Magnetron Deconstruction and Antenna Adaptation

After extensively researching magnetrons I was able to find a number of resources in

which detailed the governing equations but was unable to find anyone that had converted one

into an antenna. Our application of a magnetron was certainly atypical and thus we had to

adopt some unconventional methods. Several microwaves were purchased for <$20 a piece

from various people from Craigslist so that we would have some disposable "learning"

materials. One magnetron was used primarily for the temperature testing (see weekly journal:

week 1) and the others were taken apart to retrieve the magnetrons.

A magnetron works by supplying high voltage to a central cathode (typically Tungsten

Thorium) which causes it to emit electrons. Permanent magnets are placed on either side of the

cathode so that the electrons around a central axis. The outer shell, called the anode, has small

cavities which act as a resonant chamber for the electrons. As the newly emitted electrons pass

by the open chambers the trapped electrons vibrate at a frequency around 2.45 GHz (typ).

However, as the magnetron heats up, the size of the cavities expands and the resonant

frequency can drift. For heating applications this doesn't matter: any frequency in that range

with 900 W of power will certainly heat food rather quickly.

Hitachi 2M121A Microwave Oven Magnetron

(note: resonant cavity was vice clamped during deconstruction and deformed from its original circular

shape )

After taking apart a magnetron we realized that the central tap wire connects the oscillating

electric field from the resonant chamber to the distribution antenna. The antenna is simply a

quarter wave monopole made of a pinched copper tube that was presumably used to draw and

seal the vacuum within the resonant chamber. Therefore we determined that if we could slice a

magnetron exactly where the tap wire connects to the chamber we could convert it into an

antenna.

[NOTE DANGEROUS STEP: The white ceramic insulators that surround the antenna and power

leads are made of Beryllium Oxide. This ceramic is VERY TOXIC if broken into an inhalable

powder.]

After precisely slicing the magnetron cavity with a band saw, the tap wire was exposed. Now

we needed only to attach the SMA conductor the tap wire and the SMA panel mount to the

outer cavity. Unfortunately the panel mount was made of stainless steel and the cavity was

entirely copper. Thus, our first attempt to connect the two resulted in a very messy connection

that was fragile and inconsistent.

Later, Mr. Estevez helped us use the oxygen acetyl welder to firmly attach the SMA panel to the

outer copper cavity. Although this was vastly better than our first attempt, it still demonstrated

inconsistent results (see VNA Analyses). In the end this antenna was easily surpassed by the

professionally made antenna lent to us by Mr. Gerling (a $2500 precisely tuned and

manufactured antenna).

During this deconstruction/antenna creation process we thought it necessary to be able to

simulate the output of the magnetron in EM Pro. We had already developed simulations using a

plane wave assumption but a monopole output is significantly different that a plane wave. After

hours of measuring and molding we were able to make a reasonably accurate magnetron in

Solid Works. However we later realized that EM Pro was not suited to handle this level of

complexity nor would our computers have the processing capabilities necessary for this level of

accuracy. This avenue was quickly abandoned and all future EM Pro simulations were

performed using plane wave excitations.

We also were under the impression that 900~1000 W might be too much power for our setup.

Because we are delivering a significant amount of power outside the resonant frequency, (see

output spectrum courtesy of NASA Eagleworks) we started searching for methods to reduce the

output power.

We eventually found a circuit from an random website with all sorts of magnetron experiments,

[NOTE: These experiments are VERY DANGEROUS: http://danyk.cz/magn2_en.html]

At the time we were unable to verify the schematic due to our lack of electrical engineering

experience, however Scott Vollmer, an EE undergraduate, joined the project during the middle

of Fall 2015 and did verify the circuit and the components were purchased.

Figure 1: 50 Watt Magnetron Circuit

Figure 2. Normal magnetron circuit from manufacturer

A lot was learned about magnetrons through research and deconstruction, however we do not

believe they are an appropriate power source for an EM Drive. Due to their large and unstable

output bandwidth as well as the significant amount of heat generation they are not easy to

work into a theoretical 'low thrust' device. Ideally an EM Drive would use a very high power

source with a phase lock loop (PLL) in order to track the reflection coefficient of the cavity and

adapt the signal to match the resonance. Although we foresaw this near the beginning, we

were never able to become familiar enough with the components necessary to create a

frequency tracking power source. A high power RF source would also cost us the entire budget

of the project and we would need a host of other components to make the system work

(isolator, waveguide to coax, high power attenuator, see proposal for future experimentation).

If one were to adapt a magnetron for an EM Drive application, it would most likely only be

feasible if a circulator was used. This device uses ferrous materials to create a one-way round-

about for the EM radiation. Attaching a magnetron to a waveguide with a circulator could be

done with the S12 port toward the resonant cavity and the S23 port toward a broadband

matched load. This would result in all reflections from the cavity being absorbed in some sort of

load and no reflections traveling back to damage the magnetron.