Waves: Part 10 - Microwave Innovation

The histories of broadcasting and radar are forever intertwined as the same people worked in both fields and knowledge flowed between them. Television and radar are both imaging technologies, but only one is for entertainment.

Most electromagnetic waves are light we can’t see and like light, they can reflect from objects in their path. Sometimes the reflection is deliberate, and we will come to that when discussing antennas; sometimes it is inadvertent.

Between the wars, radio engineers discovered that airplanes could affect their signals. Fig.1 shows what happens: the received signal suffers from multi-path reception in which the direct signal and the delayed reflected signal are added. Addition with delay is the basis of the comb filter, which inserts notches in the received spectrum. Movement of the airplane sweeps the notch frequencies, so a fixed AM receiver finds the signal suffers periodic fading.

At a later date FM receivers found the noise floor rising and falling and a digital receiver saw the error rate going up and down. To this day radio systems near airports need to have more closely spaced transmitters to deal with all the reflectors flying over.

Fig.1 - Delayed reflections from an airplane turn the path from the transmitter into a comb filter.

Fig.1 - Delayed reflections from an airplane turn the path from the transmitter into a comb filter.

Going back again, in the nineteen thirties every crackpot in the UK was inventing weapons based on radio signals; the so-called death rays. The problem was that a lot of them were not crackpots; they were accomplished fraudsters and charlatans. Then as now, ignorant journalists believed the nonsense and publicized it.

Simple calculations showed that death rays could not possibly work as the amount of power needed was simply not available, but rather than an outright rejection, those who did understand the principles remembered that radio waves had been used in the past to detect distant objects but had not been able to interest the military. The prospect of war made the ideas more attractive. The fear of enemy bombing had resulted in the development of fast well-armed fighters, but they needed warning if they were to climb to the level of the bombers in time.

Given the constant fear of crackpots, a demonstration had to be made before funds could be released to develop such a warning system. An unmodified BBC AM transmitter was used as the radio source, and as an RAF bomber flew by, its course was plotted with a suitable receiver. The idea worked and development began in earnest.

Such was the urgency that the system had to be built using available parts wherever possible. There was no time to do anything fancy. The resulting system, known as Chain Home, was the world’s first military radar, built before the term had even been coined. It was disturbingly crude, but it had one redeeming feature, which is that when it was needed in the Battle of Britain, it worked.

The system operated on timing the returns from transmitted pulses so that the range could be calculated. However, the transmitting antennas were so big that there was no possibility that they could be rotated. Instead, the fixed transmitter sent forward a fan-shaped beam that covered a wide area. The direction sensing was done in the receiving system that was a separate structure some way away.

As the transmitters were not directional, a return from an airplane illuminated by one transmitter could be picked up by several receivers. This was dealt with by synchronizing the transmitters, so they produced pulses at different times. The 50 Hz AC power grid was used as the synchronizing source and each transmitter was triggered by a different phase of the AC power.

The fixed receiving antennas were a pair of dipoles crossed at 90 degrees. Each dipole fed a goniometer, which is a device that virtually rotates fixed antennas. Fig.2 shows the goniometer has two input coils at 90 degrees, one fed by each dipole. Those coils were recreating the radio field seen by the antennas. A third coil could be rotated to explore the field. It could find the angle at which a signal was at a maximum, or, more accurately, it could find the angle 90 degrees away at which a signal was nulled.

Fig.2. - A goniometer has a pair of coils at right angles, each connected to one of a pair of dipoles mounted at right angles. The search coil can find the direction of peaks or nulls just as if the entire antenna had been rotated.

Fig.2. - A goniometer has a pair of coils at right angles, each connected to one of a pair of dipoles mounted at right angles. The search coil can find the direction of peaks or nulls just as if the entire antenna had been rotated.

There was one heart stopping moment when it was discovered that the back lobe of the transmitter was sufficiently powerful to illuminate airplanes flying inland. The goniometer had a figure-of-eight directivity that would interpret the reflection as coming from over the sea. This defect was rapidly overcome by adding switchable reflectors to the inland side of the receiver dipoles. When the reflector was switched in, the directivity became a cardioid, causing a return from inland to diminish and one from over the sea to grow.

The Chain Home system was obsolete when it was built, and development of new ideas was rapid. The first requirement was to reduce the wavelength used so that antennas could be smaller. That led to the need for a more compact source of radio energy.

That source was found in the magnetron. Fig.3 shows how a magnetron is made. The tuned circuits are formed by inductors having a single turn and parallel capacitors across a gap in the turn. At high frequencies, the current runs on the surface of a conductor, so the resonant cavities could be machined from a solid block. A cathode on the axis of the device emitted electrons and an axial magnetic field caused them to spiral outwards. As they passed the gaps in the cavities, they would excite resonance, just as the air flow in a referee’s whistle does.

The magnetron allowed radar sets to be small enough to fit in aircraft. The much shorter wavelength allowed compact directional parabolic antennas to be used. These could be steered to obtain directional information from a target.

As the primary information obtained from reflected RF energy is distance, it was relatively simple to add direction to that by arranging for the entire antenna to rotate continuously. A circular display synchronized to the rotation would produce a “blip” showing the range and direction of an object.

Fig.3. - The slotted cylindrical holes of the magnetron formed resonant cavities.

Fig.3. - The slotted cylindrical holes of the magnetron formed resonant cavities.

Sometimes an additional radar was used in which the antenna rocked up and down so that altitude information could be obtained. The preferred solution in aviation was for airplanes to carry a transponder that would return altitude from their own instruments.

Continuous wave radar that effectively emitted a sine wave could not measure distance, but if an illuminated object moved, the frequency of the return would suffer a Doppler shift. Doppler radar is used in some vehicle speed enforcement systems. Another possibility is to frequency modulate the transmitter. In that case the return will differ in frequency from the transmitted signal because of the time-of-flight delay.

Pressure operated altimeters are of limited use near the ground as they rely on accurate knowledge of the atmospheric pressure at ground level. Radar allows a direct measurement of height. Most units in aviation use frequency modulation by a sawtooth wave, returning a steady frequency difference during the sweep that can readily be used to drive a display.

The magnetron was simply an oscillator whose frequency stability was not very good and it could not be modulated. It was fed with pulsed power for radar applications. Magnetrons subsequently became the most popular power device used in microwave ovens. As they cannot be modulated, the lower power levels in an oven are obtained by changing the duty cycle of the applied power as can be heard in most ovens as the hum changes in level.

The next key piece of technology was the klystron, which was essentially a tuned amplifier that could work up to microwave frequencies. The tuned cavities of the magnetron were still there, but as Fig.4 shows, they took a different form. The cylindrical cavity of the magnetron was rolled round into the shape of a donut, with the gap that formed the tuning capacitor on the inside.

Fig.4 - The klystron’s resonators are donut shaped, with the slot on the inside.

Fig.4 - The klystron’s resonators are donut shaped, with the slot on the inside.

A beam of electrons was fired through the hole in the donut. When the donut was resonating, the voltage across the gap would alternate in polarity and would either accelerate or retard the electron beam. The electrons would be bunched together or thinned out, just like the particles in a sound wave. The electron beam passed on to a second donut in which the modulation of the beam induced oscillation that was considerably more powerful.

The klystron could be used in different ways. If the first donut was fed with a low powered signal that was modulated, the klystron would amplify it. If some of the output was fed back, the klystron would become an oscillator. As well as being used as the energy source for radar, the klystron was popular as a power amplifier for television transmitters until it became possible to use transistors.

The development of semiconductors resulted in the Gunn diode, which is a diode only because it has two terminals, as it can’t rectify. Part of its transfer function displays negative impedance, which means that if it is connected to a resonant circuit, the damping will be negative and it will oscillate. It can do so at microwave frequencies and is popular in low-powered Doppler systems such as for automatic door opening and speed enforcement.

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