TRIODES & MAGNETRONS

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Early radars were based on traditional vacuum-tube technology. The basic classic vacuum tube was the “triode”, which consisted of a vacuum-sealed tube with plates at each end and a mesh or “grid” of wires in the middle, with each of the three connected to its own external lead. The “cathode” plate at one end was negatively charged and heated by a separate filament, to boil off electrons that were attracted to the positively-charged “anode” plate at the other end. If the grid was given a negative voltage, it could shut off the flow of electrons; in this way, a low-power signal fed to the grid of a triode could modulate the flow of electrons through the tube, creating the same signal at a significantly boosted power level.

Such vacuum tubes were used to construct amplifier and oscillator circuits for radars, an oscillator in this case being nothing more than an amplifier with some of its output fed back out-of-phase to its input through a filter circuit designed to pass the desired frequency. High-power triodes, sometimes with liquid cooling, were used for the high-power radar transmitter output stages. Triodes were an effective technology but could not produce very high frequencies.

* Higher frequencies, in the microwave range, could produce more focused beams with greater resolution. They were produced by a wartime invention, the “cavity magnetron”. It was actually invented roughly in parallel in a large number of countries, but the British ran with it and also passed it on to the US. It is still in widespread use, at least in microwave ovens.

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The core of the magnetron is a metal cylinder, not so different in appearance from the cylinder of a revolver-type pistol. The center of the cylinder is bored out, and a set of “cavities” are bored through the cylinder around the central hole. Each cavity is connected to the central hole through a slot. A conductive rod is placed in the central hole, to act as the negatively charged cathode, while the cylinder itself acts as the positively charged anode. The assembly is sealed inside a vacuum envelope, and magnets are placed top and bottom.

Electrons move at a right angle to a magnetic field, and so the magnets cause the electrons moving from cathode to anode to swirl around the cathode rod in a spiral. Electrons emit EM radiation when they are accelerated, and so as they are pulled around the cathode they emit EM radiation over a wide range. This radiation flowed into the cavities around the central shaft. The cavities act as “resonant chambers”, with EM radiation of a specific wavelength building up inside them. The wavelength in practice is about 8 times the diameter, or a 1.2 centimeter cavity would produce a 10 centimeter / 3 GHz signal. One of these resonant cavities is tapped out to drive microwave power to the radar system.

The magnetron’s operating wavelength can be adjusted within a limited range by lowering a set of slugs into the cavities to adjust their resonant frequency. Unlike the vacuum triode, the magnetron is not an amplifying device as such: it generates its own high-power signal at a set frequency. Initially, the rest of the radar still remained reliant on vacuum tubes.

Incidentally, at lower frequencies it is possible to generate a signal in the oscillator subsystem and send it to an antenna over a solid-wire connection, usually in the form of “coaxial cables” — consisting of a central core conductor surrounded by an insulator and then a conductive wrapper, where it excites a dipole or similar element to generate radio waves. Trying to send an electrical signal over a solid conductor is more difficult at higher frequencies, resulting in losses, so the radio energy is produced directly by the oscillator and “piped” down hollow tubes with conductive walls, known as “waveguides”, to an antenna feed horn.