Building a REAL MANS microwave - Page 1
Forum section: electronics / projects

author: Dane Bear
author_rank: Contributor
timestamp: September 09, 2015, 02:39:59 am
content: Hello, I bought the "most powerful" microwave oven available in my town. Being the kind of guy I am, I want a microwave that will cook pot pies and heat general stuff FAST. Is there anyone who has a diagram of a high powered,,, lets say 5000 watt magnetron? I can run on up to 230v 3 phase at my place. Also, how would one regulate cooking power?-Dane

author: ConKbot
author_rank: Super Contributor
timestamp: September 09, 2015, 03:12:46 am
content: Hmm, interesting request. A magnetron is a vacuum diode on the basic level, but tuned cavities set up a resonance, and an antenna inside couples the rf energy out.So are you trying to machine your own magnetron core? Or just stuffing a bigger magnetron into a normal oven?

author: rs20
author_rank: Super Contributor
timestamp: September 09, 2015, 03:18:18 am
content: It's worth noting that the innocent-looking ceramic in the diagram (i.e., the bit labelled "Ceramic") is generally constructed ofBeryllium Oxide. It's fine as an intact solid, but the moment you chip it, grind it, or break it, the dust released is very nasty stuff. So be careful with that magnetron!

author: sarepairman2
author_rank: Frequent Contributor
timestamp: September 09, 2015, 03:21:02 am
content: you can use wave guide to concentrate alot of microwaves into one place, designing your own magnatron would be pretty hard.. idk what military radar gear is available to civilians (some is very high powered)

author: Dane Bear
author_rank: Contributor
timestamp: September 09, 2015, 03:32:18 am
content: I wonder if one of those GE transmission antennas would provide enough energy to cook with? I basically want to have a range sized microwave, adjustable from warming up leftovers, to cooking turkeys and chicken pretty quickly.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 09, 2015, 04:06:25 am
content: Maybe get a used commercial use oven? Note that for small stuff, in my experience, more power is not helpful - you end up with overcooked and undercooked spots. (That's actually where the "inverter" microwave ovens shine - they can go very low in power, down to 100-200W or so.)Radar magnetrons are apparently next to useless for heating purposes. The Mythbusters tried to cook a turkey on a ship radar and they couldn't even measure a temperature increase.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 09, 2015, 04:34:21 am
content: An extremely over-powered microwave would not be useful: most of the microwave energy gets absorbed and converted to heat within the first centimeters of entering damp material. All a horribly over-powered oven would achieve is burn the exterior while the interior is still frozen - something you should already be familiar with by re-heating stuff from the fridge for two minutes, which leaves you with almost boiling-hot outside and often still cold middle, so you need to mix and re-reheat to get something uniformly hot.If you want to re-heat something unattended or do not want to stir it, you end up having to set the microwave power to 50-70% for twice as long to let the heat soak in.

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: September 09, 2015, 04:45:48 am
content: NiHaoMike and DanielS have it right. A 5kW microwave oven, for many foods, is a shortcut to a ruined meal. You can't escape physics.For your educational pleasure:https://en.wikipedia.org/wiki/Microwave_oven

author: LaserSteve
author_rank: Super Contributor
timestamp: September 09, 2015, 02:40:53 pm
content: With the right spacing on the waveguide mounting, two or more magnetrons will phase lock to each other. They have quite a broad tuning range and frequency is somewhat proportional to applied voltage. An improvised S Band Waveguide is available as Aluminum Hollow Door Frame for Industrial Doors.Two in parallel works, but three or more did not work so well on cheap waveguide due to high wall currents in the aluminum guide. If I had used silver plated guide things would have been better. I was doing that for commercial reasons as a trial project, results were OK, but not awesome.You can also injection lock them using a circulator, but that is a story for another time.Now for the caveat. For cooking food, just go buy a 2 kW commercial microwave with additional hot air convection heating. Pre cook your meat and store it in a humidified cabinet for an hour or two. Then nuke it. Your food will taste like fast food, which is the only point of using a high power microwave. Next time your at a McDonalds in the US, note the humidifier cabinets.However the fast food places have very carefully calculated duty cycles based on the amount of burger in the cavity, they do not dump 2KW CW into a single cheeseburger, because it burns the outside and breaks down the protein too much.I was an lowly EE student working my way thru school flipping burgers when they went to "Qing" ovens to allow us to keep the meat longer.. I studied this extensively. I still think this and the electric double sideded clamshell grill ruined the taste of burgers.Now for the safety warning. The first thing to go when exposed to high power microwave fields is your corneas, followed by the Testes. If you've never worked with high voltage and high power RF, consider this project a thought exercise and don't mess with it.If only takes a second or so of accidental exposure to fry corneas to an opaque white at 2.4 Ghz. Its not worth the risk.Steve

author: suicidaleggroll
author_rank: Super Contributor
timestamp: September 09, 2015, 04:12:29 pm
content: I rarely even use my 1kW microwave on full power...all you end up with is hot/cold spots. You have to give it time for the heat to conduct through the food, there's no way around that.

author: Sbampato12
author_rank: Regular Contributor
timestamp: September 09, 2015, 08:38:04 pm
content: Don't forget to make a 'bed' of SiC (silicon carbide), to spread the heat!(never is too much to say caution...)In a company I worked some years ago, they tried to make a high temperature oven, with microwave. And they used SiC tablets, to get heated. But it get really hot...

author: AF6LJ
author_rank: Unknown
timestamp: September 09, 2015, 09:03:12 pm
content: Quote from: LaserSteve on September 09, 2015, 02:40:53 pmA thoughtful post on the subject.Getting beck to basics; each way of cooking food creates a different chemical soup that we eat.I have often found that little more than pizza tastes good when it gets nuked the next day. I usually end up eating that left over pizza cold for breakfast the next morning. I generally don't like what comes of a microwave oven. (I go to In and Out Burger where they don't use those ghastly things)On a somewhat humorous note. The vision of a microwave with a pair of maggies coupled into an S-Band circulator make me laugh seeing that contraption slapped up against the side of a microwave oven. That got me to thinking.......four maggies and a Magic TA bit of trivia about microwave ovens; you can put your cell phone in one and call that phone and the call will go through. For some this becomes a head scratcher. How can a microwave oven be safe if it allows a cell phone to handshake with the system and receive a call.For those here viewing this thread that don't know the door internal shape forms a "Choke Joint. The 2.4GHZ RF cannot leak out due to the geometry of the internal face of the door.

author: Dane Bear
author_rank: Contributor
timestamp: September 09, 2015, 09:19:57 pm
content: Thanks guys for all your info!! It seems quite apparent that a commercially purchased oven is the way to go; given the fact of uneven heating/burning. I honestly thought that a magnetron was similar to an induction furnace, which I have made for heat treating in the Tool And Die field. (They are really fun to play with) I guess heat is heat, wether derived from a flame, coil, or renisasance.

author: Don Hills
author_rank: Regular Contributor
timestamp: September 10, 2015, 12:29:57 am
content: Quote from: LaserSteve on September 09, 2015, 02:40:53 pmThe original microwave patent (2495429) shows two magnetrons coupled to a common waveguide.I note that they're wired to opposite sides of a centre-tapped transformer, so they conduct (and oscillate) on alternate half-cycles.

author: TerraHertz
author_rank: Super Contributor
timestamp: September 10, 2015, 10:40:03 am
content: Another factor is that the field strength in consumer microwave ovens (when the cavity is empty) isn't all that far below the dielectric breakdown strength of air. If you doubled or tripled the power input you'd just get a permanent arc sitting in the waveguide. This is why high power military radars use pressurized insulating gas in the waveguides. Only once the beam is diverged enough via a horn can it be exposed to air at STP.With food (an absorbent load) in the cavity of an oven, field strengths are much lower.Air breakdown in the E-field component of an EM field is unrelated to the heating effect of microwaves, since the heating in that band is due to the field vibrating polar molecules (such as water, but others too.) And in induction heating it's resistive, from the induced current flowing in the material being heated.

author: ConKbot
author_rank: Super Contributor
timestamp: September 10, 2015, 11:41:56 am
content: Quote from: TerraHertz on September 10, 2015, 10:40:03 amIs it really? I didnt think that commercial S-band radars (ships), with 30-60kw pulsed outputs took any special precautions. I know some radars are lightly pressurized to keep moisture and contaminants out. No experience with any though, so I could be wrong.

author: lincoln
author_rank: Regular Contributor
timestamp: September 10, 2015, 09:02:10 pm
content: Quote from: DanielS on September 09, 2015, 04:34:21 amThen why dose butter melt form the inside out?edit> the Internet says that there is less moisture at the surface than in the center. Hmmmm... I'm not buying that with out proof....

author: rs20
author_rank: Super Contributor
timestamp: September 10, 2015, 09:10:15 pm
content: Quote from: lincoln on September 10, 2015, 09:02:10 pmBecause a) citation needed, and b) butter is only 16% water, so not "damp" by any stretch.. If you cook a bowl of beans, you have to stir halfway or else you end up with a block of cold beans surrounded by lava.

author: Zero999
author_rank: Super Contributor
timestamp: September 10, 2015, 09:54:19 pm
content: Microwaves do heat fats as well as water. It's possible that when heated near the surface more heat escapes, than further into the butter, where the heat can't escape.

author: lincoln
author_rank: Regular Contributor
timestamp: September 10, 2015, 09:56:40 pm
content: Quote from: rs20 on September 10, 2015, 09:10:15 pmSource: Every stick of butter, for the past 30 years.and b) butter is only 16% water, so not "damp" by any stretch.. If you cook a bowl of beans, you have to stir halfway or else you end up with a block of cold beans surrounded by lava.

author: TerraHertz
author_rank: Super Contributor
timestamp: September 11, 2015, 12:51:02 am
content: Quote from: ConKbot on September 10, 2015, 11:41:56 amI said that based on how easy it is to produce an arc in microwave ovens, with not much in the way of field concentrating 'antenna' structures. A grape cut in half, a small graphite ring with a gap, burnt matchhead, etc. Also small carbonized spots of grease on the mica inlet cover on the waveguide port are great for causing arcing, that usually results in the frightened owner tossing out the microwave unit.It would be interesting to actually measure the field strength, and compare to air breakdown values. I don't yet have a means to do that measurement.As for 'pressurized gas', there's the pics below. These are of a unit from a test set for a military fighter plane combat radar. It's basically just a pressure regulator and doesn't say what the gas actually is, apart from the cryptic "GX2" (or GN2") label on the pipe connector from the gas source. More interesting for the show of how low volume military test gear was constructed. The wires are all teflon insulated. Not because they needed to be, but because _everything_ in the associated gear used that wire.

author: eas
author_rank: Frequent Contributor
timestamp: September 11, 2015, 01:38:47 am
content: Quote from: AF6LJ on September 09, 2015, 09:03:12 pmI like a good piece of pizza cold the next day, but lately, I've been heating a couple of pieces up in a big skillet. I turn the stove to medium, put the slices in, sprinkle a few drops of water on the pan, away from the slices, and put a lid on it. The bottom gets heated and crisped up by contact with the bottom of the pan, and the steam transfers heat to the top. A couple of minutes before eating it, I push the lid ajar to let the steam escape and the crust crisp up a bit.

author: Someone
author_rank: Super Contributor
timestamp: September 11, 2015, 05:38:05 am
content: Quote from: TerraHertz on September 11, 2015, 12:51:02 amA localised field concentration can breakdown the air in a microwave but a well designed waveguide will easily transport 100's of kW even in the tiny sizes seen inside microwave ovens (the magnetron antenna limits the practical sizes possible).

author: hayatepilot
author_rank: Frequent Contributor
timestamp: September 11, 2015, 06:48:12 am
content: Quote from: AF6LJ on September 09, 2015, 09:03:12 pmJust don't turn the microwave on!Reminds me of this:Greetings

author: Deathwish
author_rank: Unknown
timestamp: September 11, 2015, 08:11:20 am
content: if this is a REAL MANS microwave your building are you getting inside it to test it then ?.

author: Stray Electron
author_rank: Super Contributor
timestamp: September 11, 2015, 11:32:47 am
content: Quote from: TerraHertz on September 11, 2015, 12:51:02 amWe used Sulfur Hexafluoride for pressurizing the "waveguide" in multi-mega watt search RADARs. IIRC it was only pressurized to about 7 PSIG. All of the insulators and spacers used in and around it were pure Teflon. We called the conductors "waveguide" but it was actually a rigid, round, coax and bolted together in sections.Teflon insulated wire has been pretty much standard in all military electronics and military support equipment for the last 30 years or more since the conductor is usually silver plated copper.

author: German_EE
author_rank: Super Contributor
timestamp: September 11, 2015, 11:44:44 am
content: I may have told this story before so if I have then sorry for the dupe.I was once called as a technical witness in a court case. The owner of a Chinese Takeaway had two microwave ovens that were used to heat up the cartons of food before handing them to the customer but he got tired of opening the door, putting the food in, closing the door and then starting the microwave.So...............He removed the doors and defeated the safety switches so that the microwave ovens could run continuously. Finding that the ovens did not work so well (they were no longer a cavity) he put an aluminum panel on the front with a slot just big enough for a hand holding a carton. Everything worked well until he found that all of his family that worked there had trouble moving their fingers.Two people lost all of the fingers on their right hand and a third person managed to have two fingers saved. He took the microwave oven manufacturer to court because the instructions did not tell him that removing the door and fitting an aluminum plate in its place was a bad idea. He lost.

author: rs20
author_rank: Super Contributor
timestamp: September 11, 2015, 12:14:58 pm
content: Quote from: German_EE on September 11, 2015, 11:44:44 amI'm curious to know the medical side of this -- what exactly happened to the fingers? Was it heat, or radiation? Cell death due to heat, or DNA damage? Or not cell death at all? (IIRC, microwaves don't have enough energy to enact ionization).

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: September 11, 2015, 02:37:24 pm
content: YEah, the microwave radiation is non ionising so it would just have a thermal effect on the hands. However, it would heat the finger tissue pretty evenly all the way through so by the time your hand is hot enough to activate the 'ow that's hot' nerves in your skin, the whole of your finger is probably cooked. Full thickness tissue burns = they chop your fingers off in the hospital

author: SeanB
author_rank: Super Contributor
timestamp: September 11, 2015, 04:26:42 pm
content: Quote from: TerraHertz on September 11, 2015, 12:51:02 amGN2 is dry gaseous nitrogen, typically obtained from a high pressure cylinder, commonly used for aircraft tyre inflation, and for purging of internal pitot static plumbing during maintenance. The cylinders are typically refilled ( at least in the military) by a dedicated support unit, where they have the air liquifier plant to compress and dry the air and then compress in a high pressure compressor series to get it to first drop out the water vapour, then the CO2 , then finally the O2, leaving behind the N2 and traces of other rare gases ( He, Ar, Kr, Ne Ra ) which is finally condensed into the liquid gas by evaporation. This then is stored in large Dewars, and as needed used to fill cryotanks or fed to an evaporator to generate high pressure gas for filling cylinders.Aircraft test gear often uses the same instrumentation as the actual aircraft uses, along with the same fittings, plumbing and wiring, so as to have common connectors. There you find them using internally a circular bayonet connector for a connector, despite it being a connector costing $100 or more, in an application where it never will be disconnected, just because the same connector is used on the airframe and it has enough terminals ( or at least enough, I have seen a 200 pin connector used with 5 wires in it) to handle the loom. the ground pitot static set used the same piping internally as the helicopter, which made ordering a replacement tube difficult, I eventually replaced a serviceable one on the airframe to get the used pipe, as it was considered expendable. Did not just use the airframe number but replaced the pipe, important to have the parts match the paperwork, though there was the hanger queen which at one time did have 5 3 axis gyro units in it, while there was wiring and mounting there for only 2. That one lost a lot of parts for AOG use, I took almost all the hydraulic system ( or at least the instrumentation part I dealt with) out as spares. That helicopter actually flew again..... The other hanger queen was at one point 3 broomsticks holding wiring looms, a serial number plate and 2 rows of racking with assorted hardware and panels.