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Mechanical Eng. with 2450 MHz Microwave question
3

Mechanical Eng. with 2450 MHz Microwave question

Mechanical Eng. with 2450 MHz Microwave question

(OP)
I have designed an oven to cook with several 2450 MHz microwave Magnetrons and hot air to brown the production of meat on a high temp. conveyor belt.  I need to attenuate or snubb the unused microwave from exiting the conveyor in/out port that is about 6" high and 16" wide.  How do you place these rods (pattern) inside the conveyor in/out port that is about 18-24" long at each end of the oven to absorb or snubb the excess microwave and is 1/4 wave length (1.2") satisfactory? I got this far using the internet but ran into a brick wall when it came to keeping the unused wave from coming out. Can any one help me or tell me where to find this information?
Thanks,
Leo

RE: Mechanical Eng. with 2450 MHz Microwave question

It sounds like you want to build a waveguide attenuator with probably a high level of attenuation. There are various sources of information available. The rods are probably high temperature lossy ferrite material such as emersons mf-500. I would place them at 1/4 guide wavelengths apart such to improve destructive cancelation. Also you could use a tapered vane.

Also helpful would be to introduce a trap or choke of low-high-low quarter wave transmission (waveguide)segments to provide a virtual short circuit at some reference plane. Selecting stainless steel waveguide attenuates the energy also (at least at the higher freq's).  In any case a linear simulator or HFSS should help the design somewhat. (but neither of these have models of cooking chickens on conveyer belts the last time I checked.

RE: Mechanical Eng. with 2450 MHz Microwave question

A quick reality check would indicate that your "waveguide" is operating in a higher mode (I am use to TE10 only) Yours may be TE6,2? in this case you would need an array of the ferrite rods 6 top and 6 bottom (evenly spaced), arrayed at a longitudinal spacing on the order of 1.2" You only have around 5 wavelengths - long waveguide to work with so it may be difficult to attenuate to the  power density level mandated by OSHA or FCC. Some experimentation will probably be required along with the proper power density measurement equipment and safety!!!. Also the ferrite may be highly toxic, so another lossy material (carburundum??) may be needed.  

RE: Mechanical Eng. with 2450 MHz Microwave question

You can't rely on lossy materials: You don't want to overheat them or reduce the efficiency.

The only feasible way is to change the impedance of the waveguide to reflect the energy back into the main cavity
the only problem is the space requirement.

<nbucska@pcperipherals.com>

RE: Mechanical Eng. with 2450 MHz Microwave question

Why isn't your electrical designer involved in this?

This is a potential safety of use issue.

TTFN

RE: Mechanical Eng. with 2450 MHz Microwave question

The carburundum?? spl? can take the heat I have seen less than an inch used in 500W mmw loads. (however a heat sink is needed)

RE: Mechanical Eng. with 2450 MHz Microwave question

How about a two interlocked two door scheme, 1-open, 1-closed and visa-versa. with RF gasketed doors. The chickens would need to be spaced approptiately to keep from getting the doors jambed though.

RE: Mechanical Eng. with 2450 MHz Microwave question

You need to look at designing a microwave brick and pin choke. This will allow you to move your product through the machine without using a door system.   

RE: Mechanical Eng. with 2450 MHz Microwave question

You do have a difficult problem, and searching on the internet is not going to find you a solution because the problem is too complex.

The main problem that you have is that your openings on either side of the oven are just too darned big.  Microwave energy will expand to fill that entire void, bouncing off of metal objects and travelling in funny ways.  These funny waves are called modes to a microwave engineer.  It is easy to build a filter to stop one mode of propogation.  It is not difficult to stop maybe two modes of propogation.  You probably have around 20 modes of propogation, and that is difficult to handle!

My recommendations are
1) hire a microwave engineering consultant with some waveguide filter design experience
2) get a book called "Microwave filters, impedance-matching networks, and coupling structures" by Matthaei, Young and Jones, open it to chapter 15.06 Dissipative Waveguide Filters, and build something like this contraption at both ends.  

I have built leaky wave filters like these, and they can easily handle 500 watts cw dissipation IF you use high temperature absorbers (like a ferrite) and have someplace for the heat to go (water cooled or aluminum fins/fan), and watch out for arcing.  

I CAN NOT guarantee that OSHA/FCC leakage limits will be met with this filter, but it will probably work better than some "rods", since it works on many modes at the same time.

RE: Mechanical Eng. with 2450 MHz Microwave question

Iomaglia,
One simple option would be to use water absorption. You lose 10 dB per inch of energy in water at 2450 Mhz based on my previous measurements.
For partial absorbption of energy on your entrance and exit, hang wetted cloth, similar to car wash strips of cloth. This may provide 20-30 dB of attenuation and get you to safe enough levels. You need a way to drip water onto the strips of cloth. The cloth moves out of the way when the chicken's go thru it. I'm assuming you have the belt moving continuously.

The metallized cutoff waveguide idea is ok if the chickens go in and sit there awhile, then move. As opposed to constantly moving at a slow pace. You'll need spacings closer to one inch for good cutoff loss.

Thin (0.1") Microwave absorber attached to you exit metal walls would also work, but fears of outgassing may be a problem for food safety.

there is a cheap answer to this.

kchiggins,
if you could, send a picture of the belt and opening to khiggins at Toyon dot com. I'm curious if only one belt is inside or two belts?

RE: Mechanical Eng. with 2450 MHz Microwave question

Consider a revolving door concept for input and output from your industrial sized nuke...for some resaon, the though of purchasing microwaved chicken, convection browned or not, isn't appetizing to me...then again, the I can go to the local costco and buy a freshly flame broiled whol bird for a few bucks and it is ...mmmmm tasty.  Maybe it's because my family watched chicken run last night on DVD...you aren't making PIES, are you?

Good luck with your fowl cooking system (Hmmm...I did spell Fowl right...didn't I?)

I remain,

The Old Soldering Gunslinger

RE: Mechanical Eng. with 2450 MHz Microwave question

Here is a book you probably want to own:

Understanding Microwave Heating Cavities (Artech House Microwave Library)-US-
ISBN:158053094X (Hard cover book)

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