[SOLVED] microwave wave guide terminations

I need to pick a proper termination for a 2.45 Ghz 1000 watt magnetron.
I will be passing microwaves through an oil shale sample. at the other end will be a waveguide termination.
The silly question is, what wattage do I want to absorb at the waveguide termination?

The simple answer would be - all of it. You do not want any power to be reflected thus causing standing waves as this make affect whatever it is that you are trying to do with the oil shale.

Inti Innovations said:

The simple answer would be - all of it. You do not want any power to be reflected thus causing standing waves as this make affect whatever it is that you are trying to do with the oil shale.

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Thanks for your speedy reply. I was thinking of doing just that, but I didn't want to guess. I didn't want to burn out my magnetron. Thanks for setting me straight.

kruger2013 said:

Thanks for your speedy reply. I was thinking of doing just that, but I didn't want to guess. I didn't want to burn out my magnetron. Thanks for setting me straight.

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No probs.
Out of curiosity, are you just trying to melt oil shale or is it something more complicated?

Inti Innovations said:

No probs.
Out of curiosity, are you just trying to melt oil shale or is it something more complicated?

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I want to extract the kerogen out of the shale. I have a patent pending on an antenna I've developed.
I want to measure the speed of the extraction. I am conducting an experiment from 1983 titled Microwave Heating of New Brunswick Oil Shale.
I'm hoping it can be combined with a microwave extraction process developed by Raytheon.

Sounds interesting!
Good luck with the experiment.

Thank you.
I appreciate your help.

the best thing would be to use a "water load". i.e. water is pumped thru a small cylindrical load, and the microwaves heat up the water. Anything else will, of course, require a really big cooling fan!!!

Good idea.
A pump would probably be a cheaper way to go.

I would feed the wave guide via a circulator, as your sample is certain to cause a reflection which could damage the tube.
Frank

Interesting.
Could you elaborate on feeding a wave guide with a circulator?
Does this device cool the tube?

A circulator is a fairly wideband device with three ports, when RF is applied to port 1, the RF "circulates" on to port two where it appears for use. Any reflection from port 2 "circulates" to port 3 which is terminated with a load. So no reflected RF "circulates", back to port 1 and upsets the RF source. it could be that your RF source has one fitted internally, so check its spec and see how it reacts to a badly matched load. It could be that it monitors its RF output and reduces its output (to protect the magnetron), if its operated in to a bad miss match. In which case an external one will increase the power available to piece of rock.
Frank

Having thought about your problem, introducing anything into a wave guide will cause a reflection, so a matching section could match the new impedance to the new wave guide impedance. This might need retuning as the process proceeds. I see you are using 2.54 GHZ, is this a domestic microwave magnetron? Which leads me to my next point, Domestic microwave kit seems to be VSWR proof, but not RMS power proof. what I mean is that say the magnetron is running at 1KW power input and 800 W out. In the absence of food or water, the whole 1KW would be dissipated in the magnetron, hence the instruction that with certain types of food, a cup of water is placed in the oven, so this dissipates the 1KW until the food defrosts and absorbs energy in its own right. So keeping the magnetrons anode dissipation down. I would start with a domestic micro wave oven, I think they have cracked your problem!
Frank

It is worth having a look at the actual paper describing the experiment.
View attachment JMPEE-Vol18-Pg37-Butts.pdf

The setup can be seen in the Fig1 and consists of a microwave source (i.e. magnetron) followed by a directional coupler to measure forward and reflected power a waveguide with a sample in it and the termination.

The sample is in a quartz tube in the precise location in the waveguide. To be able to calculate absorbed power you need to be able to measure the power absorbed in the termination.

The amount of reflected power will be fairly small due to a size and location of the sample - the paper mentions 90% efficiency of microwave energy absorption.

In no way can this be substituted by a microwave oven. Also using water as an energy absorber may be simple and effective but could prove difficult when trying to establish the amount of energy absorbed.

The last thing is the energy source - it is assumed it will yield at least 85% energy efficiency - this is way above the performance of a magnetron, I'm not familiar with a device capable of delivering powers as low as 100W and this efficiency...

by "water load" i did not imply you try to build one. You should buy one. They look like this:
https://www.alibaba.com/product-gs/200199070/2_45GHz_Short_Dummy_Load_with.html

VSWR will be better than 1.2:1, so no need for a circulator (which would be much more expensive and also would require water cooling).

Thanks for the paper, very interesting. the principle has been established. So the thing to do is to increment the power (100W -> 1KW) and build a model of a production plant that runs for some prolonged period - 1 week? and finding out where the real problems will be. Problems could be getting the residue (sand) to be recovered without any of the hydrocarbons. Catching the gas and oil. using the hot sand, gas and oil to preheat the incoming shale to increase the thermal efficiency of the process. The time period taken for the process is interesting. Bigger lumps of shale = longer time in microwave oven, or grind it up into fine bits = takes a shorter time (and easier feed mechanism?) but costs money in complexity.

2.54 GHZ is the resonance for water, is it a co-incidence that its the same for oil, or was the availability of suitable cheap magnetrons ?
Frank

Inti Innovations said:

It is worth having a look at the actual paper describing the experiment.
View attachment 86963

The setup can be seen in the Fig1 and consists of a microwave source (i.e. magnetron) followed by a directional coupler to measure forward and reflected power a waveguide with a sample in it and the termination.

The sample is in a quartz tube in the precise location in the waveguide. To be able to calculate absorbed power you need to be able to measure the power absorbed in the termination.

The amount of reflected power will be fairly small due to a size and location of the sample - the paper mentions 90% efficiency of microwave energy absorption.

In no way can this be substituted by a microwave oven. Also using water as an energy absorber may be simple and effective but could prove difficult when trying to establish the amount of energy absorbed.

The last thing is the energy source - it is assumed it will yield at least 85% energy efficiency - this is way above the performance of a magnetron, I'm not familiar with a device capable of delivering powers as low as 100W and this efficiency...

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Panasonic has an inverted power supply. It delivers 100% of the wattage claimed.
I'am using a conventional microwave oven. Modified of
course, my first experiment burst into flames, i knew i needed a termination, just in a hurry.
My patent should be concluded in march. My antennas drive electrons. It is a combination of Einstein's photoelectric effect and thermal run away.

chuckey said:

Thanks for the paper, very interesting. the principle has been established. So the thing to do is to increment the power (100W -> 1KW) and build a model of a production plant that runs for some prolonged period - 1 week? and finding out where the real problems will be. Problems could be getting the residue (sand) to be recovered without any of the hydrocarbons. Catching the gas and oil. using the hot sand, gas and oil to preheat the incoming shale to increase the thermal efficiency of the process. The time period taken for the process is interesting. Bigger lumps of shale = longer time in microwave oven, or grind it up into fine bits = takes a shorter time (and easier feed mechanism?) but costs money in complexity.

2.54 GHZ is the resonance for water, is it a co-incidence that its the same for oil, or was the availability of suitable cheap magnetrons ?
Frank

Click to expand...

My samples are crushed. I used a pole driver in combination with a 6ft chisel. One end of the chisel has a large 3 inch diameter nail head. Hard work but cost less than a stone crusher.
I'm trying a glass retort. I am concerned with using Pyrex. I don't want to automate the process too much right away. I have never tested my antennas with a solid before.
My antennas create a hot spot. They are basically a dipole antenna on top of a voltaic pile.

Raytheon uses radiators with their insitu microwave process. I'm trying to find out if my antenna design would be a better match. They sold their technology to Schlumberger last year.

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My fault, I misunderstood the part about microwaves absorbed by the sample. Panasonic inverted power supply claims to deliver a certain wattage all the time.
I want to pass microwave energy through a sample with my antenna present inside the sample. Do you think this could be done with a conventional microwave oven?

" my first experiment burst into flames". Thats a shame, Was the source, the PSU ?(which should be fully protected) or the RF side?. I still don't understand what you are actually doing. You have a new sort of aerial, are you driving it from a microwave magnetron?, coupling it from a hole in the side of a microwave?, or you have the "guts" of a microwave, driving a wave guide in which sample is living?
Going back to what I have said, the PSU should be able to handle, say a kilowatt, The microwave/magnetron, 800W but that power must be absorbed or it will get reflected back to the magnetron, which is cooled for 200W, which then self destructs. Now your shale I would take as a poor insulator, but it might have some lossy dielectric in it. So for it to absorb enough power in a microwave (to keep the magnetron happy), it must have a large surface area. I was thinking of a buffer load in a microwave, i.e. a posh version of the "cup of water". If you just load the cavity with a resistive agent - blocks of carbon, they will stabilize the magnetron, but will get red hot and catch fire (again!). So one idea is to drill two small holes in the case and run in a long coil of plastic tubing and pour/pump water through it. This will stabilize the magnetron and the heat will be carried away in the hot water. Also by measuring the temperature of the outlet, you can see how much heat is being absorbed by your piece of rock.
Frank

Im using the whole commercial microwave oven. Im putting the boiler flask in front of magnetron. The sample inside of boiler flask surrounding the antenna. The antenna is submerged in the sample. The termination would be just on the other side of the boiler flask. This part of the apparatus would occupy the oven cavity. Please let me know what you think of the set up. If this works then the next step would be Green River Basin Colorado.

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The burst into flames was just stupidity on my part. I had drilled a hole in the top of the microwave for the condenser part of the retort. It was too big, so I had used wedges of wood to stabilize the glass tubing.
I tried to go cheap and use a porcelain jar from hobby lobby to avoid arcing with the oven cavity. I thought this was a good idea. I shoorted out the power supply. I am a novas. Again first time submerging my antenna in a solid. This next time I spending money on a fused quartz beaker 1500ml.

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The antenna is a simple dipole on top of a voltaic pile. In other words the base is a multiple layer of aluminum, zinc plated washer and silver plate. The aerial part of the antenna can be aluminum or zinc plate, even nickle plate.
The whole combination works well with metals that have a high electro-negativity value.

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Just to clarify, the boiler flask was placed inside the porcelain jar to act as an insulator.

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Yet another clarification, this antenna can be assembled by soldering with silver solder at the base with a zinc or nickle plated nail as the aerial. The silver solder would have to be jewelers solder with a melting point of 810 degrees C. Or the antenna can be assembled with a rivet gun. The aerial would then be conveniently aluminum.

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I'm also assuming a retort temperature of 500 degrees C using 80% silver solder. Riveting would be a better assembly.

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I was thinking of how fast the water would have to be moved through the oven area with the temperature of 500 degrees C plus at the retort.
I might go with a termination. I was looking at some air cooled terminations that handle 1200 watts. Expensive, but the results of a successful experiment would be worth it.

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I agree, 2.45Ghz seems to be the magic number for most things organic. And I am very thankful for cheap magnetrons. I have destroyed quite a few of them.