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Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Are you saying that they are completely different and that the electromagnetic wave will travel differently in a resonant cavity filled with air rather than one evacuated? I am under the impression that the cavities in the magnetron function in the TE mode to supply the electric field for the electrons to rotate. Any and all help is greatly appreciated.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Coronafix wrote ...
Are you saying that they are completely different and that the electromagnetic wave will travel differently in a resonant cavity filled with air rather than one evacuated?
Well, off the top of my head, the wavelength of 2.45GHz radiation is ~12.5 cm in air and ~3 cm in the human body. The figure for a vacuum is different again, but without googling I'm not sure of the exact figure. I assume it is greater than 12.5 cm.
I believe that this is related to the fact that light travels at different speeds depending on the medium (both are EM radiation, or photons)
Coronafix wrote ...
I am under the impression that the cavities in the magnetron function in the TE mode to supply the electric field for the electrons to rotate. Any and all help is greatly appreciated.
It's the magnetic field that causes the electrons to rotate in the magnetron.
I'm also trying to learn about this subject, the one thing that immediately suggests (to me) that what you propose is incorrect is that, for example, a quarter wavelength waveguide for 2.45 GHZ in air is around 3 cm diameter, whereas a magnetron cavity in a vacuum for 2.45 GHz is around 1.2 cm diameter.
At first glance it wouldn't appear that the formula you found relates to magnetron cavities, but relates instead to waveguide cavities. (the apparatus in the paper certainly resembles a waveguide)
I don't claim to know any more than you do about this subject, that is just my 2 cents' worth.
EDIT: From what we already know from what was posted above, any formula for magnetron cavities would presumably take current and load into account, as well as the other factors.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Coronafix wrote ...
Does anyone know what the formula is to ascertain the operating frequency in a magnetron cavity? I know that the outer of the anode is the inductive part and the cavity openings create the capacitive part, but I can't seem to find any design formulas of sizes and geometry vs operating frequency. Thanks.
You need a cylindrical resonant cavity calculator.
Let's hope this zip file will get you sorted!
But there's always an empirical element - known as tinkering in scientific jargon - to get your theoretical design to resonate at the frequency you want in real life.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
You need a cylindrical resonant cavity calculator.
Let's hope this zip file will get you sorted!
But there's always an empirical element - known as tinkering in scientific jargon - to get your theoretical design to resonate at the frequency you want in real life.
Is the .M file a MATLAB file, PM?
If so, it looks like I'll have to pay $89 to open it.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Proud Mary wrote ...
You need a cylindrical resonant cavity calculator.
Let's hope this zip file will get you sorted!
But there's always an empirical element - known as tinkering in scientific jargon - to get your theoretical design to resonate at the frequency you want in real life.
Is the .M file a MATLAB file, PM?
If so, it looks like I'll have to pay $89 to open it.
Oh dear! But I suppose if you're going to start machining precision cavities in copper stock, $89 will only be the start of your expenses.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Proud Mary wrote ...
You need a cylindrical resonant cavity calculator.
Let's hope this zip file will get you sorted!
But there's always an empirical element - known as tinkering in scientific jargon - to get your theoretical design to resonate at the frequency you want in real life.
I believe that the formula for the cylindrical resonant cavity is in the paper I posted, but it does indeed seem that the space between anode and cathode plays a part in frequency also. I asked a lecturer about this as well and he told me a story about a WW2 english plane that was shot down and the magnetron removed to back engineer, I think he said it was the the russians. Anyway, they found that the strapping had all kinks in it, and when they copied it they made the strapping straight and hence couldn't get it to work. By kinking it, they were changing the inductance slighty on each sector and hence tuning it. This is the tinkering that is needed, but it pays to start somewhere in a dimensional sense. I want to thank you Ash and Stella for all the help provided so far. @ Ash, my mistake, of course the magnetic field provides the rotation, I should have said acceleration or being ac, the velocity modulation. Therefore the field must be perpendicular to the cavity axis or parallel to the opening/throat. @PM, I also found that zip file but didn't bother as it is a calculator rather than a formula, I like to see what I am working with. Indeed the copper stock is expensive but the tools to machine i already have.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Coronafix wrote ...
Proud Mary wrote ...
You need a cylindrical resonant cavity calculator.
Let's hope this zip file will get you sorted!
But there's always an empirical element - known as tinkering in scientific jargon - to get your theoretical design to resonate at the frequency you want in real life.
I believe that the formula for the cylindrical resonant cavity is in the paper I posted, but it does indeed seem that the space between anode and cathode plays a part in frequency also. I asked a lecturer about this as well and he told me a story about a WW2 english plane that was shot down and the magnetron removed to back engineer, I think he said it was the the russians. Anyway, they found that the strapping had all kinks in it, and when they copied it they made the strapping straight and hence couldn't get it to work. By kinking it, they were changing the inductance slighty on each sector and hence tuning it. This is the tinkering that is needed, but it pays to start somewhere in a dimensional sense. I want to thank you Ash and Stella for all the help provided so far. @ Ash, my mistake, of course the magnetic field provides the rotation, I should have said acceleration or being ac, the velocity modulation. Therefore the field must be perpendicular to the cavity axis or parallel to the opening/throat. @PM, I also found that zip file but didn't bother as it is a calculator rather than a formula, I like to see what I am working with. Indeed the copper stock is expensive but the tools to machine i already have.
The thing is to make the cavities slightly too small - so that you can lower the frequency of resonance to the desired point by means of a micrometer tuning slug, and alter it once again when you match it into a load, a backwards and forwards empirical process easily mistaken for tinkering.
I'm having problems getting Octave to install. I think I've downloaded the right version. I'm running Windows Vista with Norton, but Norton doesn't seem to be blocking it.
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Magnetron cavity formula
