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Help with 2.45GHz high power application.
3

Help with 2.45GHz high power application.

Help with 2.45GHz high power application.

(OP)
After posting on the "Antenna & Propagation engineering" forum, I have decided that this forum may offer the answers I need. I am involved in a post grad research project for material testing (welds in particular), and need assess the viability of several RF techniques.

My first question: I understand that Ansoft and HP HFSS are the industry standard for EM simulation, but have rather steep licensing costs. Are there any cheaper packages, or even assessment freeware, that will at least begin to allow me try out some initial design ideas?

My next question: I have looked through the RSGB and ARRL for suitable books to learn about microwave design techniques, there seem to be quite a few titles. Are there any in particular that are recommended? I'm OK with vect calc (from fluids), but am still getting up to speed with the Maxwell equations.

Finally: Since my research is going to need a lot of power (>1kW) into very low impedances, I have decided that the most sensible frequency to use is 2.45GHz. A lot of existing hardware seems to be available for this frequency, at the power levels I will require. Are there any techniques I should be familiar with (including the obvious safety requirements!)? I am interested in designing the apparatus to be as efficient as possible, since it is likely to become portable.

Graham

RE: Help with 2.45GHz high power application.

HP has sold HFSS to Ansoft.  There is a free student version available (don't know how crippled it is).

http://www.ansoft.com/products/hf/hfss/

Then click "contact."

RE: Help with 2.45GHz high power application.

(OP)
Hmmm, interesting - have to think up a good commercial justification for "trying out" this code...

RE: Help with 2.45GHz high power application.

***I understand that Ansoft and HP HFSS are the industry standard for EM simulation

This may sound like a statement from a commercial issued by a respective software vendor, but unlikely represents a real engineering practice. This is no similarity to MS Windows here. HFSS is based on FEM, but modern computational electromagnetics has real alternatives to this method, for instance, FDTD. Many researchers and engineergs find the latter one to be more efficient/convenient for designing systems and components in many microwave applicaitons. In particular, for microwave power engineering, the benefits of FDTD modeling (and respective simulators) have been explicitly formulated - see, e.g., http://www.wpi.edu/~vadim/project3.html

RE: Help with 2.45GHz high power application.

I do not think HFSS will be of real help in this case as is never really seemed to model loss in  systems all that well, or at least when I tried, It took to long and I was skeptical of the results. You are not very clear of you application, when I was A pipe weld grinder in wyoming, they would always have the welds X-rayed for defects. Is that what you are after? I do not think that microwaves have the resolution, other, than to tell that a welded section is maybe present, Now good weld from bad weld? that is a stretch, I know that PHd's need to find a new nitche to hang your hat, but this is going to be very tough. I would suggest a wide band linear FM or psuedorandom signal such that pulse compression can be employed to maybe, maybe get your resolution sub-inch. This would seem your only hope, I think, However really, I know nothing of nuclear magnetic resonant imagining, if which this is. Forget every thing I just said, and laugh that I just made an ass of myself again, and have fun with you superconduting magnets.

RE: Help with 2.45GHz high power application.

(OP)
Thanks for the info IMMG. Are there any codes you recommend particularly? Again cash is tight so just something for an initial looksee will do me.

Thanks for the ideas GOTWW. Sorry I can't say more about my application. May be subject to intellectual property rights, and is likely to generate a patent. By disclosing too much information I shoot myself in the foot.

RE: Help with 2.45GHz high power application.

I guess the question at this point is what do you want to do?, simulate the fields in a cavity, coaxial resonator, transmission line or what. As I said, HFSS is somewhat limited in evaluating the skin effects, or discontinuities of conductance. Basically S-parameters, and modal field distributions. The conductivity, skin effects, & associated loss is troublesome for me with the simulators, especially if I have to wait. If your goal is to basically establish a resonant scenerio with alot of surface current, just dig into the old rad-labs, etc. Pick a mode that produces J, that suits your need.

RE: Help with 2.45GHz high power application.

(OP)
Simulation of fields in a cavity is a good summation. The ability to, say, predict interaction of an aerial with the cavity while simulating skin effect would also be useful. At this point I'm assuming (perhaps falsely) that I won't have to simulate the generator and feedlines. Smith chart manipulation should be good enough for this.

Most of it, as you say, could probably done by picking the appropriate mode and assuming that this will be dominant. Without saying too much, the test piece will be put at one node, and aerial/reciever at another.

RE: Help with 2.45GHz high power application.

I guess my training is different, and taylored be applied to different problems. Using network analysis, scattering parameters w/ clearly defined ports, that I estimate to what you refer to as aerial. And true, if a calibration proceess is employed, the error network that is between the "idealized" measurement instrument and test ports can be "de-embeded" or removed from the raw data collected.

RE: Help with 2.45GHz high power application.

3
A few notes on high power stuff.
Dielectric is lossy, if you can build something without it, please do.
If you make a stripline power divider, use either a very thin printed circuit board (0.005", 0.010") with 2 Oz. copper and space it with plastic spacers, or use solid metal parts with foam spacers under and above to avoid dielectric.  Hexcel Honeycomb is a good spacer material. Use Hexcels Texas representative if you order any, Hexcel is too expensive with their minimum order requirements.

If you need to fill something with liquid, buy Castor Oil, it's totally lossless at this Microwave Oven frequency range. If you put Castor Oil in a normal microwave oven, you can't even feel any heat generated, super low loss. It's good for cooling too.

If you have a cable with dielectric on it, and transition to something without dielectric, there can be breakdown arcing at the surface of the dielectric at much lower powers than you'd ever imagine, so tapering dielectric to air is a good way to help that situation.

I used a Microwave Oven as a source in a Cancer study to heat tumors through the skin, it was a cheap source of power. We purchased a waveguide to coax transition and spaced it away from the antenna in the microwave oven to vary our CW power.  Added a new door with type N transition, and voila, had a nice variable CW power. We of course got the extra warranty on the oven, but didn't use it.
Loss in water is about 10 dB per inch at that frequency (measured).

kch

RE: Help with 2.45GHz high power application.

(OP)
Some good pointers Higgler. Thanks

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