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Helical antenna in a magnetron

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altimicus

Sat Nov 06 2010, 11:25PM

Registered Member #3403 Joined: Sat Nov 06 2010, 11:21PM
Location: SÃ£o Carlos, Brazil
Posts: 11

Hello everyone,

I've been reading and watching this forum for a long time, now finally I made an account...
The question is, is it possible to attach a helical antenna on a simple microwave magnetron? is there any other way to generate a high power EM field in helical pattern?

Thanks
Carlos

Proud Mary

Sun Nov 07 2010, 12:02AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

You must first make or buy a waveguide-to-coaxial transition adapter appropriate to the frequency and power of your magnetron.

At the other end of the coaxial line, you will then need to make a matching network to match the impedance of the unbalanced line to the impedance of the axial-mode helical antenna.

The need to ask such very basic questions suggests to me that you should gain experience with low power microwave devices, by becoming a licensed radio amateur, for example, rather than risk injury and interference to yourself and others by illegally transmitting at high power with a magnetron.

altimicus

Sun Nov 07 2010, 12:34AM

Registered Member #3403 Joined: Sat Nov 06 2010, 11:21PM
Location: SÃ£o Carlos, Brazil
Posts: 11

I have a strong theoretical background my undergrad course, but I lack the practical part, that's mainly why I'm here...

EDIT: Mainly with magnetrons since I never got one in my hands and don't know the connections...
And I'll not be transmitting, this is antennas will be in a closed environment manly for heating a fluid flux to plasma temperature...

Proud Mary

Sun Nov 07 2010, 08:56AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

altimicus wrote ...

And I'll not be transmitting, this is antennas will be in a closed environment manly for heating a fluid flux to plasma temperature...

If you'd like to say which of the processes in the paper below is most like your own project, we can take it from there:

H Conrads and M Schmidt Plasma generation and plasma sources Plasma Sources Sci. Technol. 9 (2000) 441â€“454.

Steve Conner

Sun Nov 07 2010, 12:39PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The little probe on top of a microwave oven magnetron is essentially a 1/4 wave antenna, it radiates fairly well into free space or a waveguide. It's not designed for direct connection of any sort of cable.

A waveguide-to-coax adapter is, as far as I know, a piece of waveguide with a hole for this antenna at one end, and another 1/4 wave probe at the other end, hooked up to the centre conductor of a coaxial socket. Maybe there'll be a couple of tuning screws in the middle bit of the waveguide.

A helical antenna won't necessarily generate a field in a helical pattern.

A fusor hobbyist in the USA made a very nice plasma ion source powered by a microwave oven magnetron. You should probably check out his project for some inspiration.

Whenever anyone talks about plasma experiments like this, I ask whether they couldn't simply stick the experiment inside an old microwave oven. Either on its own, or using some kind of metal resonant structure to concentrate the energy into the experiment. It's so low risk that it's worth trying. Worst case, you destroy an oven that you paid $20 for at the thrift store. You should get an old microwave anyway and take it apart, so you can see how the magnetron and waveguide are made. (watch out for those charged capacitors!)

Proud Mary

Sun Nov 07 2010, 01:38PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Steve's suggestion recalls the microwave oven based microwave plasma reactor below, taken from a 1996 Belljar project-

altimicus

Sun Nov 07 2010, 04:25PM

Registered Member #3403 Joined: Sat Nov 06 2010, 11:21PM
Location: SÃ£o Carlos, Brazil
Posts: 11

Well, the project that I'm going to use this is not the current one but the next, both are Magnetoplasmadynamics Thrusters (MPDT's).

The first one that I'm doing is simpler "railgun like" model, where the "projectile" is the gas or liquid to become plasma.

And the second I'll headup the plasma with a magnetron or other source of high power RF to avoid the use of electrodes that have very shot life on continuous use.

Thanks for the pdf Proud Mary, I already read some, but this one is very good, I'd say that the project I'm looking for is something like "1.4 Microwave discharges".

Steve, would you have a link for his project?

Thanks everyone!

Proud Mary

Sun Nov 07 2010, 05:04PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

altimicus wrote ...

Thanks for the pdf Proud Mary, I already read some, but this one is very good, I'd say that the project I'm looking for is something like "1.4 Microwave discharges".

I thought it was a very helpful review of the current art too!

I came across it in the course of building up a small pdf library on hollow cathode Grimm glow discharge and capillary soft X-ray sources, which I'll be in a position to investigate with the X-ray spectrometer project on which I've just started work.

radiotech

Mon Nov 08 2010, 05:57AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Does your Magnetoplasmadynamic device involve proton precession
resonance? What fluidl are you using?

altimicus

Mon Nov 08 2010, 07:55PM

Registered Member #3403 Joined: Sat Nov 06 2010, 11:21PM
Location: SÃ£o Carlos, Brazil
Posts: 11

I didn't choose the fluid yet, but I'm not aiming the resonance effect since it's hard to trimming it, it would be RF heating of the fluid

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