Microwave Communication Microwaves are the ultra-high, super-high, and extremely high frequencies directly above the lower frequency ranges where most radio communication now takes place and below the optical frequencies that cover infrared, visible, and ultraviolet light.
Microwave Frequencies and Bands The practical microwave region is generally considered to extend from 1 to 30 GHz, although frequencies could include up to 300 GHz. Microwave signals in the 1- to 30-GHz have wavelengths of 30 cm to 1 cm. The microwave frequency spectrum is divided up into groups of frequencies, or bands. Frequencies above 40 GHz are referred to as millimeter (mm) waves and those above 300 GHz are in the submillimeter band.
Benefits of Microwaves A major approach to solving the problem of spectrum crowding has been to move into higher frequency ranges. Initially, the VHF and UHF bands were tapped, however, today most new communication services are assigned to the microwave region. At higher frequencies there is a greater bandwidth available for the transmission of information. Wide bandwidths make it possible to use various multiplexing techniques. Transmission of high-speed binary information requires wide bandwidths and these are easily transmitted on microwave frequencies.
Disadvantages of Microwaves The higher the frequency, the more difficult it becomes to analyze electronic circuits. At microwave frequencies, conventional components become difficult, if not impossible to implement. Microwave signals, like light waves, travel in perfectly straight lines and therefore communication distance is limited to line-of-sight range. Microwave signals penetrate the ionosphere, so multiple-hop communication is not possible.
Microwave Communication Systems Like any other communication system, a microwave communication system uses transmitters, receivers, and antennas. The same modulation and multiplexing techniques used at lower frequencies are also used in the microwave range. The RF part of the equipment, however, is physically different because of the special circuits and components that are used to implement the components.
Transmitters Like any other transmitter, a microwave transmitter starts with a carrier generator and a series of amps. It also includes a modulator followed by more stages of power amplification. The final power amplifier applies the signal to the transmission line and antenna. A transmitter arrangement could have a mixer used to up-convert an initial carrier signal with or without modulation to the final microwave frequency.
Receivers Microwave receivers, like low-frequency receivers, are the superheterodyne type. Their front ends are made up of microwave components. Most receivers use double conversion.
Receiver Operation The antenna is connected to a tuned circuit, which could be a cavity resonator or microstrip or stripline tuned circuit. The signal is then applied to a special RF amplifier known as a low-noise amplifier (LNA). Another tuned circuit connects the amplifier input signal to the mixer. The local oscillator signal is applied to the mixer. The mixer output is usually in the UHF or VHF range. The remainder of the receiver is typical of other superheterodynes.
Transmission Lines Coaxial cable, most commonly used in lower- frequency communication has very high attenuation at microwave frequencies and conventional cable is unsuitable for carrying microwave signals. Special microwave coaxial that can be used on bands L, S, and C is made of hard tubing and this low-loss coaxial cable is known as hard line cable. At higher microwave frequencies, a special hollow rectangular or circular pipe called waveguide is used for the transmission line.
Antennas At low microwave frequencies, standard antenna types, including the simple dipole and quarter-wave length vertical antennas, are still used. At these frequencies antennas are very small; for example, a half-wave dipole at 2 GHz is about 3 in. At higher microwave frequencies, special antennas are generally used.
Microwave Transistor Amplifiers Although vacuum and microwave tubes like the klystron and magnetron are still used, most microwave systems use transistor amplifiers. Special geometries are used to make bipolar transistors that provide voltage and power gain at frequencies up to 10 GHz. Microwave FET transistors have also been created. Monolithic microwave integrated circuits (MMICs) are widely used.
Microstrip Tuned Circuits At higher frequencies, standard techniques for implementing lumped components such as coils and capacitors are not possible. At microwave frequencies, transmission lines, specifically microstrip, are used. Microstrip is preferred for reactive circuits at the higher frequencies because it is simpler and less expensive than stripline. Stripline is used where shielding is necessary.
Microstrip Transmission Line used for Reactive Circuit
Equivalent Circuits of Open and Shorted Microstrip Lines
Microwave Transistors The primary differences between standard lower- frequency transistors and microwave types are internal geometry and packaging. To reduce internal inductances and capacitances of transistor elements, special chip configurations, known as geometries are used. Geometries permit the transistor to operate at higher power levels and at the same time minimize distributed and stray inductances and capacitances.
Microwave Transistors (Continued) The GaAs MESFET, a type of JFET, uses a Schottky barrier junction and can operate at frequencies in excess of 5 GHz. A high electron mobility transistor (HEMT) is a variant of the MESFET and extends the range beyond 20 GHz by adding an extra layer of semiconductor material such as AlGaAs. A popular device known as a heterojunction bipolar transistor (HBT) is making even higher-frequency amplification possible in discrete form and integrated circuits.
Small-Signal Amplifiers A small-signal microwave amplifier can be made up of a single transistor or multiple transistors combined with a biasing circuit and any microstrip circuits or components as required. Most microwave amplifiers are of the tuned variety. Another type of small-signal microwave amplifier is a multistage integrated circuit, a variety of MMIC.
Transistor Amplifiers A low-noise transistor with a gain of about 10 to 25 dB is typically used as a microwave amplifier. Most microwave amplifiers are designed to have input and output impedances of 50 Ω. The transistor is biased into the linear region for class A operation. Radio frequency chokes (RFCs) and ferrite beads (FB) are used to keep RF out of the supply and prevent feedback which can cause oscillation and circuit instability.
Monolithic Microwave Integrated Circuit A common monolithic microwave integrated circuit (MMIC) amplifier is one that incorporates two or more stages of FET or bipolar transistors made on a common chip to form a multistage amplifier. The chip also incorporates resistors for biasing and small bypass capacitors. Physically, these devices look like transistors. Another form of MMIC is the hybrid circuit, which combines an amplifier IC connected to microstrip circuits and discrete components.
Power Amplifiers A typical microwave power amplifier is designed with microstrip lines which are used for impedance matching and tuning. Input and output impedances are 50 Ω. Typical power-supply voltages are 12, 24, and 28 volts. Most power amplifiers obtain their bias from constant-current sources. A single-stage FET power amplifier can achieve a power output of 100 W in the high UHF region.
Waveguides Most microwave energy transmission above 6 GHz is handled by waveguides. Waveguides are hollow metal conducting pipes designed to carry and constrain the electromagnetic waves of a microwave signal. Most waveguides are rectangular. Waveguides are made from copper, aluminum or brass. Often the insides of waveguides are plated with silver to reduce resistance and transmission losses.
By Definition… The electric field is at a right angle to the direction of wave propagation, so it is called a transverse field. An interconnection of two sections of waveguide is called a choke joint. A T section, or T junction is used to split or combine two or more sources of microwave power. Waveguides and their fittings are precision-made so that the dimensions match perfectly. Directional couplers are used to facilitate the measurement of microwave power in a waveguide and the SWR.
Cavity Resonator A cavity resonator is a waveguide-like device that acts like a high-Q parallel resonant circuit. A simple cavity resonator can be formed with a short piece of waveguide one-half wavelength long. Energy is coupled into the cavity with a coaxial probe at the center. The internal walls of the cavity are often plated with silver or some other low-loss material to ensure minimum loss and maximum Q.
Circulators A circulator is a three-port microwave device used for coupling energy in only one direction around a closed loop. Microwave energy is applied to one port and passed to another with minor attenuation, however the signal will be greatly attenuated on its way to a third port. The primary application of a circulator is a diplexer, which allows a single antenna to be shared by a transmitter and receiver.
Isolators Isolators are variations of circulators, but they have one input and one output. They are configured like a circulator, but only ports 1 and 2 are used. Isolators are often used in situations where a mismatch, or the lack of a proper load, could cause reflection so large as to damage the source.
Small Signal Diodes Diodes used for signal detection and mixing are the most common microwave semiconductor devices. Two types of microwave diodes are: Point-contact diode Schottky barrier or hot-carrier diode
Point-Contact Diode The oldest microwave semiconductor device is the point-contact, also called a crystal diode. A point-contact diode is a piece of semiconductor material and a fine wire which makes contact with the semiconductor material. Point-contact diodes are ideal for small-signal applications. They are widely used in microwave mixers and detectors and in microwave power measurement equipment.
Hot Carrier Diodes For the most part, point-contact diodes have been replaced by Schottky diodes, sometimes referred to as hot carrier diodes. Like the point-contact diode, the Schottky diode is extremely small and therefore has a tiny junction capacitance. Schottky diodes are widely used in balanced modulators and mixers. They are also used as fast switches at microwave frequencies.
Frequency-Multiplier Diodes Microwave diodes designed primarily for frequency- multiplier service include: Varactor diodes Step-recovery diodes
Varactor Diodes A varactor diode is basically a voltage variable capacitor. When a reverse bias is applied to the diode, it acts like a capacitor. A varactor is primarily used in microwave circuits as a frequency multiplier. Varactors are used in applications in which it is difficult to generate microwave signals. Varactor diodes are available for producing relatively high power outputs at frequencies up to 100 GHz.
Step-Recovery Diodes A step-recovery diode or snap-off varactor is widely used in microwave frequency-multiplier circuits. A step-recovery diode is a PN-junction diode made with gallium arsenide or silicon. When it is forward-biased, it conducts like any diode, but a charge is stored in the depletion layer. When reverse bias is applied, the charge keeps the diode on momentarily and then turns off abruptly. This snap-off produces a high intensity pulse that is rich in harmonics.
Oscillator Diodes Three types of diodes other than the tunnel diode that can oscillate due to negative resistance characteristics are the: Gunn diode IMPATT diode TRAPATT diode
Gunn Diodes Gunn diodes, also called transferred-electron devices (TEDs), are not diodes in the usual sense because they do not have junctions. A Gunn diode is a thin piece of N-type gallium arsenide (GaAs) or indium phosphide (InP) semiconductor which forms a special resistor when voltage is applied to it. The Gunn diode exhibits a negative-resistance characteristic. Gunn diodes oscillate at frequencies up to 50 GHz.
IMPATT and TRAPATT Diodes Two microwave diodes widely used as oscillators are the IMPATT and TRAPATT diodes. Both are PN-junction diodes made of silicon, GaAs, or InP. They are designed to operate with a high reverse bias that causes them to avalanche or break down. IMPATT diodes are available with power ratings up to 25 W to frequencies as high as 30 GHz. IMPATT are preferred over Gunn diodes if higher power is required.
PIN Diodes A PIN diode is a special PN-junction diode with an I (intrinsic) layer between the P and the N sections. The P and N layers are usually silicon, although GaAs is sometimes used and the I layer is a very lightly doped N-type semiconductor. PIN diodes are used as switches in microwave circuits. PIN diodes are widely used to switch sections of quarter- or half-wavelength transmission lines to provide varying phase shifts in a circuit.
Microwave Tubes Vacuum tubes are devices used for controlling a large current with a small voltage to produce amplification, oscillation, switching, and other operations. Vacuum tubes are widely used in microwave transmitters requiring high output power. Special microwave tubes such as the klystron, the magnetron, and the traveling-wave tube are widely used for microwave power amplification.
Klystron A klystron is a microwave vacuum tube using cavity resonators to produce velocity modulation of an electron beam which produces amplification. Klystrons are no longer widely used in most microwave equipment. Gunn diodes have replaced the smaller reflex klystrons in signal-generating applications because they are smaller and lower in cost. The larger multicavity klystrons are being replaced by traveling-wave tubes in high-power applications.
Magnetron A widely used microwave tube is the magnetron, a combination of a simple diode vacuum tube with built-in cavity resonators and an extremely powerful permanent magnet. Magnetrons are capable of developing extremely high levels of microwave power. When operated in a pulsed mode, magnetrons can generate several megawatts of power. An application for a continuous-wave magnetron is for heating purposes in microwave ovens.
Traveling-Wave Tube One of the most versatile microwave RF power amplifiers is the traveling-wave tube (TWT), which can generate hundreds and even thousands of watts of microwave power. The main advantage of the TWT is an extremely wide bandwidth. Traveling-wave tubes can be made to amplify signals in a range from UHF to hundreds of gigahertz. A common application of TWTs is as power amplifiers in satellite transponders.
Microwave Antennas Because of the line-of-sight transmission of microwave signals, highly directive, high gain antennas are preferred because they do not waste the radiated energy and because they provide an increase in gain, which helps offset noise at microwave frequencies.
Low-Frequency Antennas At low microwave frequencies, less than 2 GHz, standard antennas are commonly used, including the dipole and its variations such as the bow-tie, the Yagi, and the ground-plane antenna. The corner reflector is a fat, wide-bandwidth, half- wave dipole fed with low-loss coaxial cable. The overall gain of a corner reflector antenna is 10 to 15 dB.
Horn Antenna The problem with using a waveguide as a radiator is that it provides a poor impedance match with free space and this results in standing waves. This mismatch can be offset by simply flaring the end of the waveguide to create a horn antenna. Horn antennas have excellent gain and directivity. The gain and directivity of a horn are a direct function of its dimensions; the most important dimensions are length, aperture area, and flare angle.
Parabolic Antenna A parabolic reflector is a large dish-shaped structure made of metal or screen mesh. The energy radiated by the horn is pointed at the reflector, which focuses the radiated energy into a narrow beam and reflects it toward its destination. Beamwidths of only a few degrees are typical with parabolic reflectors. Narrow beamwidths also represent extremely high gains.
Helical Antennas A helical antenna, as its name suggests, is a wire helix. A center support is used to hold heavy wire or tubing formed into a circular coil or helix. The diameter of the helix is typically one-third wavelength, and the spacing between turns is approximately one-quarter wavelength. The gain of a helical antenna is typically in the 12- to 20-dB range and beamwidths vary from approximately 12 to 45 degrees.
Slot Antenna A slot antenna is a radiator made by cutting a half- wavelength slot in a conducting sheet of metal or into the side or top of a waveguide. The slot antenna has the same characteristics as a standard dipole antenna, as long as the metal sheet is very large compared to 1 λ at the operating frequency. Slot antennas are widely used on high-speed aircraft where the antenna can be integrated into the metallic skin of the aircraft.
Dielectric (Lens) Antennas Dielectric, or lens antennas use a special dielectric material to collimate or focus the microwaves from a source into a narrow beam. An example of a lens antenna is one used in the millimeter-wave range. Lens antennas are usually made of polystyrene or some other plastic, although other types of dielectric can be used. Their main use is in the millimeter range above 40 GHz.
Patch Antenna Patch antennas are made with microstrip on PCBs. The antenna is a circular or rectangular area of copper separated from the ground plane on the bottom of the board by the PCBs insulating material. Patch antennas are small, inexpensive, and easy to construct. Their bandwidth is directly related to the thickness of the PCB material. Their radiation pattern is circular in the direction opposite to that of the ground plane.
Phased Arrays A phased array is an antenna system made up of a large group of similar antennas on a common plane. Patch antennas on a common PCB can be used, or separate antennas like dipoles can be physically mounted together in a plane. Most phased arrays are used in radar systems, but they are finding applications in some cell phone communication systems and in satellites.
Microwave Applications Radar Satellite Wireless computer and personal area networks Wireless broadband access to the Internet Cell phones Heating
Radar The electronic communication system known as radar (radio detection and ranging) is based on the principle that high-frequency RF signals are reflected by conductive targets. In a radar system, a signal is transmitted toward the target and the reflected signal is picked up by a receiver in the radar unit. The radar unit can determine the distance to a target (range), its direction (azimuth), and in some cases, its elevation (distance above the horizon).