can microwave be focused?

[stevenz]

I am considering to use focused microwave energy to heat ceramics materials?I don't want to put the ceramics in oven, instead i want to put it in open space.Is this idea possible.

Tks,

 

[IRstuff]

Of course it can be done, BUT:

> You'll consume way more energy, as you cannot take advantage of a resonant cavity

> You'll be spraying microwave energy all over the place, which you and your co-workers might find a bit annoying

> You'll probably cause massive interference with electronic devices within a 2 mile radius

TTFN

 

[logbook]

You could focus the beam with a large lens (not of glass, mind you) but that would probably be inefficient. A parabolic reflector (metal) would focus the beam.

You would then need absorbers surrounding the setup to stop the beam killing birds, people and other living things in the area.

 

[stevenz]

Thks! I have another question,what is the minimum diameter of microwave beam that can be focused if a parabolic reflector (or other device for focusing microwave)is used ,say for 2.45GHZ microwave?

Best Regards,

 

[IRstuff]

Depends on the diameter of the reflector. You can essentially apply the Rayleigh criteria of 2.44 lamda/diameter to get the angular spread of the diffraction spot.

A 6-ft dish has an diffraction spot spread of about 9.4 degrees.

TTFN

 

[biff44]

I would worry about safety issues! Ceramics are generally low loss, so you will be radiating a lot of power. Don't want you to grow two heads!

 

[Higgler]

I've made what you describe a few years ago for medical reasons. Essentially was an external microwave oven (2.45 ghz)intended to heat cancerous tumors within the skin. We used an elliptically shaped one foot diameter reflector with a notch antenna feed to heat/cook a one inch diameter center of chicken breast. We used microwave oven energy and tapped off the energy with a waveguide to coax. transition, then waveguide and another waveguide to coax. transtion to coax. cable to the elliptical reflector primary feed notch antenna.
It worked pretty well, our next step was to create a smaller hot spot using a ceramic dielectric filled elliptical antenna, though our funding ran away from us (literally, it was inappropriated by a scoundrel co-worker).
kch

 

[biff44]

I was looking for something else and found this link. Maybe it is helpful:

http://www.talisman.org/~erlkonig/misc/microwave-metal-casting.html

 

[biff44]

BTW, if you actually try this stuff use some common sense safety preparations:
cotton, wool, or leather clothing. NO Synthetic clothes.
No cuffs on your pant legs
Leather boots INSIDE of pant legs
eye protection
leather welders gloves.
Many metals give off poisonous gasses when melted, such as lead, zinc, mercury, etc. Beware.

 

[ProfK]

Found this thread by search engine. Hope someone's listening.
Interested in focusing 2.4 Ghz uwave for materials processing as well. Saw response from Higgler that begs the related question: can one deliver 2.4 Ghz uwave energy, say starting from an ordinary oven waveguide, by flexible cable (RG8??) to some sort of applicator, say an antenna/horn/dish such that ~1 kw or so of energy is directed to a dielectric workpiece for heating?
Such technology is not in common literature but I haven't seen any such arrangement so guess it's not feasible?? Is principle ok but needs tweak?

 

[biff44]

I remember seeing an article of some HAM guys doing just that. Try searching at the ARRL, or some Ham message boards. I think basically you have to find the output of the Klystron tube, which is a metal surface that probably looks like a window with a tiny slit in it, and somehow clamp the waveguide flange to the metal surface. You should be sure that their is a good metal to metal seal all the way around the window, or you will be leaking high power energy into the air (a bad thing).

Also, watch out for the HIGH VOLTAGE at the klystron. In today's world of transistors and IC's, most people no longer know how to work with high voltage. Find some safety tips somewhere. What I can remember: Don't wear any jewlery, especially rings. When testing, keep one hand in you pants pocket. If you want to work on the circuitry, take a long screwdriver with a plastic handle, ground the screwdriver to the chasis with an aligator clip/wire, and discharge the high voltage capacitor terminals! Do this even if the project has been sitting around for weeks unused (the blead off resistors sometimes fail and you have a lethal voltage on the capacitor).

Also, nobody has answered the fundamental question: how do you prevent damage to your eyes when experimenting? I would imagine there is a big chance for developing cataracts.

 

[biff44]

Oh, by the way, if you are going to discharge the capacitor with the screwdriver, obviously be sure the AC power is off at the time!

 

[ProfK]

I have prepared myself by purchasing a uwave survey meter to monitor leaks and I know that metal screen and foil will stop the beam. One just needs to work in a Faraday cage of sorts. Another protection is distance. Also the cataracts may form from chronic exposure and not a brief accident.
I have some expertise working with electronics including HV, digital, x-ray equipment etc. just not uwave RF of this power and my E&M classes don't help understanding waveguides vs. coax vs. antennas.
I plan to use a commercial magnetron with adjustable power so experiments can be done at low power first. With added confidence, I may just tie into a cheap oven waveguide. Will see where ham literature takes me. Thanks.

 

[IRstuff]

Just remember that the wavelength you are interested in will probably also work on meat, e.g., you.

TTFN

 

[hacksaw]

a low power magnetron, that will be a real first....only on eng-tips

if you find out how to do it you better register the patent,
there are other more controllable sources.

highly recommend use of special grounding equipment to discharge the caps...screw drivers are not considered adequate safety equipment

 

[Comcokid]

What you are trying to do is similar to this previous discussion thread.

thread247-93662

Also check the web - here is an industrial microwave heating industry that does a lot more than food, and there may be application papers out there.

 

[biff44]

I once read a paper where the authors were using an Liquid Crystal display to measure RF energy. Basically, the liquid crystal will change states due to heat by the microwaves. So if you take a big LCD display, remove all the external wiring, put it in the position that the hotdog will be in, and turn on some medium power microwaves, you can see how the fields are focused by watching the LCD flat panel. You can empirically design some sort of horn or lens to focus the microwaves and see what is going on. Don't know if it actually works, or how much power you need to see the effect, but it might be useful.

 

[ProfK]

biff44 neat idea re lcd. I use thermal printing paper to map hot spots from uwaves. This may be more dynamic and hopefully more than one-shot!
I saw the hot dog thread and mentally smirk. I can't see how it can be done with one cycle because the material properties are so different. I can see someone making a flexible reflector tied to a multipower source such that a good focussed zap will heat the dog and after it's warmed up, defocus and reduce power for the bun/bread combo.
I am trying to heat materials up to 1200 C. It can be done and is not a secret, but my need is not merely to being a box on the factory floor, rather to direct the energy to a target and perhaps focus it a little to raise flux. Wavelength can be anywhere, 1-100 GHz, but cheap tests are clearly best at 2.45