What are Microwave Tubes?
Microwave tubes are a type of vacuum tube that generate and amplify high frequency microwave signals from 300 MHz to 300 GHz. They can generate high output power levels from a few hundred watts to more than 10 MW. Microwave tubes are commonly used in military radar, electronic warfare, civilian radar-like weather detection, highway collision avoidance.
Conventional vacuum tubes can also provide high efficiency, high power signals, however they are limited by frequency. As you go higher up in frequency the efficiency of conventional vacuum tubes drops significantly. Microwave tubes replaced conventional vacuum tubes like triodes, tetrodes, and pentodes as they can generate high power with efficiency at frequencies up to 300 GHz. There are basically two types of vacuum tubes - Linear Bean Tubes (TWT and Klystron) and Cross Field Tubes (Magnetrons). We go in to more details about each type of tube later.
How do Microwave Tubes Operate?
Microwave Tubes consist of a vacuum tube with a resonant cavity and an electron gun. The most fundamental and critical component of a microwave tube is that cavity resonator. Electrons are emitted from a source into the tube and the cavity resonator inside the tube amplifies the signals. A collector is then used to collect the amplified electrons at the other end of the tube. Microwave tubes that use this operational structure are called linear-beam tubes, as the electron beam and the circuit elements of interaction are arranged linearly. Travelling Wave Tubes (TWT) and Klystron are examples of Linear-beam tubes.
Travelling Wave Tubes (TWTs)
Travelling Wave Tubes (TWTs) are linear beam vacuum tubes that amplify microwaves by converting the kinetic energy of an electron beam into microwave energy. These tubes are usually used in very high power RF/Microwave amplifiers. The main benefits of this tube is the wide operational bandwidth and the ability to generate high power levels. TWTs can operate in both continuous and pulsed modes. Click here to see TWT Products from the leading manufacturers on everything RF.
Klystron is another type of linear-beam tube. In this case the electrons flow is in a straight line and a magnetic field is used to focalize the electron beam. They work on the principle of velocity modulation. Velocity modulation is the variation in the velocity of an electron beam when electrons in the beam are being accelerated and slowed down. This variation is normally achieved by varying the velocity of the beam, which causes the electrons to bunch up and generate some amount of RF current. Klystrons typically applies the transit-time effect principle by varying the velocity of an electron beam to form an electron bunch. The resonant cavities in the tube modulate the electric field along the axis. Depending on the number of the resonant cavities, klystrons are categorized into two-cavity klystrons, multi-cavity klystrons, and reflex klystrons. Click here to see Klystrons from the leading manufacturers on everything RF.
A Cross Field Tube is another type of microwave tube that use a combination of a vacuum tube with cavity resonators and an extremely powerful permanent magnet. The most common type of cross field microwave tube is the Magnetron. Here the direction of electrons is altered by the application of some strong magnetic field with the help of a C-shaped magnet. This type is also known as cross-field MWTs. It is called so, because here the applied magnetic field is at right angles to the applied electric field.
The magnetron is cross field microwave tube. These tubes are normally circular in appearance. In magnetrons, the electric field and magnetic field run perpendicular to each other. The crossed electric and magnetic fields are used to generate high output power, which in turn is used in applications such as radar equipment. Magnetrons combine a vacuum tube with cavity resonators and a powerful magnet. A magnetron consists of an electron gun/cathode that emits the electrons and multiple anode cavities. A permanent magnet is placed on the backside of cathode and the space between the anode cavity and the cathode is called interacting space. There are two main types of magnetrons; pulsed magnetron and CW magnetron. Pulsed magnetrons are used in radars and CW ones are used in applications like microwave ovens. Click here to see Magnetrons from the leading manufacturers on everything RF.
Microwave Tubes vs Solid State Devices
Microwave Tubes have been used for many years and were the most efficient ways to generate high power, high frequency signals. However, over time and with the advancement of semiconductors and solid state devices, high power transistors and diodes started becoming a possibility. Originally these semiconductor devices were only used for low power applications. But, with new materials like GaN and SiC, generating KW level power at high frequency started becoming a reality.
Solid State high power devices are considerably smaller than microwave tubes. They are also more reliable, have a longer operational life, are easier to handle and provide higher levels of control. Due to this Microwave Tubes have been replaced by Solid State devices in most applications. However, there are still a number of cases where these tubes are used and continue to operate well.