Talk:Cavity magnetron/Archive 1

This is an archive of past discussions about Cavity magnetron. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.

Archive 1

Corrected a common myth concerning the heating modus operandi of the microwave oven. It is not just the water molecules that heat up, but other materials as well. In fact if you overcook a water bearing food product, once the water evaporates off, the temperature increase is much more rapid until the product actually catches fire. It is the molecular vibration due to dielectric loss that creates heat. Water happens to have a relatively high diectric loss, but is also capable of easily absorbing the heat produced.

Added some more historical info and corrected errors. I have to say, Wikipedia suffers from not having authors who know obscure technical information. The magnetron's history is easily researched.

Regarding the above comment, a look at the page history will reveal that it has been considerably cleaned up and improved from its original form (e.g. electron "sloshing"). My changes were based on improving the article from personal knowlege and training (late 1950's Navy tech school), as I understand that it is a wikipedia way to add improvement where they are needed. It was not my intent to write the killer historical article on the magnetron, and without the preceding authors I would have had no basis to start from and would probably not have attempted to write an article from scratch. In other words, without someone starting this page and its subsequent authors, there would be no magnetron article (unless, of course, the above author wrote the whole from scratch). I expect that others (such as the unknown author above), will add their corrections, knowlege and, if interested and capable, the products of their research. The only way wikipedia will suffer is if people do not contribute their knowlege (even if with some errors or lacking a high literary style) because they are discouraged by comments such as the above.

Will a knowlegable author please contribute the effect of magnet strength upon the output of the magneton? Thanks, Leonard G. 18:07, 26 Apr 2004 (UTC)

It is complicated. Magnetron cavities have many cavities, so they also have many modes. Every magentron that I know of, however, is designed to operate in the pi mode, where the phase of the RF electric field switches by 180 degrees each cavity. When operating in this mode, the velocity of the electrons, which is equal to E/B for DC electric field E and DC magnetic field B (this is the traditional E cross B drift), must be chosen so that the electrons travel past exactly one gap in half of an RF period (the time it takes for the field to switch by 180 degrees). So if you change the B field, you also have to change the E field (by changing the voltage) to maintain synchronism of the electrons and the RF fields. If you don't change the E field along with the B field, you will get mode competition - at some point, the velocity of the electrons will change enough that you will excite a different mode more than the pi mode. This is bad for performance, since typically the most power is produced at the pi mode. In addition, you can get a mix of modes, which makes phase information difficult to use and increases the noise of the signal. Dont think twice 04:25, 16 August 2007 (UTC)

Pehaps somone might mention that kids who fool about with magnetrons scavenged from old microwave ovens tend to get cataracts, burns etc.

What is the diffrence between a "cavity magnetron" and a "magnetron"? Philip Baird Shearer 00:52, 14 Oct 2004 (UTC)

1/3 or a year later and no answere to the question. I am going to move this page to "Cavity magnetron" unless anyone objects with good reasons in the next few days Philip Baird Shearer 22:57, 31 Jan 2005 (UTC)

Gleaned from the reference [1] was a mention of early "two pole" and more later "six pole" magnetrons, incapable of producing significant power. As the "cavity magnetron" is what is most commonly thought of when "magentron" is mentioned, I think that it stands best as it is. Leonard G. 02:47, 10 Mar 2005 (UTC)

Mention could be made of the Japanese invention of the cavity magnetron see "The main reason the Japanese effort remained unknown was because it lagged so far behind Allied and German efforts. Ironically, the Japanese had good technical minds, and had even developed an effective cavity magnetron in 1939, well before the British." http://www.vectorsite.net/ttwiz7.html

There's an episode of the 1977 BBC programme The Secret War called "To See for a Hundred Miles" on YouTube here with both Randall and Boot re-creating their initial experiments with an RCM: [2] - towards the end of this part at 08:43 — Preceding unsigned comment added by 86.112.68.219 (talk) 15:54, 18 June 2011 (UTC)

BTW, the experimental equipment they used was mostly old gear and looks like something out of James' Whale's Frankenstein — Preceding unsigned comment added by 86.112.68.219 (talk) 16:22, 2 July 2011 (UTC)

What is the diffrence between a "cavity magnetron" and a "magnetron"? - the difference between an ordinary magnetron and a resonant cavity magnetron is relatively easily explained. A normal magnetron is like a whistle, where the audio note produced is dependant on the size of the whistle's internal dimensions. A resonant cavity magnetron however is like a whistle with a pea in it, the rotary motion of the pea produces multiple notes of a much higher frequency. In the magnetron the 'pea' is replaced by a rotating electric field that 'brushes' around the centre slots causing the connected cavities to resonate at the required radio frequency, the operating frequency of the magnetron being governed by the physical size of the cavities - the 'cavities' are the multiple circular-shaped holes seen in a plan view. The RCM device is only called a 'magnetron' because like the simpler two-pole earlier types, it operates within a magnetic field. The early RCMs only had a very short life, around a few hours at full power, but because the signal was pulsed the peak power was only used for fractions of a second, so the short life was relative and did not affect the usability of the systems. Early British RCMs were hidden away under the 'Common Valve' (CV) designation system, making them look like normal radio thermionic valves. — Preceding unsigned comment added by 80.4.57.101 (talk) 20:22, 5 August 2011 (UTC)

• Have a look at http://www.radarworld.org/england.html and the de WP article. You are giving too less credit to Heinrich Greinacher and Albert W. Hull.
• "Microwave ovens also produce low frequency electromagnetic radiation from the transformer used to generate high voltage for the magnetron and this may possibly also be harmful, so one should not closely approach the device when it is operating." I would say that is complete and utter nonsense because there are no studies to proof it! Since a encyclopaedia is about facts and not about assumptions I see no point in keeping the sentence. --Paddyez 13:12, 15 May 2005 (UTC)

This is one of those things where people who believe in something don't seem to be swayable by facts; I'm on your side and would support removing the language, but don't be surprised if this becomes a revert war.

Atlant 16:58, 15 May 2005 (UTC)

Hm, I do not really like the idea of that :-( I was trying to go through things with en wp people. I need the article authors to try to stick to scientifically proven facts though first. My intention is not working on stuff, that ends up in an edit war. --Paddyez 23:49, 15 May 2005 (UTC)

this is not just one of these things some people believe and others don't. the point is most electrical devices do have transformators that emit 50/60Hz radiation. this is in no way specific to the magnetron, so it does not make sense to write about it in every article about any electrical device taking its current from the plug. -- ∂ 00:37, 17 May 2005 (UTC)

But not all devices have transformers and the one on a microwave oven is a high current device with lots of output at 60hz. Standing back in any event seems like a good idea to me. By the way, the highest 60hz I ever measured was from a cordless toothbrush stand with the toothbrush out - 56 miligauss! -- 05:04, 17 May 2005 Leonard G.

So what? I know you may burn people alive if you get resonance frequency with the body of a person. I do not know that 50/60 Hz done harm to anyone yet. People who stand next to large transformers that are being switched get a little flicker disturbing the sight but AFAIK have no permanent damage, at least it is not proven. 56 miligauss is nothing compared to a large Transformer used for industrial purposes btw. --Paddyez 11:44, 20 May 2005 (UTC)

Cataracts form in the lens from overheating, not the cornea. The lens contains crystallin protien which is normally clear, but can coagulate if denatured, causing a cataract. The cornea contains some crystallin but is thin enough that coagulation doesn't significantly distort vision. The cornea is also wide and flat, giving it a large surface area in contact with the vitreous humor of the eye and thus relatively good cooling. The lens on the other hand is thicker and thus easier to overheat, causing the crystallin to begin to coagulate. Both the lens and cornea lack blood vessels. --Anuran 01:08, 27 Jun 2005 (UTC)

Nature disputes the accuracy of this article; see http://www.nature.com/news/2005/051212/multimedia/438900a_m1.html and Wikipedia:External_peer_review#Nature. We're hoping they will provide a list of the alleged errors soon. —Steven G. Johnson 01:51, 15 December 2005 (UTC)

The article on microwave ovens claims that Litton introduced a no-load oven in the 1970s -- that is, the oven can be turned on with no food in it and not suffer damage. This article claims otherwise, suggesting that modern devices still need a sink. Which is correct? Maury 14:45, 17 December 2005 (UTC)

The results of what exactly Nature suggested should be corrected is out... italicize each bullet point once you make the correction. -- user:zanimum

• Microwave ovens were not completely unanticipated before World War II. Radio-frequency heating of foods had been tried on an experimental basis, and I have heard anecdotally that Germans were experimenting with microwave cooking before 1940.
• While the mass production of magnetrons was of great benefit to the Allies, German and Japanese radars were also developed. If the history section contains material on the Allied effort, mention should be made of the Axis work (at least a sentence).

The General electric company of england (Now Marconi) was NOT the same firm as General Electric of the USA. This is a common misconception, due to the simalarity of the names.

cmacd 17:16, 2 June 2006 (UTC)

Radio frequenct heating was used almost as soon as electronic tubes were developed. One step in making most tubes is to use an RF heater to heat the elements red hot to drive off gas before the pumping is finished

Pre war RF heaters were also used for medical purposes

I am not sure if heaters running below Microwave frequencies would offer even enough heating for food.

cmacd 17:21, 2 June 2006 (UTC)

It is said that the phyisics of electromagnetic wave formation inside the magnetron cavities are among the most complicated areas of classical physics both from theorectical and practical viewpoint. Only the most experienced scientist-engineers can design a reasonably efficient magnetron. Is that true? 195.70.32.136 15:56, 11 August 2006 (UTC)

I'll be the first to admit that I don't design magnetrons, but this claim sounds dubious to me. What part of their operating principles is difficult to explain or understand?

Atlant 00:14, 13 August 2006 (UTC)

The claim is only half true. Designing a magnetron is pretty straightforward, and it doesn't take a rocket scientist to do it. However, the physics behind the actual functioning of magnetrons is very complicated, to the point that it is nearly impossible to provide an accurate model of magnetron performance that doesn't rely on empirical data (very different than most vacuum microwave devices, which can be modeled easily). Most modeling of magnetrons is done with particle-in-cell codes, which is basically a brute force method of pushing electrons around to see what happens.

There is a famous quote by the inventors of the cavity magnetron, Boot and Randall: "If this point [the possible operating conditions of a magnetron] had been appreciated in 1939 it is doubtful that the magnetron would ever have been made." Basically, if they had understood how complicated the magnetron actually is, they never would have bother to try to make it work. Dont think twice 04:25, 16 August 2007 (UTC)

Cavity magnetrons(Or travelling wave type) negative resistanve type and cyclotron frequency type

Under health hazards it says some magnetrons contain beryllium oxide ceramics, sometimes purplish color like in the "pictures above", however one shows a magnetron from a domestic microwave oven. Do those really sometimes contain beryllium? There's http://www.defra.gov.uk/environment/waste/topics/electrical/pdf/weee-aeatreport-june06.pdf which says "As discussed in section 4.1, the ceramic material around the filament in a magnetron may contain beryllium, but whilst this may be used in large radar magnetrons it is unlikely to be present in the types of magnetron found in domestic microwave ovens. Analysis of the filaments from two microwaves showed that neither filament contained any beryllium." So anyone have a reference for the contrary? Anyone from e.g. Witol mind sharing this info? —The preceding unsigned comment was added by 213.243.189.251 (talk) 19:12, 3 January 2007 (UTC).

I'm not sure I understand your point. Both photos show the ceramic noticeably coloured (so, presumably, containing beryllium).

Atlant 01:19, 4 January 2007 (UTC)

One can make ceramics coloured or color coated any way you want. BeO itself is white (example insulator pics e.g. Google Images). Generally insulators and RF components etc that contain Be or BeO are clearly labeled so (at least IME, in the EU).

The DEFRA weee-aeatreport-june06.pdf report Figure 3 page 18 (pdf page 26) shows the same consumer microwave oven magnetron with pinkish insulator as in wikipedia, and DEFRA found there was no beryllium in the insulator nor in the filament.

So i'm suspecting, wikipedia article saying/implying that consumer magnetrons contain Be might be just repeating an old internet myth. Unless there's some hard reference to the contrary, then, might change article to warn about high power magnetrons and Be labeled ones instead?

jwagnerhki

I opened my old microwave when it broke and it's magnetron looked almost similar to the one on this page (the one with pink thingy). And it doesn't say anything about beryllium anywhere. I however put some electric tape over the pink ceramic part just in case. 84.250.45.172 15:26, 23 July 2007 (UTC)

• Briefcase 'that changed the world'
• http://news.bbc.co.uk/2/hi/science/nature/6331897.stm
• --Ling.Nut 00:45, 6 February 2007 (UTC)

[3] Interesting story on the garget —The preceding unsigned comment was added by Wk muriithi (talk • contribs) 13:49, 6 February 2007 (UTC).

Um, it looks like the wikipedia article just lifted a few sentences from this BBC story. Seems like plagiarism to me... maybe they should be reworded, and the article cited? --140.247.171.79 16:09, 6 February 2007 (UTC)

I seem to remember from an interview with a bod from the Science Museum (London) on TV some time ago that an 8-cavity version was sent to the US during WWII as that was all they had available while the British were working with a 6-cavity version, both of which functioned perfectly. The Americans were concerned that the documentation and the device did not match. I believe that throughout the war it was possible to tell the original manufacturer by the number of cavities. I am not sure if this is interesting enough to include and am uncertain of where to get confirmation. Purple Aubergine (talk) 21:14, 5 October 2008 (UTC)

IIRC, one of the early hand-built prototype magnetrons had been built using the chambers (cylinder) of a Colt revolver as a simple drilling guide for the multiple cavities in the anode block. When a proper method of manufacturing had been developed the number of cavities had been revised for the later prototypes, presumably to the eight that you mention. — Preceding unsigned comment added by 80.4.57.101 (talk) 23:02, 8 April 2012 (UTC)

Some info on the UK and later US magnetron development here: [4] (pdf) — Preceding unsigned comment added by 80.7.147.13 (talk) 16:46, 6 July 2012 (UTC)

removed: This is far more efficient than the klystron, which typically operates at around 30% efficiency.

This just isn't true. Any modern klystron designed for a single frequency will achieve around 65% efficiency. I think modern magnetrons have much higher efficiency, approaching 95%, but I dont have the data on hand to back that up. Dont think twice 04:25, 16 August 2007 (UTC)

small contribution on the topic: a magnetron can in some circonstance egmite x-ray throu 2 differante process ,(I guess).If power with hy voltage (above more less 40 kv)throu electron colition on the copper surounding and possibly by cynclotron radiation du to the facte that the electron path is bend by the magnetic feeld.the first hypotese is proven(I did the experiment in hi scool) at 120 kv the magnetron was emiting a fair amont of hard x-ray mainly escaping from the ceramic.note:with this amont of voltage the magnetic feeld provide by the permanant magnet are probably not bending the path of electron any more very much and remouving them was providing similar result.for the second hypotese it's just a guess I am making.if any body know anything about this topic feel free to complet or modify my alegations. —Preceding unsigned comment added by 82.92.34.118 (talk) 14:08, 11 November 2007 (UTC)

The article claims that the USA created H2S airborne radar. This is factually wrong. —Preceding unsigned comment added by 77.123.173.183 (talk) 07:58, 3 August 2008 (UTC)

Due to a lack of sources to establish independent notability for James Sayers (physicist), I propose that the entirety of the article be merged into an appropriate section in this article. I can find no non-trivial sources that would help expand the Sayers article; he is notable only in relation to his work on the cavity magnetron and the information available is not sufficient to create a full, neutral biography on this individual. There is nothing currently there that would not be appropriate if stated in the history section of this article. If there is consensus to do so, or if no one comments within a week, I will undertake the merge myself. Cheers, CP 16:56, 15 September 2008 (UTC)

There is currently discussion on this at Talk:James Sayers (physicist). Cheers, CP 20:48, 24 September 2008 (UTC)