In 1999, the Internet went nuclear over an Macromedia Flash animation that allowed the user to microwave a cartoon gerbil until it exploded!
Joe Cartoon was a website created by Joseph Shields that featured crude cartoons animated using Macromedia Flash, a browser plug-in that allowed for the creation of simple applets. Flash was a revolution at a time when the web consisted mainly of images and text, with the occasional low-resolution video, although it was notoriously unstable and prone to security issues. But Gerbil in a Microwave, while grotesque, did demonstrate its potential effectively, and users flocked to the Joe Cartoon website to microwave the poor gerbil millions of times, generating over $US25,000 in advertising revenue for Shields at the peak of its popularity.
But how do microwaves really work? What is a ‘microwave’?
Microwaves are radio waves that have short wavelengths of between one metre and one millimetre, frequencies between 300MHz and 300GHz, above those of ordinary radio waves and below infrared light. Unlike lower frequency waves, microwaves can only travel by line-of-sight; they do not diffract around hills, reflect from the ionosphere or curve around the earth’s surface, so they are limited to the distance of the visual horizon (about 64km).
Although at the lower end of the band waves can pass through walls, usually objects must be avoided to ensure transmission. Higher frequency microwaves can be absorbed by moisture or even gases in the atmosphere, limiting their range to around a kilometre. Above 100GHz the absorption of electromagnetic radiation by the atmosphere is so great, microwaves are consumed almost instantly!
And so, microwaves are useless for broadcasting. But they are great for narrowcasting! If you want to send a signal somewhere with a reduced risk of it being intercepted by someone else, microwaves are the way to go. Even better, because they are so small, you don’t need a huge antenna to either send or receive them – in fact, you can use a parabolic dish, which means that you can transmit on a certain frequency without worrying about interference from other transmitters, or their being interfered with by you.
Before the advent of fiber-optic transmission, microwave transmission was commonly used for long-distance telephone calls, which were carried by networks of microwave radio relay links, up to 70km away from each other. Using frequency-division multiplexing (a way of shifting signals into different basebands in order to carry multiple signals on a single frequency) up to 5400 telephone calls could be carried simultaneously on a single microwave radio channel, with up to ten channels broadcast by each antenna!
But microwave transmitters weren’t just useful for making phone calls…
They were also perfect for use in radio and television broadcasting, either to transmit a signal for broadcast from the studio to the transmitter for distribution to the public, or from remote units to the studio, either directly or by way of a remote pickup unit (RPU), usually placed on a local mountaintop. In the US, these microwave signals are usually sent in the 2Ghz band.
Because the transmission needs to be largely unobstructed, remote transmission vans generally hoist their transmitters up on telescopic masts in order to get over buildings and trees. This capability and others led to the rise of Electronic News Gathering (ENG), the modern process of reporters creating stories largely in the field, either live or recorded, that could be transmitted instantly or near-instantaneously. This allowed for events to be covered in real-time, rather than having to wait for a reporter to return to a newsroom before broadcasting.
Wait, I have a 2.4Ghz wireless modem!
This doesn’t mean that you’re going to be cooked like the gerbil. In small amounts, microwaves are harmless. And in fact, the microwave spectrum is used for all sorts of things, not just for WiFi signals but also Bluetooth and cellular phone signals, the Global Positioning System, and satellite television. Microwaves allow these devices to work while staying clear of the crowded UHF spectrum. Wherever microwaves are practical, they are used, in order to save spectrum for applications that really need it.
How can microwaves cause a gerbil to explode?
So, now that we know all about microwaves, how do they cook things? Well, remember that water vapour absorbs microwaves – what do you think happens to that energy? It’s converted into heat! And what are meat and vegetables mostly made out of? Water! So, if we expose food to microwaves, in theory it should get hot and cook, right?
Around 1890 French physician Jacques Arsene d’Arsonval , who had been studying medical applications for electricity, discovered that frequencies of alternating current above 10kHz did not cause electric shock but instead warming. However, using electrodes in contact with the skin eventually caused burns in subjects, but he discovered that insulated capacitive plates and inductive coils also produced the warming effect – that is, the radiation from the plates or coils caused the warming effect in tissue without direct contact. In 1908 German physician Karl Nagelschmidt coined the term diathermy to describe electromagnetic heating, and by the 1940s microwaves were being used experimentally.
Meanwhile, at the 1933 Chicago World’s Fair, American manufacturer Westinghouse demonstrated a system for cooking food that consisted of two metal plates hooked up to a 10Kw 60MHz shortwave transmitter. Food placed between the plates cooked in minutes. But it took a lot of power!
However, the development of radar during World War II necessitated the creation of a microwave generator, in order to improve radar’s resolution, and decrease the size of the antenna required. A number of efforts were made by scientists in a number of countries, and they eventually developed the cavity magnetron. A magnetron uses a magnetic field to direct a stream of electrons from a cathode past a metal cavity. to an anode. By tuning the magnetic field precisely you can cause some of the electrons to not quite make it to the anode, and circle back toward the cathode. As they circle around they generate radio waves in the cavity, much like blowing air over the mouth of a bottle causes it to whistle.
But it was initially difficult to tune the magnetic field precisely, and only small numbers of electrons experienced what is now called the cyclotronic effect. To be practical, many more microwaves needed to be generated compared to the power expended. This problem was solved by creating a ring of cavities around a chamber inside a solid copper cylinder. At the centre of the chamber is the cathode, which runs the length of the cylinder.
The anode is actually the copper cylinder itself. A magnetic field causes the electrons to cycle around the inner chamber. As electrons strike the anode, they repel other electrons, causing an oscillating current to form, which as it passes by the cavities generates a large amount of microwaves. These microwaves can be captured using a wire at one end of the cavity, or by opening the cavity into a larger channel called a waveguide.
The cavity magnetron allowed for radar systems that could be compact enough to be used on and detect small airplanes. Invented by the British, it was offered to the US in exchange for help with the war effort. Contracts were awarded to Raytheon and other companies for mass production of the magnetron. The advanced radar capabilities it provided would be crucial in winning the war.
But the frequency of the microwaves can vary, making them impractical to use in more advanced phased-array radar introduced after the war. What to do with all these magnetrons? Well, in 1945 an engineer at Raytheon named Percy Spencer discovered that the radar set he was working on melted the chocolate bar in his pocket! Percy created a metal box which reflected microwaves injected into it around inside.
He cooked popcorn with it, and inadvertently exploded an egg! (something children have done ever since). Raytheon filed a patent on the process and in 1947 introduced its RadarRange. It was very expensive, but over time advances in manufacturing and safety drove costs down and made microwaves more appealing for home use.