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purpose of odd dual aperature in oven

purpose of odd dual aperature in oven

purpose of odd dual aperature in oven

(OP)
I just examined a compact microwave oven and it had an odd pair of openings at the magnetron waveguide end. The waveguide funneled out and then the larger of the two openings was half covering the lower portion of the funnel and the second opening, about level with the magnetron, was almost a slit of 1/2 x 2 inches. Both aperatures were cut into the sheet metal wall and niether was as large as the usual single waveguide opening of about 3 x 2 inches I am used to.  I want to couple to this waveguide with another waveguide but this geometry is confusing.  I feel like opening up the larger aperature and closing the smaller one before proceding. What does this odd configuration mean?

RE: purpose of odd dual aperature in oven

Sounds odd, is it at 2.45 Ghz?
Maybe they needed to match the Magnetron last second, and instead of redesigning their probe, added what seems like an inductive iris.
It could also purify the polarity of the rf energy to help cut down rf leakage out the door. Maybe they flunked a leakage test and couldn't sell the unit until they added a quick fix.
The oven I used to couple RF energy out only had a rounded probe and a short tapered waveguide section. Waveguides are cheap, maybe buy another one.

Are you planning on attaching your waveguide at the 0.5x2 inch opening?

kch

RE: purpose of odd dual aperature in oven

(OP)
Interesting take on what was going on. I am trying to learn about these fine points so I can take advantage of them later. Your assesment causes me to consider mounting the magnetron to my own waveguide.
 
As I don't really require a waveguide, and don't care to start another thread, I'd like to know know what is wrong with using a round, spun metal mixing bowl as the applicator where the stem of the magnetron pokes through the base. I am trying to direct the energy unidirectionaly 'downwards' into soil to warm it and don't care about the wave front albeit uniform energy distribution is a desirable trait. This is not a naive attempt to make a parabolic dish style of applicator. I would also consider a wide, shallow angle cone with the antenna at the apex. My reading on the subject of waveguides never addresses these kinds of geometries, just rectangular and cylindrical end fed tubes.

RE: purpose of odd dual aperature in oven

You can try what you're planning, it'll probably work and be your cheapest solution. It might arc though if the metal your place around the probe isn't shaped properly. Plus there's alot of high voltage danger and high RF danger to consider.

I've done a project similar to what you are planning, except I was heating chicken with my antenna. Chicken being a substitute for a human cancer.
I purchased two Waveguide to coax. transitions, added a cable between them and you have an RF power transfer capability from your microwave oven to the ground, you could add an antenna if you need a wider spot of the ground. The cable loss will help protect your oven because the soil will reflect alot of power. There was 3 interlocks we needed to cheat to get the microwave oven to turn on. For our protection, and to minimize RF leakage, we built our own metal microwave oven door and put a coax. feedthru on it to get the energy thru the door. We only took out about 100 watts CW by spacing the coax. to waveguide transition inside the oven away from the rf aperture. Full power might have melted the feed thru and maybe the coax. to wg transitions. I added a tupperware of water to the oven to help absorber rf too.

There is a thread on this website where someone has built a multi element array of waveguide ovens with attached antennas specifically to heat the ground and kill weeds.

kch

RE: purpose of odd dual aperature in oven

(OP)
I subscribe to that journal with the weed killer. The array is used only to heat the vegitation above the ground and no intent to heat the soil. There was work in the past specifically to heat the soil to sterilize it. I want to heat dry soil and the low loss tangent should make the beam travel pretty deep...I don't know but I hope not to get much reflection.  I have rewired several microwaves already as the physics of electricity is well known by me but I have no feel for how microwaves behave near their source. I do want to couple all energy to the soil. To start, I am using a bucket of water because I expect no reflection from that. I read about your coax escapade earlier and still find it intrigueing. I could use a box waveguide and assume there is a uniform plane wave emanating from the end of it, but I can't help thinking I can treat the waves from the magnetron like optical propegation instead of RF and reflect them off a metal cone/dish/mirror.

RE: purpose of odd dual aperature in oven

Water has a huge reflection unless you shape it. Dielectric of water is 81, hence it's impedance is 377(air)/(square root of 81=9)= 42 ohms.
Waveguide is usually 100-200 ohm type impedance, so if you had a waveguide with water at the end, there's alot of reflections. Loss is water is about 10 dB per inch travel. That was a measurement I did way back when in a water tub.

Regarding penetration thru the soil. One way to estimate the penetration is to take some soil, arrange it in tupperware of various thicknesses and place that in the microwave oven pressed up against the rf source end and see what happens. If it heats alot on one side and not at all on the other end, that'll give you the propagation length.

Clay is the highest loss soil, or so my memory says.

One problem that many people are working on is ground penetrating radar. Big problem since most soils are very lossy. Low frequencies like 30 Mhz are what's needed to penetrate soil. What depth are you trying for? I've heard of 900 Mhz microwave ovens in the rumor mill, that would be better than 2450 Mhz.

Raytheon made 50 kilowatt applicators for Shell Oil's Shale oil project way back when. The antennas needed to be 50-100 feet long and spaced 50 feet apart to heat the soil enough to liquify the oil in the shale. I think these were 10 Mhz units. Shell Oil advertised that for a long time after they knew it was a useless project that didn't get any results.

kch

RE: purpose of odd dual aperature in oven

(OP)
My 'soil' is more like fine crushed dry rock; no water and no clay to speak of. I will rethink water 'trap'. What if waveguide is a foot or so (> 1 lambda) away from 'tub' as you say. For now waveguide opening is wall-of-microwave oven aperature.  I seek 1-2 feet penetration with heating and because funds are low, staying with 2.45 for now.  Rock properties change less with wavelength than with temperature so I hope that assumption will hold.   

RE: purpose of odd dual aperature in oven

12-24 inch depth sounds like alot, I'd suggest you try a large tupperware full of soil (fill half the volume of the oven) plus a small cup of water in the microwave. Have the soil next to the radiating element.

Way back when, I'd considered using Ham Radio friends as a source of multi hundred of watts of RF power. Many of their transmitters are much lower in frequency if you need to penetrate further. You'd need the proper applicator/antenna which may be easier and cheaper to make than buy.

kch

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