An oscillator is an amplifier with a selective feedback path to define the frequency and an amplitude limiting stage to define the amplitude. You therefore need an active device with power gain at 1GHz.

A "very high current capability" is a meaningless statement. Gives us a number. In power terms, do you want 1mW, 1W or 1MW? Then what is the load impedance? One would typically have a load of 50 ohms but you are saying it will be "very low". This impedance is going to be crucial to the current output since you will probably need some sort of reactive matching network.

RF devices will be characterised for 50 ohm operation, so it is probably going to be easier to design the oscillator to drive a defined 50 ohm load then put on a matching network to change the impedance to match the load. Given that the system is running at a fixed frequency, the bandwidth of the matching network should not be a problem.

If you can say you want 100mA into 1 ohm, for example, that gives a good starting point.

Well, the application is the subject of research and may lead to a patent for industrial application. It is a non-destructive testing technique for various types of non-porous weld. As I say, it will be an enclosed system, so impedance is likely to be very low. I am interested in inducing very high currents in the test piece, so the oscillator will only be used to sustain the currents long enough to map the field.

The current required, and this is the hard part, may well need to be as high as a kiloAmp. Since the impedance will be very low (although may be reactive), there is no power requirement as such. Ideally I would like the system to be efficient, so have already ruled out Cavitron type devices.
I have some familiarity with Power Mosfets, and hoped I might be able to use a number of these in parallel.

Since the frequency will be fixed, resonant techniques are fine. I did have some papers by Peter Baxandall, who had similar ideas using bipolar transistors for UHF oscillators, but seem to have misplaced them. I realise this is a difficult challenge, but my topic is as much to determine the feasibilty of the system as to develop it.

At this stage I am as open to scathing rebuttals as I am helpful suggestions...

I think the power is going to rather higher than you might like.

Let’s take 1000A and 1milliohm. That’s 1kW.
1kW at 1GHz sounds like a magnetron to me.

… but wait a minute. 1GHz currents are not going to penetrate a metal very far.

9mm at 50Hz
0.002mm (2 microns) at 1GHz.

… assuming non-ferrous metal. And I am not sure if anyone has a good handle on the relative permeability of ferrous materials at GHz frequencies.

I was going to say that eddy current NDT was done at lower frequencies (100Hz to 100kHz) but I just found a paper (dated 1981) where the guys (Auld & Winslow) were using 1GHz frequencies and a ferromagnetic resonance probe to look at surface cracks.

Thanks for your feedback so far, logbook.

Agreed about the 1kW estimate. I was figuring about that. Eventually i will need higher still! The thing i don't like about magnetrons is that they are only 50% efficient. I was hoping to use a transistor in switch mode to minimise heat dissapation in the unit itself. Seeing as it is not modulated, i hoped this would help - doesn't even need to be a perfect sine wave.

This is really the crux of my question. Is there any way to design a solid state device capable of oscillating at these power levels and frequencies? This is a very tall order, but I can't believe that there isn't some way of doing it. Maybe there is, at least, a cutoff frequency that perhaps I could go down to to improve my chances.

Another related question is how can i minimise skin effect in a conductor at these freqs? I was thinking along the lines of a cross section of concentric copper rings, laminated in such a way as to insulate each concentric layer. The idea being to stop the charge migration to the outer layer. Each could be 2um foil - although it could be a manufacturing pain. There would be a high hall effect voltage across each layer, but the whole section would still be conducting.

Any ideas?

I don’t see how saturated switching is going to be possible at these frequencies. Typically RF devices quote ft and not turn on & off times. The older saturated switching transistors were of the order of several nanoseconds as I recall. SiGe would be the best but the currently available parts are too weedy (<200mW from Infineon). In Silicon the biggest Infineon offers is 2W. That would take an awful lot to get up to the power levels you are talking about, even if they were able to switch fully on and off, which I doubt. Given the low ft (5GHz) the base current requirement would not be trivial.

People do make kW Rf transmitters using semiconductors rather than valves, but the frequency is much much lower. The efficiency is good though.

I would think your best bet using semiconductors would be an emitter coupled switch with push-pull transformer coupling from both collectors. This should be fast and more efficient than a Class A stage.

Can you force a current to flow down the inner part of a coaxial cylinder if you laminate coaxially with coaxial insulators? I wouldn’t like to say definitively no, but I certainly think the answer is no. One of the earliest experiments on skin effect was passing a current through a gun barrel with a wire running down the middle of the gun barrel. "No current" went down the wire. It was a crude experiment and open to criticism but there have been other similar ones. I think it has been done with wire cages, producing a similar result.

Thanks for your help logbook - you really are making a big dent in my thought processes!

"People do make kW Rf transmitters using semiconductors rather than valves, but the frequency is much much lower. The efficiency is good though."

Maybe this would be a better starting point for me. I had a Baxandall circuit for an emitter coupled switch with push-pull transformer coupling from both collectors, but I have lost the paper. I'm quite used to machining my own inductor parts so I like this approach. Recommend any books/sites on the subject? - Circuit description and theory ideally. I know I'm going to have to do a lot of reading and experimentation...

"One of the earliest experiments on skin effect was passing a current through a gun barrel with a wire running down the middle of the gun barrel. "No current" went down the wire."

With a travelling wave I can definately see this, since current will escape to outside wherever the inner and outer conductors contact. I was thinking more along the lines of having the transistors feed each coaxial cylinder independantly, so no chance existed for hall effect to push current to outside conductor. Alternately if the line had a standing wave then as long as inner and outer conductors contacted at voltage nodes only (ie no current) then again hall effect voltage would not exist, so no skin effect current.

I appreciate that this is all a brute force approach to persuading the electrons to go where I want them. What can I say, I'm a mean guy to electrons!

Can you use 2.45 GHz instead of 1.0 GHz ? The advantages include easier licensing (ISM frequency) and readily available and incredibly cheap 1kw sources (microwave ovens).

Well ElectLect, you are making my brain creak! So we now have two isolated circuits driving individual isolated coaxial conductors. What happens? I couldn't think of this in terms of solving Maxwell's equations, but it occurs to me that we now have a coaxial cable. Whether this is an inapplicable quasi-static approximation to the problem is not yet clear.

I believe your association of the Hall Effect and the skin effect to be entirely spurious. The Hall coefficient in copper is 5 orders of magnitude smaller than in Silicon, for example, and the Hall effect in Silicon is already pretty weak. The Hall Effect also does not increase with the square root of frequency, if at all.

In a coaxial cable the inner is coupled very tightly to the outer by the mutual inductance, the mutual inductance being equal to the self inductance of the inner conductor. If I first energise the inner circuit with an AC signal the resulting voltage drop from end to end will depend on the product of current and impedance. At high enough frequencies the reactive portion must ‘win’. I think the self inductance is relatively constant so the reactive drop increases linearly with frequency. The resistive drop is only increasing as the square root of frequency due to the skin effect.

Most of the drop is in the self inductance, but this couples to the outer conductor due to the mutual inductance acting as a 1:1 transformer. There is therefore the same volt drop in the outer conductor, both magnitude and phase. When I now connect up my second circuit, having a generator with the same magnitude and phase as the first generator, which circuit wins? Well the outer conductor has less self inductance and less resistance, although the difference is relatively small.

I started off with a current creating a voltage, now I prefer a voltage creating a current. The current that would be created in the outer is greater than the current that would be created in the inner if the circuits were only present one at a time. If the outer current flows, the volt drop created across the mutual inductance is then greater than that which would have been created by the inner current, and I have already argued that most of the volt drop is reactive. I think the inner circuit impedance is therefore bootstrapped, increasing the inner impedance by orders of magnitude. No current will therefor be allowed down the inner conductor; it has a built-in generator backing off any applied signal.

The easiest option must be the magnetron.

50% efficient or not, they are pretty robust, a lot more so than any semiconductor you are likely to find.

To obtain the equivalent amount of power from semiconductors, it appears to be necessary to use many low power (100W?) stages and use combiners to sum the outputs.

By the time you take the losses in the combiners into account, I'll bet my left leg that there isn't much difference in efficiency compared with the magnetron.

Also 2.4GHz magnetrons are cheap, which is not something you can accuse power RF silicon of being.

See if you can find anything on the web about semiconductor radar transmitters.

rgds
Zeit.

"Can you use 2.45 GHz instead of 1.0 GHz?"

I don't see why not, but my concern here would be the safety implication of using microwave frequencies. Is anything in the GHz range dangerous or does 2.45GHz require extra precations?

How do antannae work at these frequencies? I have never been quite sure how microwaves are launched. I may need to study Maxwells equations in greater detail.

"I believe your association of the Hall Effect and the skin effect to be entirely spurious...
...No current will therefor be allowed down the inner conductor..."

Well explained, i see your point. So any conductor may as well be hollow then. To force current down a thin central wire requires infinite voltage. This is bad news, I may have to consider superconductors too - yuk! Is there any way to maximise the conducting area? This could well force me down to lower frequencies.

OK, howabout oscillators? I'm still interested in the emitter coupled switch with push-pull transformer coupling from both collectors, you mentioned. Got any circuit details for this? I would still like to read up more on this topic. This too will limit the frequencies I can play with. Maybe I need to think about lower frequencies first.

Thinking a little more about this, I may be able to use magnetrons. They are the most cost effective option. I'll need to do some serious reading on design an application. In particular I need to understand in some detail how the waves are launched into any kind of conduit.

Any suggestions? Books etc...

There isn't much difference in danger between 1 GHz and 2.45 GHz - both are equally dangerous at 1kw level.  Do a Google search on "Safety Code 6" to find an excellent reference on human safety w.r.t. RF.

For this sort of application you'll likely be forced to stay on ISM frequencies.  Even with very good shielding, a kilowatt would have the potential to cause plenty of illegal interference to other communications users.  It is a bad assumption that you don't have to worry about frequency assignments.

You shouldn't have to go back to Maxwell's Equations to independantly derive antenna design.  I think that you'd benefit from a more recent and more practical reference.  Search on ARRL and RSGB (for examples) and have a look at their quite practical guides to microwave.

Here's another good read on the subject of patents:
www.tinaja.com/glib/casagpat.pdf

Another option not discussed is a resonant loop of waveguide using circulators and a low powered source. The low power CW source circulates in the ring, accumlating power. I have seen this for testing components at power levels not available, the actual numbers were quite impressive. It has been a while and I do not remember the details, but it is real, no bull.

Agree about the patent thing VE1BLL. Thanks for the refs.

Interesting GOTWW. Got any papers links, books etc?

I started to look at the IEEE periodicals site, but apparently my sub has expired, I know there is some ref's out there.

The one I saw was (millimeterwave) many wavelengths around.  (I believe) It was feed from one port of a circulator, with the other two ports connected to the loop. The trick was I believe that the source port was coupled to the circulator input using a small iris. therefore when the signal tried to exit this port, the majority of it bounced off and continued going around the loop. I never had a chance to analyze it, and am alittle confused about the the true effect of the iris. The fact that the system is resonant must have play a part. I have seen a paper, so I know one exist.  Hope someone can clear this up so we will both know.

http://www.lns.cornell.edu/public/ERL/2003/ERL03-15/ERL03-152.pdf

http://www.astec.ac.uk/rf/resonant%20ring%20poster.pdf

basic google search

Thanks GOTWW - Interesting read.

I'm very interested to know what EM simulation packages are recommended. Seems to be the norm for this sort of research now, and has to make life a lot easier than using the Maxwell equations directly - even though my vect calc is ok...

Ansoft and HP HFSS is pretty much the industry standards for waveguide systems, but the license and price are a bit steep. There are others, but am really only familiar with HFSS

By choosing 2.45 GHz, you have made your application very similar to many other high power industrial microwave applications. Do a search on "Industrial Microwave Heating". You will find many companies that do development of high power microwave systems for non-communication purposes.

Thanks Comcokid. This forum really is every bit as usefull as i had heard! Changed my approach to the problem completely!

Howabout freeware for EM simulation? Are there any limited versions i can get hold of for some initial design studies? If i am going to microwaves I will need to (potentially radically) rethink my initial design. May very well work better, but will take more doing.

High power RF is most efficiently generated with a valve operating hard into class-C.  In fact a big triode will oscillate so happily you'll be hard pressed to stop it.  An output network which uses a tuned line made of silver plated copper tube, grounded at one end and with the low Z connection being a tapping point close to the ground should be quite efficient.

Reducing the losses in the impedance matching network to feed your low impedance load will be helped by lowering skin effect through using the most conductive metal available such as hard silver plating.  Copper plating is OK but not as good however both metals suffer from a rise in skin resistance if surface corrodosion develops, particularly copper as its oxide is a semiconductor.  Some kind of surface stabilisation might help but few products are suitable for high power 1GHz use, possibly a thin PTFE coating but there are probably laquers available as well but I don't know which.  Use only ceramic insulators and air spaced capacitors to minimise your losses.

The military disposals market does have some sources of fairly powerful 1GHz RF that while not suitable for your final product might be a useful development aid.
Eg, an AN/APX-76 IFF Interrogator generates between 2-8KW pulse power at either 970 or 1030 MHz, I cannot remember which.  Similarly ex military TACAN beacons use Klystrons producing many KW of pulse with a high duty cycle at approx 1GHz.  The klystron could be reconfigured as a CW amplifier if you need CW.

Interesting BrianR, some very useful pointers...

Any books I should read to get me up to speed on this sort of design, using valves in particular? Do you have any links, for sites dealing in military disposals market?

Very useful design and construction data is found in the ARRL Handbook.  Also (buy as well as - ) their specialty handbooks, http://www.arrl.org/catalog/?category=VHF%2FUHF%2FMicrowave&words=

EIMAC Industrial RF heating tubes at http://www.cpii.com/eimac/index.html
Application note "Care & Feeding of Power Grid Tubes"

http://www.qsl.net/kk5dr/Tubes.html

1296MHz 500W amplifier http://www.qsl.net/dl4mea/23gi7.htm

Some really useful info BrianR - Thanks.

The overiding impression I am getting is that I was way overoptomistic to assume I could use transisors, even GaAs MOSFETS. Howabout designing a fixed frequency power resonator? Is there a better method than simply using feedback though a Power amp designed for signals? I'm not overly familiar with valves, so don't know if there is any advantage in running them off --> saturated --> off...
I appreciate that this is what magnetrons really do anyway.

You could use MOSFETS or nearly any other type of transistor to do this job, with enormously varying degerees of difficulty.  However the desing is not something a competant engineer could do even with a good understanding of the maths, as there is a lot of practical stuff that only comes with experience.  The effects of stray reactance, changes in material properties vs frequency and power, heating effects and the behavior of RF currents on surfaces must be well understood also.

You do not need feedback around an amplifier, (oscillating amplifiers are very unstable with regard to frequency), you could drive an amplifier with a low power oscillator and have fine frequency and power control.  

Valves work very efficiently in Class-C transferring huge gulps of power to the resonant load and relying on the flywheel effect of the load to sustain the waveform.

Yours is an industrial heating task which places varying low impedance loads on the RF power source and this further complicaes the design.  All in all I would suggest that you should see what you can buy for this job.  It will be less costly in the end.

Thanks BrianR, you really are helping me to think this thing through...

A few years ago I bought a 500 watt average power solid state S band power amplifier. It cost $800,000, was 30% efficient, and weighs 120 pounds. I sat it next to our microwave oven which put out 600 watts average, 40% efficient, and cost $90. (I also developed tube transmitters.) Of course the magnetron is not a coherent amplifier, but you don't care in your application.

Another trick we used to do is a resonant ring, where you add power into a loop of waveguide with a circulator, and it just builds up to infinity (minus the losses) instead of dumping into a load at the end of your test cell. It was fun at megawatt levels. This could reduce your power requirements 100X.

BTW IFF systems interrogate at 1030 MHz and respond at 1090 MHz.

Final advice: forget Maxwell, get the ARRL handbook on microwaves.

I guess I would go the Maggie route, however if you wish to eschew the lowly magnetron, consider a reflex klystron.

The advantage of the magnetron is that it is a high powered oscillator and only needs a DC supply for the B+ and an AC filament supply.  Simplicity of design.

A reflex klystron is also a high power oscillator, but the power supply requirements aren’t as simple as for the magnetron.

Other systems, like IOTs or TWTs require more and more complex external support electronics.

Yes, it all could be done with solid state; however at frequencies over .75 GHz, the efficiencies of high power hollow-state devices begins to exceed those of their solid state brethren.

High power UHF TV transmitters are usually klystron amplifiers driven by solid state exciters.

I remain,

The Old Soldering Gunslinger

Electlect (Graham),
I've used a microwave oven before as a power source. I didn't need the full power so I placed an antenna inside the oven to couple off power at various levels. If you need full power, you would attach a waveguide to the inside of the oven and easily bring out the energy.
The opening of the waveguide is 2.84" WIDE X 1.34" tall for WR284, although you may need a slightly larger size like WR430 (4.3" wide, 2.15" tall) to better match the opening in the oven where the energy exits.

You'd probably get over one kW our your waveguide. Trying to operate it in an "enclosed" area may reflect too much power into the oven and damage it. You must be concerned with that reflection.
kchiggins