Posts Tagged ‘wave guide’

The Microwave Oven Becomes Reality

Sunday, July 31st, 2011
     The evolution of cooking methods has been interesting indeed, from the open fires of primitive man, who must have decided at some point that cooked meat tasted better than raw, on to wood fired stoves, fossil fuel-based cooking, whether coal, propane, or gas, and let’s not forget electric range tops.  Standing on its own in the modern kitchen is the microwave oven.  What will be next?  The space age food pill dispensing stations of the futuristic cartoon family, The Jetsons?

      We’ve been talking about resonant cavity magnetrons and the purpose they serve within a microwave radar system.  We also learned about Dr. Percy Spencer’s discovery and how microwave radar transmissions emanating from a magnetron can cook food, not to mention melt candy bars.

Figure 1- Microwaves Melt Candy Bars and Cook Food

     Although the technologies used in microwave radars and microwave ovens are similar, they do have important differences.  It would be both unsafe and impractical to install microwave radar systems into kitchens.  Radiation emitted from radar wave guide lacking proper containment would bounce aimlessly around the kitchen, posing a threat to human safety.  You see, microwaves don’t know the difference between our bodies and the food we wish to cook.  They’ll heat up human tissue just as readily as a bowl of chicken soup.  Another issue is that runaway microwaves lose much of their effectiveness through their aimless bouncing about, and much of it would not be directed to the food itself.  Dr. Spencer would learn how to corral that energy, making microwave cooking a commercial success.   

     The biggest problem for Dr. Spencer to overcome was containment of the microwaves.  They needed to be directed towards food in order to efficiently heat them.  His first microwave oven was a metal box containing an opening at the top into which a magnetron wave guide could be inserted.  This would then introduce microwaves into the box, and the metal construction of the box would safely contain them.  The safety issue had now been resolved because the waves couldn’t escape, they would simply bounce around inside the box and most of their energy would be transferred into any food placed inside.

     Dr. Spencer’s employer, the Raytheon Corporation, produced the first commercial microwave oven in 1954, and it was appropriately named the “Radarange.”  It was huge, almost six feet tall, and weighed in at about 750 pounds!  Hardly something that could fit into a home kitchen.  Despite its massive size, the Radarange wasn’t all that powerful and couldn’t compete against the compact countertop microwave ovens in use today.

     It wasn’t until 1967 that technology improved enough to give us the smaller, more efficient units affordable to consumers.  This improvement involved using a newly developed semiconductor device called a “diode” within the high voltage electric circuitry that powers the magnetron.  We’ll learn more about these technologies in our next post.

     Also in our next post, we’ll see how high voltage circuits can pose electrocution hazards in a way you‘d never expect.  I discussed one of these instances in my recent appearance on The Discovery Channel program, Curious and Unusual Deaths, soon to be aired.


Microwave Radar and Melted Candy

Sunday, July 24th, 2011
     Ever discover a melted candy bar in your pocket?  You immediately start to think about the sources for the heat that had caused the mess.  Did you stand too close to the stove, were you outside in summer heat too long, or did you simply sit on it?  Or was it perhaps caused by being in proximity to a whirring device, something which does not seem to generate any heat at all?  If you’ve been reading along with us, you know what device I’m talking about.

     Last time we talked about an effect known as cavity resonance and how a sound is created, much like a musical note, when we blow across the top of a glass pop bottle containing some air space.  Our breath causes the air molecules to bounce in and out of the bottle’s cavity, producing the sound.  Microwave technology works in much the same way, making use of an electronic device called a resonant-cavity magnetron.  But instead of generating a musical tone, like our breath does over the bottle, the magnetron produces short wavelength radio waves, known as microwaves, and it was initially developed to generate these microwaves for radar systems.  So, how does the magnetron work?

     The magnetron contains a series of cavities arranged in a circle, their openings pointed towards the center as shown in Figure 1.  Engineers determined that when a high voltage, say 4000 volts, is applied to the magnetron, it begins to boil off electrons through a filament, called a cathode, located at its center.  Once free of the cathode, the electrons want to flow to a part of the magnetron called the anode.  This is because the cathode is positively charged and the anode is negatively charged, and as we know, electrons like to flow from positive to negative.  The anode is also the part of the magnetron containing the cavities, and we’ll see the significance of this in a moment.

Figure 1 – Interior View of a Resonant-Cavity Magnetron

     Before the electrons can take their desired straight path to the anode, they are deflected by powerful magnets located on either end of the magnetron.  These magnets force the electrons to move in a circular pattern over the openings in the cavities.  Like the air molecules passing over the top of a pop bottle when you blow across it, the electrons move over each cavity opening in the magnetron, creating not musical tones, but microwaves.  The microwaves are then collected from the magnetron using an antenna and directed along a tube called a wave guide.  The microwaves leave the wave guide when they are transmitted by radar systems.  The radar system then transmits the microwaves towards moving objects they wish to track.  These tracked objects are as diverse as airplanes, ships, and weather patterns.  See Figure 2.

Figure 2 – Microwave Radar Transmission

     So, how does the microwave oven fit into our discussion?  In 1946 an engineer by the name of Percy Spencer was working on a radar magnetron for the Raytheon Corporation, a producer of electronic technology for industry and defense.  During the course of his work he unexpectedly exposed himself to microwaves from a wave guide, and he couldn’t help but notice that the candy bar in his pocket had melted.  Putting two plus two together, he realized the microwaves had caused the candy bar to heat up.  Dr. Spencer further experimented and came to the conclusion that microwaves can cook foods far more quickly than conventional ovens, and the modern day kitchen appliance was soon born.

     Next time we’ll look at how Dr. Spencer’s microwave cooking discovery was developed into the microwave oven we find in most kitchens.  We’ll also see how even an unplugged microwave oven can pose an electrocution hazard, as I explained in the Discovery Channel program I was recently featured on, Curious and Unusual Deaths.