Phase locking a microwave oven
for EME use?

The following discussion about the posibility to phase locking a microwave oven for  EME use took place in the Moon-Net reflector. I found the subject so interesting that I decided to put together all the messages in this page.

On 12 October 1998 N4IP wrote:
Has anyone tried phase locking a microwave oven for EME use. I've heard of people using them for AM transmitters and other uses. Where do I hook up the keyer????

On 13 October 1998 OH2AUE wrote:
Haven't tried it myself, but there has been at least one article on phase locking a microwave oven magnetron by controlling the cathode voltage ( anodes are usually grounded ) via a high voltage valve voltage regulator. The principle is based on the typical power supply pulling of the magnetron frequency. The tuning range is quite small, so the magnetron cannot be pulled far off from its nominal frequency of 2450 MHz. Plenty of CW power though !!!

Keyer ? Thats a tough one. I have used two types of PLL oscillator modulation to achieve "CW sounding" signals from microwave beacons; one is to FM modulate the carrier with noise when the "key is up" to spread the carrier over a wide spectrum and the other is to FM modulate the carrier with a crystal controlled 10 kHz audio source at such a modulation index that the carrier power is distributed into sidebands at the first Bessel Zero. Crude, but sounds good !!!

BTW. If you have a suitable circular waveguide horn with a phase locked magnetron plugged into the side of it, you can fit a second, free running magnetron probe onto the opposide side protruding into the same electric field and this will spontaneously injection lock onto the PLL controlled carrier for 3 dB more power.....

In fact, you could couple several magnetrons onto a longer section of waveguide to.....oh boy, I better stop....

On 13 October 1998 G8WRB wrote:
I've not tried it, but in principle a small stable signal at the frequency of interest, injected into the cavity (small loop), should cause the magnetron to lock to the stable signal.

I assume you would have to switch the HT on/off to key it. My guess is that if the HT is turned on, it would take some time to lock to your stable source, during which time the frequecy could be anywhere.

These are just some ideas - I have not tried any of them.

On 13 October 1998 GM4JJJ wrote:
I find the easiest way to key one is just to open and close the door :-)

On 13 October 1998 AL7EB wrote:
>Keyer ? Thats a tough one. I have used two types of PLL oscillator modulation
>to achieve "CW sounding" signals from microwave beacons; one is to FM modulate
>the carrier with noise when the "key is up" to spread the carrier over a wide
>spectrum and the other is to FM modulate the carrier with a crystal controlled
>10 kHz audio source at such a modulation index that the carrier power is
>distributed into sidebands at the first Bessel Zero. Crude, but sounds good !!!

BTW this method of producing cw was originated (or at least promoted) by the San Bernardino Microwave Society, years ago, for freq. locked polaplexer klystrons (recently gunnplexers). The key was input to a logic inverter so that (as you stated, above) when the key was up a tone was modulated on the oscillator control voltage at a modulation index to produce a nulled carrier and wide sidebands, but when the key is down the modulation is removed and the oscillator is not modulated thus producing a pure carrier. The advantage of course is that the oscillator is not started and stopped thus staying in lock and stable. The signal to the receiving station is passed thru a narrow filter so that the wide fm tone is not detected; only the carrier.

>BTW. If you have a suitable circular waveguide horn with a phase locked magnetron
>plugged into the side of it, you can fit a second, free running magnetron
>probe onto the opposide side protruding into the same electric field
>and this will spontaneously injection lock onto the PLL controlled
>carrier for 3 dB more power.....
>
>In fact, you could couple several magnetrons onto a longer
>section of waveguide to.....oh boy, I better stop....

My boss, Dick Kolbly, when I was working at the Goldstone Tracking Facility (NASA), experimented with using oven magnetrons to produce a cw carrier. He's a member of the SBMS, still, and you may be able to contact him thru the SBMS web page. As I remember, he was able to generate a fairly clean signal with them.

On 13 October 1998 K2LME wrote:
Communications Quarterly's Winter 1991 issue has an article by WA6EXV describing his 13 cm magnetron implementation.

On 16 October 1998 S57UUU wrote:
Many years ago, I came across an article (I think it was in the IEEE proceedings) about orbiting solar power stations. One of the ways considered to transfer power to Earth was an active phased array of millions of KW class phase locked magnetrons. In this article, the author(s) wrote that a cooking magnetron produces quite a clean signal, when fed with smooth DC. This made me think, but I haven't had the required rectifier, WG couplers, QRO loads etc etc.

About a year ago, Pavle S57RA upgraded his 'Frankenstein laboratory' with some HV transformer brutes, so we decided to try this out.

The basic circuit was a 5kV conventional rectifier with an improvised constant current source made with an GI-7 russian triode. The magnetron was a Toshiba 2M240, connected to a standard waveguide with tuning screws.

For phase locking, we tried to use the circuit from the S53MV 13cm satellite receiver. It has about 2MHz reference frequency with a few hundred kHz of loop bandwidth. The magnetron was used as a 'CCO' (current controled osc).

The results:

(Note that this magnetron has 2460 MHz nominal frequency, so results would be better with a 2450 MHz type)
The frequency can be changed both by the impedance that the magnetron 'sees' and by the current. (DC-wise the magnetron is almost like a 4kV zener diode, so it must be connected to a current source, not a constant voltage supply!) By both methods, single or combined, the frequency can be pulled down to abt 2415..2420 MHz where the magnetron starts to 'mode' (the single line spectrum changes to a forest of crazy dancnig lines). We haven't tried to change the DC magnetic field of the permanent magnets.

At the 2424 japanese band, it gave abt 100W of power at 50..60mA, at 2434 abt 250..300W at cca 150mA and at 2460 it gave 800W at 350..400mA.

The spectral line width of the unlocked magnetron was abt 1..2 MHz, sitting on a wideband (10's of MHz) 'noise hill' some 30-40dB down.

The noise gets worse as you pull the magnetron away from its design frequency. The noise seems to be generated inside the magnetron, you can see it if you monitor the DC current through the magnetron with an oscilloscope. The height of the grass on the 'scope grows as you insert the tunning screws.

We weren't able to get a phase lock, the magnetron was simply jumping around too fast for our PLL circuit to catch it. The circuit we used is a 'divide down' PLL, and it was to slow. With a 'mixdown' PLL with a MHz or so loop bandwidth things might have worked. Also the current source should have very high impedance and fast regulation. A choke is probably no solution, since it would slow down the PLL. Also, removing the heater filter caps in the magnetron case base might help.

About modulation:
The best solution is to use FSK. If the shift is a kHz or so, the receiver can be tuned to hear only the 'mark' frequency - it will sound exactly the same as a 'real' CW transmitter. If tuned to 'space' frequency, the receiver would hear 'inverse morse', but I think that's no problem. In fact, the communication theory says that a receiver using BOTH frequencies could gain up to 3dB in sensitivity. (if we don't switch the TX off between dots and dashes, it is better to USE that energy than to throw it away by
spreading or noise modulation.)

I had some slides and circuits with me at the Paris EME conference, but the time schedule was so tight, that I forgot about it...

On 19 October 1998 AL7EB wrote:
This was the project we were involved in, that I commented earlier (10/13/98) about. One thing I remember: Dick (Kolbly) experimented with the magnetron by turning off the filament supply after initial oscilation began with a resulting narrowing of the output noise spectrum (apparently the filaments continued to be heated by back bombardment). I was a jr. engineer at the time and my part was designing and building a w/g transition to couple the Litton magenetron to our (NASA-DSN) standard WR-430 w/g. Soon after I was transferred to the Microwave Receiver Group and wasn't involved in the tests. Could you list the IEEE article/proceedings issue? I would be interested in obtaining a copy of them.


On 15 November 2004 LU8YD wrote:
I am looking information about 23 cm and up amplifier design using microwave oven magnetron.

On 15 November 2004 WA6PY wrote:
Hi Alex, Microvawe oven works at 2.45 GHz. I've seen description of the injection lock amplifier for 13 cm band using magnetron. I was probably in QST many years ago, but I do not have a copy. I hope this helps to find a copy of this interesting paper.

On 15 November 2004 KJ6HZ wrote:
I think it was in 73 magazine. The December issue usually had an index of articles for the year which should make it easier to find the article.

On 15 November 2004 K2TXB wrote:
In February of 2003, Frank, WB6CWN, wrote in the Microwave Reflector:

> Jerry, Ken and all, 
> At the last meeting of the San Bernardino Microwave 
> Society, microwave pioneer Chuck Swedblom WA6EXV received 
> a Lifetime Achievement Award. On the long list of Chuck's 
> designs mentioned was an injection locked CW keyed 
> 2.4GHz microwave oven multiple cavity magnetron. He 
> used it with a waveguide made from Sears metal drain 
> pipe in a near kw eme generator in the 70s to get huge 
> signals back from the moon. There was an article about 
> the oven generator in either Ham Radio or CQ at the time 
> with a picture of the business end of the drain pipe as 
> I recall. The design produced a pure signal when locked, 
> and it was as ingenious as you could imagine. Anyone 
> interested today can e-mail Chuck to follow up on the details. 
> Frank WB6CWN

Hope this helps, sorry I have no email address for WA6EXV.

On 15 November 2004 OH6NVQ wrote:
Marko s57uuu has some interesting stuff on his site ! http://lea.hamradio.si/~s57uuu/mischam/magnetr/index.htm

On 15 November 2004 GW4DGU wrote:
A very long time ago, I was involved in my work with 2.45GHz magnetrons used for medical hyperthermia (ie. microwave heating, not hypothermia, which is what happens when you get cold for an extended period of time...)

My feeling, based on a few games, which I was able to play while my boss wasn't looking, is that it's not easy to electrically tune a magnetron. So, phase locking isn't really practicable. You'd expect that from the physics. It would be entirely possible to reliably injection-lock a magnetron using a few watts and a circulator, but it wouldn't be possible to move it very far in frequency. Injection locking has been well understood for seventy years, and (as the so-called 'Goyder lock') was used in amateur HF transmitters in the 1920's and '30s.

If we were to move 13cm EME activity to close to 2.450GHz, cooker or industrial magnetrons could make an attractive PA, particularly for constant envelope modes. Tubes with power outputs of up to several kW are easily available, and the power supplies aren't difficult. Attempting to 'bend' them, in the way we used to with low-power klystrons, (anyone remember polaplexers?) to the existing EME frequencies isn't likely to be fruitful.

FWIW the UK licence allows operation at frequencies up to 2450MHz, although QRM from ISM sources might be a problem!

On 15 November 2004 F6DRO wrote:
This amp was described in the QEX magazine , one of the 1st issues. Be careful , the oven magnetron used is not running in EME band...

On 14 February 2006 VK4KKQ provided the following information and articles. Many thanks!

On 16 July 2007 Barry Priestley (ex G3JGO) wrote:

A lot of people don't seem to know that if a moderate VSWR won't PULL an oscillator on to the desired frequency then injection locking with a sensible amount of power won't lock it there either.

The saying was "Goyder Lock is fine; until you lost the key ! " i.e. to lock successfully the locked oscillator needs to be working at a very low Q say 5 or 10 and so unstable and pullable. I suspect a cooking magnetron has a higher loaded Q than this ; maybe one could alter the load coupling to reduce the Q, but perhaps this would cause loss of power.

On 7 August 2016 Bruce Tiemann / N6URH wrote:

Because of their use in consumer electronics, an effort is made to make magnetrons as safe as possible. The entire outer metal structure, including the vanes for the heat sink, are all grounded. Only the two terminals for the filament are at high DC voltage, and of course the probe where the microwaves come out is energized at RF in operation.

But the vanes contain ferrite magnets, and all that stuff is grounded. It should be possible to wrap a few turns of wire around the magnets, an equal number at each end, and send in a DC current which makes a DC magnetic field which, depending on the sign of the current, either adds to or subtracts from the field the magnets make, in the spirit of the FM coil in a YIG oscillator. This would alter the cyclotron frequency of the electrons, and change the frequency they would by trying to make. Of course the cavities would not be changed by this operation, and so gross changes in operating frequency would still be impossible to make, but it is possible that the frequency of mode splitting could be shifted, and, in any case, the instantaneous operating frequency could be changed within the controllable bandwidth as quickly as the current in these coils could be changed - which might be at 10 kHz or 100 kHz rate. Sufficient for phase locking without use of an injection signal, and much more quickly (and/or safely) than attempting to make high-speed adjustments to the HV input. Noting again that the location the coils would be wound are all near grounded metal, and so, not especially risky on that particular account.

Has anyone done this?


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1 user comments


#1 - Victor - 05/Feb/2010 - 21:27
What are the potential dangers of doing such a thing? Sounds cool though.

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