Re: High voltage standing waves with a magnetron?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 

When propagating a field, the power is proportional to the field strength
squared.  In free space the ratio of the electric and magnetic fields is a
constant (377 ohms). If you raise the E field, you raise the B field by the
same amount (read voltage for E field and current for B field).

If you want high fields you need high powers (or small dimensions.. but
there IS a limit (waveguide cutoff)). You can make a waveguide that is the
right width (a few inches for 2.4 GHz) but very short to create high fields
with limited power (This is one way to study RF breakdown or multipaction).

Or, you can create high powers with a low power source by setting up a
resonant cavity, and pumping it with a low power source. The magnetron from
an oven isn't a particularly good low power source though.

The resonant ring is a situation like this.  It's a racetrack of waveguide
with some couplers and a phase adjuster. You launch the wave into the ring
and it circulates around.  You adjust the phase/length of the WG so that
it's exactly an even number of wavelengths around. If you put a pulse in, it
would circle around until it decayed due to the losses in the waveguide
walls.  (just like a tesla coil secondary).  If you continuously drive it,
the power circulating will increase until the losses in the WG walls equal
the power you're putting in.  You can get very high Q's with a circuit like
this, and correspondingly high fields.  The
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, January 31, 2004 8:19 PM
Subject: Re: High voltage standing waves with a magnetron?


 > Original poster: "mercurus2000" <mercurus2000-at-cox-dot-net>
 >
 > Yes, but I'm looking for the high voltage output, which is why I wanted to
 > try to drastically underpower the magnetron, I was hoping to get the high
 > voltage field without the extreme heating effects. Are you sure I can't
 > underpower it in any way?
 > Thanks,
 > Adam
 >
 > ----- Original Message -----
 > From: "Tesla list" <tesla-at-pupman-dot-com>
 > To: <tesla-at-pupman-dot-com>
 > Sent: Saturday, January 31, 2004 4:57 PM
 > Subject: Re: High voltage standing waves with a magnetron?
 >
 >
 >  > Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net>
 >  >
 >  > What you're describing is a fairly standard operation.. you're talking
 > about
 >  > using a microwave oven magnetron to excite a resonant cavity of some
sort.
 >  > It's a bit trickier than just coupling it in, because as the cavity
power
 >  > goes up, power starts to flow back to the magnetron.  What you'd really
 > want
 >  > is called a "resonant ring", which is something we use for high power
 >  > breakdown testing.  Conceptually very simple, it's hard to get to work:
as
 >  > it heats up (and it does), the phase shifts change, so you have to keep
 >  > adjusting it to keep it working.
 >  > The trick is getting small power out of a magnetron. You can't just
limit
 >  > the current and keep the voltage constant (because that's not the way
 > tubes
 >  > work).  And, there's a certain threshold below which the magnetron
won't
 >  > oscillate.
 >  >
 >  > If you want to fool with microwave at low powers, there's a couple
 >  > inexpensive and easy ways.
 >  > #1... use one of those 2.4 GHz wireless TV thingies.  You can improvise
a
 >  > probe with a suitable diode and antenna.  You can enjoy all the nifty
 > stuff
 >  > fooling with a slotted line.  The problem is that 2.4 GHz is 12cm, so
the
 >  > waveguide is big!
 >  >
 >  > #2.. scrounge up a 10 or 24 GHz source as in a doppler door opener or
 >  > burglar alarm.  Then the wavelength is a more reasonable 1-3 cm.  You
can
 >  > easily fabricate cavities and such with coffee cans, pipe, etc.
 >  > http://home.earthlink-dot-net/~w6rmk/radar10g.htm has some info on what I
did
 >  > with one. (if the links are broken, let me know... I'm in the midst of
 >  > rearranging all my websites)
 >  >
 >  > #3 look around for surplus microwave gear... it's there, and often
quite
 >  > inexpensive.  Very few people are willing to make measurements with
 > slotted
 >  > lines and sweepers anymore, since vector network analyzers make life so
 > much
 >  > easier.
 >  >
 >  > If you drop a bit lower in frequency (say, to 144 MHz), then you can
build
 >  > open wire line out of copper pipe and such, and it's easy to come by
 > sources
 >  > of several watts to 50Watts.
 >  >
 >  >
 >  > ----- Original Message -----
 >  > From: "Tesla list" <tesla-at-pupman-dot-com>
 >  > To: <tesla-at-pupman-dot-com>
 >  > Sent: Saturday, January 31, 2004 10:27 AM
 >  > Subject: High voltage standing waves with a magnetron?
 >  >
 >  >
 >  >  > Original poster: "mercurus2000" <mercurus2000-at-cox-dot-net>
 >  >  >
 >  >  > I was just curious if anyone experimented with high voltage standing
 > waves
 >  >  > from a magnetron and trying to create a resonant rise from them? My
 > idea
 >  >  > for a safe experiment, would be taking a small microwave oven
 > magnetron,
 >  >  > power the filament at that standard 3 volts ac or dc, and applying
 >  >  > EXTREMELY small power HV DC current to the entire device, like 4000
 > volts
 >  >  > at a half a milliamp, to keep the power output at about 2 watts
rather
 >  > than
 >  >  > the normal 1KW, would a circuit like this work? Or would the heating
 >  >  > current to the filament have to be reduced as well?
 >  >  >
 >  >  >
 >  >
 >  >
 >
 >