Microwave Tubes Making a Comeback

Microwave Tubes Making a Comeback.

Scientists are looking back at the microwave tubes for high-power and high-frequency applications because only these can handle a power of up to 300 megawatts at a frequency of 1 Ghz.

A high-power microwave system consisting of a high-power microwave tube, high-voltage measurement chamber and power conditioning unit

In 1904, J.A. Fleming introduced the vacuum tube diode. After the second world war, electron tubes were used to develop the first generation of computers but these computers were impractical due to the  large sizes of the electronic components. In 1947, John Bardeen, Walter Brattain and William Shockley demonstrated the amplifying action of the firsttransistor at Bell Telephone Laboratories. They received a Nobel Prize for it. 

Bipolar transistors and digital integrated circuits (ICs) were made fist. Analogue ICs, large-scale integration (LSI) and very-large-scale integration (VLSI) followed by the mid-1970s. A VLSI design consists of thousands of circuits on a single chip with transistors acting as on/off switches or gates between them. Transistors are good for low-power and low-frequency applications. Microcomputers, medical equipment, video cameras and communication satellites are all examples of devices made possible by using ICs. 

From the day of invention of vacuum tube till today, when millions of transistors are fabricated on a single chip, technology has advanced a lot. But scientists are looking back at the microwave tubes for high-power and high-frequency applications because only these can handle high power (nearly 300 megawatts) at high frequency (nearly 1 GHz). 

The term ‘microwave’ denotes the techniques and concepts used as well as a range of frequencies. Microwaves travel in matter in the same manner as light waves. These are reflected by metals, absorbed by some dielectric materials and transmitted through other materials without significant losses. 

The helix slows down the propagation of electrons as these travel down the tube. The electrons bunch, and reinforce the voltage in the helix, which creates amplification (Courtesy: Thales Electron Devices)

Many R&D centres in India are actively inrolved in the advancement of microwave tubes. These include:

1. Central Electronics Engineering Research Institute, Pilani
2. Central Scientific Instruments Organisation, Chandigarh
3. Central Glass and Ceramic Research Institute, Kolkata
4. Central Mechanical Engineering Research Institute, Durgapur
5. Bharat Electronics Ltd, Bengaluru
6. Vacuum Electronics Devices and Application Society
7. Microwave Tube Research and Development Center, Bengaluru
8. Center of Research in Microwave Tubes, BHU
9. College of Engineering and Technology, Moradabad
10. Institute of Plasma Research, Gandhinagar
11. Society for Advanced Microwave Electronics Engineering & Research, Mumbai 

Microwave tubes have potential applications in radar, electronic warfare and communication systems. Air-traffic-controlradars, military radars, ground penetrating radars, imaging radars, UWB radars, cloud radars and space debris radars are some types of radars that use microwave tubes. Multi-beam jammers, phase-array jammers, and ultra-high-frequency and ultra-wide-bandwidth jammers used in electronic warfare also use microwave tubes. Clouds can be seen by microwave. These tubes play a role in climate forecasting and some medical applications (used in the diagnosis of hyperthermia) as well. Besides, microwave has its utilities for common man in the form of microwave heating and microwave protective gear/wall paper/furnishing.


Basic principle of microwave devices

The efficiency of conventional tubes is largely independent of the frequency up to a certain limit. When the frequency increases beyond that limit, several factors combine to rapidly decrease the tube’s efficiency. The high-frequency effects in conventional tubes are circuit reactance (inter-electrode capacitance, lead inductance), transit-time effect, cathode emission, plate-heat dissipation, power loss due to skin effect, radiation and dielectric loss.

Microwave devices

Conventional devices. Klystrons, magnetrons, traveling-wave tubes, backward-wave oscillators and crossed-field amplifiers New-generation devices. Cyclotron resonance devices (gyrotrons, gyro-TWTs and klystron-BWOs), Cerenkov radiation devices (magnetrons/BWOs/TWTs, orotrons, magnetically insulated line oscillators), Doppler effect-based devices (free-electron lasers, ubitrons, cyclotron auto-resonance masers), space-charge devices (virtual cathode oscillators) and multi-beam devices

Tubes that are efficient in the microwave range usually operate on the theory of velocity modulation. The microwave tube uses transit time in the conversion of DC power into radio-frequency power. The interchange of power is obtained by using the principle of electron velocity modulation and low-loss resonant cavities in the microwave tube.

Velocity modulation is define as the variation in the velocity of a beam of electrons caused by the alternate speeding up and slowing down of the electrons beam. This variation is usually caused by a voltage signal applied between the grids through which the beam must pass. The directions of the electron beam and the static electrical field are parallel to linear beam tubes. Against this, the field sinfluencing the electron beam stand vertically by the electron beam at the crossed-field tubes.

Magnetron—a microwave device
Magnetrons are a special form of diodes. Electrons move between the cathode and anode in a curved fashion, and thus the electric field and the magneti field are normal to the electron beam. When an electron is slowed down by an electric or magnetic field, it gives up energy, making the field stronger. If an electron’s speed is increased by an electric or magnetic field, it weakens the field.

An amplifier tube circuit that generates radio frequency signals is called an oscillator. A resonant circuit consisting of an inductor (coil) in parallel with a capacitor determines the frequency and wavelength of the oscillator. The lesser the number of turns in the coil, the smaller the capacitor plates and the higher the radio frequency that the oscillator generates. 

Magnetrons have a central cylindrical cathode surrounded by an anode in the form of a thick cylindrical shell. Top and bottom plates form the remainder of the vacuum envelope. These plates are placed between the poles of a strong magnet.

The controlled resonant circuit problem at microwave frequencies is solved by using hollow metal cylinders as a resonator. The round cylinder walls are similar to a one-turn coil and the cylinder end plates are like a very small capacitor. These cylindrical resonators are called microwave cavities. By moving one of the cavity’s end plates in or out of the cylinder, the frequency of the oscillator can be tuned. The cylinder must be about a wavelength in diameter and about one-half wavelength long at the resonant frequency. Energy can flowin and out of the resonator through a hole in the cylinder wall. 

Future technology

1. Multiple-beam klystrons for synthetic-aperture radars and missile seekers 2. TWTs for towed decoys 3. Microwave power modules based transmit/receive modules for phased-array radars 4. High-power microwave devices for directed-energy weapons 5. Gyro TWTs for radar applications 6. Vacuum microelectronics based microwave devices (TWT on a chip) 7. Tera-hertz devices for secure high-data-rate communication, imaging and radar 8. Microwave power beaming rectennas 9. Microwave propulsion 10. Microwave plasma chemistry 11. Microwave-generated artificial ionospheric mirrors (over-the-horizon radars and battlefield illumination)

The microwave fields from the resonators extend into the region between the cathode and anode. A strong magnetic field makes the highspeed electrons move such that these don’t reach the anode and return to the cathode, unless slowed down by giving up energy to a cavity electric field.

If the electrons arrive at the wrong time, these take energy from the microwave field, speed up and spiral back t the cathode. At the correct voltage and microwave frequency, the electrons move at the correct velocity to continue to loose speed and give up most of the energy to the microwave field before the impact on the anode. 

Other devices

Magnetron oscillator was the first device developed that was capable of generating large powers at microwave frequencies. Later, improved devices such as travelling-wave tube amplifiers (TWTAs) were developed for use in microwave systems. Yet, magnetron production continues for use in micro-wave ovens. 

Cross-field amplifier (CFA) is another microwave power amplifier. It is a cross between TWTs and magnetrons in its operation. It has a magnetron structure to provide interaction between crossed DC electric and magnetic fieldson one hand and RF field on he other. It also uses a slow-wave structure, as in TWT, to provide a continuous interaction between the electron beam and a moving RF field.

The backward-wave oscillator (BWO) is also a microwave-frequency and velocity-modulated tube that operates on the same principle as the TWT. However, a travelling wave that moves from the electron gun end of the tube towards the collector is not used in the BWO. Instead, the BWO extracts energy from the electron beam using a backward wave that travels from the collector towards the electron gun (cathode).