Featured Image description: Raytheon microwave oven on the ”NS Savannah”, Baltimore, Maryland, USA – Author Acroterion 2012-05-19
Introduction
Imagine standing in front of a powerful radar set in 1945. A chocolate bar in your pocket suddenly melts — with no heat source nearby. That single moment, born from secret WWII radar technology, changed kitchens forever.
This is the true story of how the cavity magnetron — a device invented in 1940 to help win the war — went from battlefield radar to your kitchen countertop. As a former merchant navy radio officer who worked on Raytheon digital radar in 1983, I’ve lived with this technology at sea. Today it quietly heats your coffee every morning.
1. Early Radar: The VHF Era and Its Limitations
Before the magnetron, radar was big, clumsy, and low-resolution. Britain’s pioneering Chain Home system (1930s–1940s) operated in the VHF band at 20–55 MHz. Transmitters blasted out massive pulses — peak power up to 200 kW or more — with pulse lengths of 5–25 microseconds.
These huge wooden-and-steel towers could detect aircraft 120 miles away, but the long wavelengths meant giant antennas and poor detail. You couldn’t fit that tech into an aircraft or small ship. Something revolutionary was needed: compact, high-power microwaves.
2. The Breakthrough: Randall and Boot Invent the Cavity Magnetron (1940)
In February 1940, two physicists at the University of Birmingham — John Randall and Harry Boot — solved the problem. Working under wartime secrecy, they built the first working cavity magnetron.
It was a simple but brilliant copper cylinder with a central cathode and multiple resonant cavities around it. In a strong magnetic field, electrons spiralled and generated powerful microwaves — around 9.8–10 cm wavelength (S-band) at 400 watts continuous (and far more when pulsed).
Their prototype (demonstrated 21 February 1940) was a game-changer. Britain shared the design with the US via the Tizard Mission. Suddenly, compact centimetric radar became possible — smaller antennas, higher resolution, and the ability to detect U-boats and aircraft that VHF radar missed.
Image file of RAYCAS V 1660/12SS Museum of Maritime Navigation and Communication – preserving Staten Island’s history and technology since 2010.
3. Radar Is Really Just a Super-Accurate Stopwatch: The 12.35 μs Radar Mile
Here’s the clever bit most people miss: a radar set is fundamentally a timing device.
Radio waves travel at the speed of light (~300 metres per microsecond). A nautical mile is 1,852 metres. For the signal to travel to a target one nautical mile away and echo back (the round trip), it takes exactly 12.35 microseconds.
Formula: Range in nautical miles = elapsed time (μs) ÷ 12.35
That’s why your ship’s radar screen has range rings calibrated in “radar miles.” In my 1983 Raytheon digital radar, the system measured those tiny time delays electronically and painted the echoes on a crisp digital display — a huge leap from the old analogue sets.
4. The Big Engineering Challenges: Taming a Monster Pulse
Generating that pulse created serious problems:
- Transmit side: Enormous power bursts at the Pulse Repetition Frequency (PRF). One moment the antenna is blasting kilowatts; the next it must listen for echoes a million times weaker.
- Protecting the receiver: A device called a duplexer (early versions used gas-filled TR/ATR tubes) switched the antenna between transmitter and receiver in microseconds. Without it, the receiver would be fried instantly.
- Receiving side: Super-sensitive superheterodyne design. A local oscillator mixed the incoming echo down to a lower Intermediate Frequency (IF) band (typically 30–60 MHz) for amplification. The receiver had to detect signals so faint they were buried in noise — yet survive the next massive transmit pulse.
The cavity magnetron made all this practical at microwave frequencies. Marine radar uses 3cm (X-band) and 10cm (S-band) radar. The lower frequency 10cm radar was less affected by weather i.e. rain/cloud.
5. The Accidental Discovery: Percy Spencer and the Melting Chocolate Bar (1945)
A 1971 Radarange microwave oven – Creative Commons Attribution-Share Alike 4.0 International
Fast-forward to Raytheon’s lab in the US. Engineer Percy Spencer was working on high-power magnetrons for radar. One day in 1945 he noticed the chocolate bar in his pocket had melted while he stood near an active radar set. (don’t try this at home!).
He wasn’t the first to notice the effect, but he was the first to investigate. He tried popcorn (which popped dramatically), then an egg (which exploded). Raytheon immediately saw the potential and filed a patent for “microwave cooking.”
Spencer received the standard company gratuity for inventors at the time — a one-time $2 payment. No royalties. The company owned the patent.
6. From Radar to Radarange: The First Microwave Oven (1947)
In 1947 Raytheon launched the world’s first commercial microwave oven: the Radarange.
- Stood nearly 6 feet tall
- Weighed 750 pounds
- Cost about $5,000 (over $70,000 in today’s money)
- Required water cooling and 220V power
It was built for commercial kitchens — hospitals, ships, military bases. The same magnetron technology that had helped win WWII was now cooking food in seconds. Home models only arrived in the 1950s–60s once the technology shrank and became affordable.
7. From Flickering Analog Traces to Digital Precision: My 1983 Raytheon RAYCAS V
When I first stepped onto the bridge in 1983 and saw the Raytheon RAYCAS V (RAYCAS V 1660/12SS variant), it felt like the future had arrived. Before this, merchant navy radio officers and deck officers worked with purely analog radars. The display was a classic rotating Plan Position Indicator (PPI) — a sweeping arm on a round, glowing green screen with long-persistence phosphor that left fading trails. Echoes often flickered, smeared in rain or sea clutter, and you had to tune the gain, sea clutter, and rain controls constantly by hand.
Collision avoidance was entirely manual. You plotted targets using a grease pencil on a reflection plotter or a separate plotting sheet, then calculated Closest Point of Approach (CPA) and Time to CPA (TCPA) yourself. In busy shipping lanes or poor visibility, this was exhausting and left plenty of room for human error.
The RAYCAS V changed that overnight. It was one of the first widely fitted digital ARPA (Automatic Radar Plotting Aid) systems on merchant ships. Instead of a flickering radial scan, it used a bright raster-scan CRT display — stable, TV-like, and much easier to read day or night. Early digital signal processing cleaned up the picture, reduced clutter, and allowed automatic tracking of multiple targets (up to 20–40 depending on configuration).
The system measured those tiny time delays we discussed earlier (the 12.35 μs radar mile) electronically and painted crisp vectors on the screen showing each target’s course and speed. It calculated CPA, TCPA, and gave audible alarms if a collision risk developed. True-motion display (with own-ship moving across the screen as in reality) became practical, and you could switch between north-up, course-up, or head-up modes far more easily.
Specs-wise, it ran with a 50–60 kW peak transmitter (very close to what I remembered as 50 kW), range scales out to 96 nautical miles (though like most officers, I usually worked on the practical 48 NM scale), and a modest ~256 kB of memory in its microprocessor brain. That was enough in 1983 to handle automatic plotting that would have taken a whole watch team manually on the old sets.
It still used the same cavity magnetron technology descended from Randall and Boot’s 1940 invention — blasting out powerful microwave pulses — but now a computer turned those raw echoes into actionable intelligence. No more constant manual plotting in heavy traffic. The workload dropped, situational awareness soared, and safety improved dramatically. That same Raytheon lineage lives on today in RTX, one of the giants of modern aerospace and defence.