  Too bad the chamber was too small.

I spent the weekend designing a larger vacuum chamber (Many thanks to Abbess!) and here is what I came up with:

The one on the left is the 30″ O.D. (27″ I.D.) chamber and the one on the right is the 24″ O.D. (21″ I.D.) chamber.  Each chamber holds a mock-up of the copper cavity and includes a 3″ square section at the cavity’s narrow end we will need for tuning.  Unfortunately, the 24″ chamber is too small as the tuning part at the end of chamber gets cut off and it still doesn’t include enough space for a scale.

I could rotate the chamber and cavity so it sits upright:

The pipe is made out of HDPE (High Density Polyethylene) which, depending on  wall thickness, can handle up to 267psi.  The door is a 2″ thick clear cast-acrylic sheet which should be sufficiently thick because Abbess uses a similar width for a 36″ wide chamber:

The back and front supports shown in green are 2″ thick HDPE plate and the front support has an O-ring embedded in it (shown overly large) and will be clamped like the picture above.

The walls of the pipe are about 1.5″ thick which conforms to a common “DR 17″ type of HDPE pipe which can handle 100psi, well in excess of the 15psi necessary for a vacuum under full atmospheric pressure at sea level.  The chambers are three feet long which, for just the pipe, would make the 30″ O.D. weigh (68lbs/ft) 204lbs and the 24″ O.D. weigh (43lbs/ft) 129lbs.   This doesn’t include the pump (35lbs), the two plates of HDPE nor the acrylic door, which will likely put the 30” chamber over 300lbs.

The costs are still being figured out, but they will be roughly:

  • 30″ diameter, 1.7″ wall thickness, HDPE pipe – $500
    • 24″ diameter with a thicker wall (DR 11), quoted at $390CND for 3ft
  • 30″ square 2″ thick HDPE plate (x2) – $300 each – $600 (hopefully cut)
  • 2″ thick clear cell-cast acrylic – $600 to $800
  • 29″ diameter O-ring – $40 – The “Viton” o-rings are apparently the best for Vacuum chambers and I am probably going to need three of these, one between the back support and the pipe, the front support and the pipe and the front support and the door.
  • Hinges and fasteners – $50
  • 30″ wide cart that can handle 400lbs (these are actually quite common with chrome wire carts that can handle 800lbs) – $300
  • 3/8″ threaded pipe to hook up to the pump – $15.
  • Port and N-type parts to get the microwave signal into the cavity – ???
  • (PURCHASED) 7CFM Vacuum pump – $400

Total  for 30″ (not including the pump): $2300CND (based on estimates)
For 24″ (not including the pump): $1530CND (based on quotes)

This doesn’t include fabrication costs, most of which I will do myself, although getting the HDPE heat welded will probably add more.  I have also been thinking I can bolt everything together with O-rings, but I want to talk to the plastics supplier first.

Breakdown Voltage Test

Yesterday, Kevin and I tested the model to make sure that the V/m did not exceed the breakdown voltage of air and the highest V/m measured was 115,000 which means there should be no arcing.

I also increased the number of tetrahedrals to the maximum allowed by the 8GB of memory to slightly more then 600K. Because the cavity is large, 15″ long and 11″ in diameter and is made up of round surfaces including the probes, it requires a large number of tetrahedrals. Unfortunately, it changed the results enough that the length needed to be modified in order for resonance to be achieved.

Because we are bumping into the maximum memory limit, I looked up the cost for adding another 2GB of memory for a total of 10GB. The best price I could find for a single 4GB memory stick is $187CND which would replace one of the four 2GB sticks already in the machine. The Kingston memory referenced is also vendor certified to work with the motherboard which is something we have had problems with before. Because our simulations are working and we can reduce the number of tetrahedrals if necessary, I have held off purchasing the extra memory.

I also got an acceptable quote for shipping the vacuum chamber from Orlando, Florida to Edmonton, Alberta for $220US from a UofA recommended carrier (Reimer/Roadway/YRC). For customs, because the cargo is freight and of a commercial nature, most brokerages require an account, which costs upwards of $200 just to create because of the paper work. I am going to attempt to do “self-clearance” by getting the paper work done myself.

As for the vacuum chamber, even though the price is only $57US when checked last, I have decided it is worth bidding $900 to win the cavity. I have done extensive searches online and through various databases, but for the size, nothing comes as close in price. Most cavities over 24″ square are $4K US or more. I still have not heard back from the seller about the inside dimensions though.

I won the vacuum pump for $222US (eBay link removed), which is about half of what it would have cost new ($390 – http://www.climatedoctors.com/items/item.aspx?itemid=928826 ). Shipping was $60 via UPS and I expect to pay $40 for customs and brokerage charges, plus I will replace the oil in the pump ( http://www.jbind.com/faq.asp – JB is the brand of pump )

(While the simulations were running, I also helped Gary with fabricating his antenna. FYI – The link to the brazing propane torch is here: removed)

Shippng Quotes & Simulations

Here is what was accomplished today:

  1. Worked a number of hours contacting shipping companies to get quotes on shipping the vacuum chamber from Florida. The vacuum chamber I hope to win is here: (eBay link removed)
    Unfortunately, I’m pretty sure that if I do bid the $600US I plan to (upon verification the inside dimensions are over 18″ square), that I will be outbid. Because it is an “altitude test” chamber, it likely has cooling or heating elements inside making it worth more then I am willing to pay.
  2. Kevin and I found a tuning mechanism among the spare parts in the lab, that will work for the tuning plate at the back of the cavity. It is sitting beside my monitor now.
  3. I ran four simulations and varied the tuning plate at the back of the cavity by 0.25mm increments and resonance is unfortunately very sensitive. Getting this cavity to resonated properly is going to be difficult.
  4. Kevin and I also talked over how to mount the cavity inside the vacuum chamber such that it would have enough freedom to move given the expected force is in milli-newtons.
  5. I also did a search of all the instruments and chambers at the UofA through this interface – http://www.inventory.ualberta.ca/ and found the university has a lot of “vacuum ovens”, however they are too small. I will search again tomorrow for a proper vacuum chamber we can use. The University of Waterloo has one, lol – http://www.mhtl.uwaterloo.ca/L3vacuum.html