A History of Microwave Ovens

A History of Microwave Ovens

Special thanks to Cory at Radarange.com for<br>preserving so much valuable history!

Feel free to skip the science nonsense (especially in the beginning) and<br>skim through the pictures.

People started seriously thinking about heating things with<br>radio waves<br>circa 1940.

This happens to be the same period when people started thinking about<br>blowing things up with atomic nuclei.

Some molecules (notably water) are tiny magnets. Waves and magnets interact in<br>spooky ways. Certain radio<br>wave frequencies cause molecules to spin; spin is angular velocity is kinetic<br>energy is heat. Many smart people call this<br>"dielectric heating".

Lots of cool radar technology (e.g. the<br>cavity magnetron) was<br>developed for WWII. After the war, some military technology was reimagined for<br>commercial/domestic use.

1947: The Radarange 1132 was the first commercially available microwave<br>oven. That behemoth provided 1600W output<br>via a water-cooled permanent-magnet magnetron -- obviously<br>not a domestic machine.

But Raytheon's basic idea was simple: put food in a<br>Faraday cage and blast it with 2.4<br>GHz radio waves to spin the water molecules.

Yes, 2.4 GHz is conspicuously the same frequency as Wi-Fi and Bluetooth…

At the time, it wasn't clear if dielectric heating was a good fit for food.<br>Wetter foods cook at higher intensities, which is surprisingly unintuitive. Even<br>worse, regions of non-homogeneous foods (i.e. mostly everything) cook unevenly,<br>which is generally undesired behavior in culinary arts.

One early Raytheon discovery was that putting a "stirrer" in front of the<br>magnetron to scatter microwaves led to more even cooking. More on that<br>later.

General Electric was secretly in the kitchen too, experimenting with similar<br>technology at lower frequencies (~915 MHz).<br>Due to science, longer<br>wavelengths penetrate deeper, but shorter wavelengths carry more energy. To be<br>specific, Raytheon's 2,450 MHz penetrates ~1.5cm, while GE's 0.915 GHz<br>penetrates ~4cm.

"This is not entirely correct, Raytheon were going for a slightly higher<br>frequency (2.6 IIRC) and the FCC ordered them down (forcing the recall of<br>their trial ovens) because GE wanted to use crystal oscillators (klystrons)<br>in the 1.2GHz range, and they wouldn't need to worry about the awful<br>klystrons harmonics if they could bleed into 2.4."<br>via anotherhue

Because the FCC was establishing a frequency allocation procedure, Raytheon<br>and GE both petitioned the FCC for a microwave-oven frequency-Raytheon<br>favoring S-band or 2450, and GE L-Band/UHF or 915 MHz. It was argued by<br>Raytheon that the higher frequency permitted better coupling to small loads<br>like a frankfurter and the greater number of modes in a given cavity permitted<br>better randomization (uniformity) of heating patterns. GE argued the<br>advantages of penetration and less thermal runaway in defrosting. The net<br>result was the allocation of two frequencies by FCC, 915+25 MHz and 2450+50.

-- A History of Microwave Heating Applications

Anyway, it turned out that 2.4GHz devices were cheap to manufacture and worked<br>great for food (even when frozen). Today, all consumer microwaves ovens use<br>2.4GHz, but many industries still use 915MHz ovens for specific applications<br>that demand longer wavelengths.

1954 : The Radarange 1161 is 1600W like the previous model, but now<br>completely air-cooled.

The air in commercial kitchens is usually hot, grease laden, and frequently<br>full of lint. … Cooling air, therefore, is drawn in at the front, where the<br>air is most apt to be cool and free of grease … and forced over the<br>electrical components and out through openings in the back of the cabinet.

Popular Mechanics, 1955

Why is the air in commercial kitchens "frequently full of lint"? Yuck.

Microwaves cook meat well. At the time of writing, you, dear reader, are<br>unfortunately<br>made of meat.<br>It is therefore of utmost importance that the box keeps microwaves inside the<br>box, while also offering a convenient door to open the box.

The cooking chamber is similar to a Faraday cage to prevent the waves from<br>coming out of the oven. Even though there is no continuous metal-to-metal<br>contact around the rim of the door, choke connections on the door edges act<br>like metal-to-metal contact, at the frequency of the microwaves, to prevent<br>leakage.

-- Wikipedia

For a heated discussion on microwave door seals, I refer you to<br>this forum thread from 2015,<br>which has been archived<br>here.

As an additional layer of protection, opening the oven oven door breaks the<br>magnetron's power circuit. This pretty much guarantees that microwaves stay<br>inside the metal box, where they belong.

Speaking of heating meat--

1954: James Lovelock et al.<br>reanimated cryogenically-frozen<br>hamsters using an ad-hoc microwave oven.

I highly recommend watching<br>Tom Scott's interview with James Lovelock<br>for more details.

Meanwhile, weird microwave experiments<br>continued throughout the fifties:

They ranged from economically unfeasible...