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Air Defence Radar Museum, RAF Neatishead, England

52° 42′ 52.2″ N, 1° 28′ 13.6″ E

It’s Bigger Than You Think

RAF Neatishead is the site of the first secret air defense radar in Britain, and today houses a museum that covers the history of radar from its invention through the Second World War and up to the end of the Cold War. Everything in the museum is authentic, including the Cold War operations room where you are invited to sit at one of the terminals and try to deal with a four-minute warning of a nuclear attack. The early-warning equipment was still in use in 1993.

The history starts with the Second World War. You’ll see the operation rooms that were in use during the Battle of Britain and the Blitz, where information from early radar systems was translated as fast as possible into positions marked on maps on large tables.

Britain’s first early-warning system, the Chain Home coastal radar stations, consisted of 110-meter-high towers that broadcast radio signals with a wavelength of 10 to 15 meters. The radio signals bounced off approaching aircraft, and by timing the delay between transmitting a pulse of radio waves and receiving the bounced signals, it was possible to calculate the distance. The Chain Home system was also capable of determining the direction of approaching aircraft by comparing the bounced signals received at two different antennas. The differing signal strength could be used to determine the direction, and a second pair of antennas could find the elevation.

Once you knew the distance, elevation, and direction, it was possible to accurately detect aircraft up to 200 kilometers away, and Chain Home was used to great effect during the Battle of Britain and the Blitz. (For more on radar, see Chapter 100.) The museum also contains a timeline describing the events that led to the development of radar, and a collection of original radar equipment and vehicles.

Outside the museum there’s a Bloodhound missile system. The Bloodhound was an early surface-to-air missile system guided by radar. It used semi-active radar to find its target—instead of carrying a complete radar system itself, the Bloodhound relied on a ground-based radar to illuminate the target. The missile would pick up the reflected radar signal and follow it to the target without having to actually create the radar signal in the first place.

The Cold War exhibit is the eeriest part: it’s unchanged since 1993, and with the power on and the background sound effects provided, it appears that the operators might have just stepped out for a cup of tea. There’s also one of 800 bunkers, where red telephones stand by to report on a nuclear strike.

Much of the museum is hands-on, and you can try out the fighter control computers that were used until the early 1980s by the RAF.

What makes this museum a gem is the combination of authentic equipment and knowledgeable and friendly staff. The free, two-hour guided tours are strongly recommended, as they explain in fascinating depth the equipment on display.

Practical Information

Visiting information is available from the Air Defence Radar Museum’s website at http://radarmuseum.co.uk/.

The Cavity Magnetron

The original Chain Home system used a long wavelength, and therefore couldn’t distinguish small objects. In addition, the transmitters were large and had to be ground based. In early 1940, two scientists working at Birmingham University invented a device that could produce a powerful radio signal on a very short wavelength—the cavity magnetron.

With the cavity magnetron, which was small enough to fit into a briefcase, it was possible to install radar in aircraft and to detect very small objects such as a submarine periscope sticking out of the water. The cavity magnetron produced radio waves with a wavelength in centimeters and was also used for mapping terrain to provide accurate information for bombing, tracking Nazi German V-1 flying bombs, and directing the artillery fire.

The cavity magnetron (Figure 36-1) works by creating a rotating electrical field using a combination of a hot cathode to emit electrons, and permanent magnets to make the electrons rotate inside a cavity.

Figure 36-1. Cavity magnetron

The hot cathode is in the center of the cavity magnetron and is connected to a DC supply. The cathode heats up and emits electrons that are then attracted to the anode, which is ring-shaped and surrounds the cathode. Between the anode and the cathode is an empty space.

A magnetic field, produced by permanent magnets, passes through the cavity and acts on the electrons—instead of flying straight off the cathode to the anode, the magnets force the electrons to swirl around in the cavity. As the electrons swirl around, they create a pattern similar to the spokes of a bicycle wheel.

The anode has small circular holes cut into it, which open onto the main cavity (Figure 36-2). As the electron spokes swirl around in the main cavity, they create a charge at the openings of the holes, and current flows around each hole.

Figure 36-2. A single hole

Each hole acts like a resonant LC circuit (for more on resonance, see Chapter 62): the circular portion is an inductor with current flowing around it, much like a coil of wire, and the gap where the hole opens is a capacitor. Together the inductor and capacitor resonate to produce an electromagnetic field with a very high frequency. This field can be used in a radar system by extracting it—a simple extraction method is a small loop antenna placed inside one of the holes, which picks up the field and can be connected to a large radar antenna.

Most people have a cavity magnetron in their homes without even realizing it—it’s the device that makes a microwave work.