Microwave Technology: Applications, Devices, and Transmission Lines
Applications of Microwaves||Common applications (any 5–6 can be written)||–
Radar (Radio Detection and Ranging)||– Satellite communication||– Terrestrial microwave links (point-to-point communication)||–
Microwave ovens (domestic and industrial heating)||– Remote sensing and radiometry||– Medical applications (diathermy, cancer treatment, imaging)||– Radio astronomy and deep-space communication||– Military EW (jamming, guidance, seekers)||Brief explanation of any two:||1.Radar (Radio Detection and Ranging)||Radar uses microwaves to detect the presence, distance, speed and direction of targets (aircraft, ships, vehicles, rain clouds etc.). A high–
frequency microwave pulse is transmitted through a directional antenna; it travels, hits a target and a small portion is reflected back to the radar receiver.||Time delay between transmitted and received pulse gives range (R = c·t/2). Doppler shift between transmitted and received frequency gives radial velocity of the target. Narrow beams obtainable at microwave frequencies (using reasonably small antennas) provide good angular resolution.
Hence microwaves are ideal for air-traffic control radar, weather radar, military surveillance and tracking, missile guidance etc.||2.
Satellite Communication||Microwaves in bands like L, S, C, X, Ku, Ka are used to provide long-distance telephone, TV, data and internet via satellites. In satellite links, an uplink (earth station → satellite) and downlink (satellite → earth station) operate at microwave frequencies because:||– They suffer less attenuation in the atmosphere (in selected “windows”)||– Antennas can be made highly directional (parabolic dishes) with small physical size||– Wide bandwidth is available to support high data rates||Microwave transponders on-board the satellite receive, amplify, frequency-translate and retransmit the signals back to earth. This enables global coverage for broadcast TV, VSAT networks, GPS-related systems, mobile/satellite phones and backbone links for communication networks.
Microwave Signals – Introductory Note||Microwaves are electromagnetic waves with frequencies roughly from 1 GHz to 300 GHz (wavelengths 30 cm to 1 mm). At these frequencies, circuits behave as distributed networks, so we use waveguides, coaxial lines, microstrip and stripline instead of simple lumped RLC. Small wavelength allows highly directive antennas with small physical size and provides large bandwidth, making microwaves very suitable for high-speed communication, radar and sensing applications.||Practical Applications of Microwaves (any 4–5)||1) Radar (Radio Detection and Ranging)||Microwave pulses are transmitted and echoes from targets are received to find range, speed and direction using time delay and Doppler shift. Used in air-traffic control, weather radar, military surveillance, missile guidance, automotive radar etc.||2.Satellite Communication||Microwave bands (L, S, C, X, Ku, Ka) are used for uplink and downlink between earth stations and satellites. Provide high directivity, large bandwidth and low attenuation windows. Used for TV broadcast, telephone, internet, DTH, VSAT networks.
||3.Terrestrial Microwave Links||Line-of-sight point-to-point links using parabolic antennas. Provide high-data-rate backbone between cities and cellular base-station backhaul, especially where laying cables is difficult.||4.Microwave Heating / Ovens||Operate typically at 2.45 GHz.
Microwaves cause dielectric heating in materials containing water. Used in domestic ovens, drying and curing in industries, rubber vulcanisation, etc.||5.Medical Applications||Microwave diathermy and hyperthermia for localised tissue heating and cancer treatment; some specialised imaging and sensing systems also use microwaves.
COMPARISON-Microstrip Line||– Structure: A conducting strip on top of a dielectric substrate with a ground plane on the bottom.
||– Fields: Quasi-TEM mode; E-field mainly between strip and ground plane (fringing in air).||– Impedance control: Characteristic impedance depends on strip width, substrate thickness and εr; easy to design for 50 Ω or 75 Ω.||– Excitation: Naturally unbalanced (one conductor + ground), easily fed from coaxial connectors.||– Losses: Conductor + dielectric + radiation losses; radiation higher at very high frequencies or sharp bends.||– Fabrication: Very simple PCB technology; compatible with standard RF/microwave integrated circuits.||– Typical examples/applications:
Microstrip patch antennas, microstrip filters and couplers, impedance transformers, matching networks, RF amplifiers and oscillators on PCB.||Slot Line||– Structure: A narrow slot cut in a ground plane (usually on one side of a dielectric substrate); the metal around the slot acts as the conductor. No separate strip above.||– Fields: Quasi-TEM; strong E-field across the slot; complementary to microstrip (duality).||– Impedance control: High characteristic impedances possible for narrow slots; depends on slot width, substrate thickness, εr.||– Excitation: Naturally balanced across the slot; often fed by probes, microstrip-to-slotline transitions, or CPW structures.||– Losses: Can have higher radiation and dispersion; useful mainly at microwave and millimetre-wave where integration advantages outweigh losses.||– Fabrication: Also done on a single metal layer PCB, but needs careful slot cutting and transitions to other lines.||– Typical examples/applications: Balanced mixers, slot antennas, broadband phase shifters, shunt elements in MICs, microstrip–slotline hybrids and couplers
MASER (Microwave Amplification by Stimulated Emission of Radiation)
||MASER is the microwave equivalent of a laser: it uses stimulated emission in an active medium to produce coherent, very low-noise microwave amplification or oscillation.
||Typical active media:
ammonia gas, ruby crystal, paramagnetic ions placed inside a high-Q microwave cavity resonator tuned to the transition frequency.||When population inversion is achieved between two energy levels and RF is applied, stimulated emission produces high gain with extremely low noise figure.
||MASERs were historically important as front-end amplifiers in radio astronomy and deep-space communication (e.G., tracking spacecraft signals).||They are more complex, bulky and expensive than semiconductor amplifiers, so in modern systems they are used only where ultra-low-noise performance is critical.||
Tuned Detectors (Microwave Detectors)
||Tuned microwave detectors are used to detect, rectify and measure microwave signals at or around a particular frequency.||They typically consist of a Schottky (crystal) diode detector combined with a tuned circuit or cavity (e.G., resonant waveguide or coaxial cavity) that selects a narrow frequency band.||At low signal levels the diode operates in its square-law region, so the output DC voltage is proportional to input power;
At higher levels it approaches linear detection.||They are widely used in power meters, VSWR measurements, frequency monitoring, slotted-line measurements and receiver test set-ups.
||Because the detector is tuned, it offers improved sensitivity and selectivity compared to a broadband detector, but only over a limited frequency range.
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BARITT Diode (Barrier Injection Transit-Time Diode)
||BARITT (Barrier Injection Transit-Time) diodes are microwave negative-resistance devices similar in purpose to IMPATT diodes but based on minority carrier injection rather than avalanche breakdown.||They use structures like p–n–p or n–p–n;
Carriers are injected across a forward-biased junction, drift through a depletion/drift region, and arrive at a junction with a transit-time phase delay that creates negative resistance at microwave frequencies.||Compared to IMPATT diodes, BARITT diodes have lower noise because they do not use avalanche multiplication, but they generally offer lower efficiency and lower output power.
||They are used mainly as microwave oscillators in the GHz range for local oscillators in receivers, small low-power radars and communication links where low noise and moderate power are sufficient.||Their tuning range is moderate and biasing must be controlled so that the device operates in the desired transit-time regime.||
TWT Amplifier (Traveling-Wave Tube Amplifier)
||A TWT is a broadband vacuum-tube microwave power amplifier in which an electron beam interacts over a long distance with an RF wave traveling on a slow-wave structure (typically a helix or coupled cavities).||The slow-wave structure reduces the phase velocity of the RF wave to nearly match the electron beam velocity; this allows continuous interaction so that electrons transfer kinetic energy to the RF wave along the tube.||Key advantages:
very wide bandwidth, high gain (40–60 dB or more)
, and significant output power at microwave and millimetre-wave frequencies.||Because of these properties, TWTAs are widely used in satellite transponders, high-power radar transmitters, long-distance troposcatter or microwave links, and ECM/EW systems.
||They need associated subsystems like electron gun, focusing magnets, RF input/output couplers and a collector, and operate at high voltages, so they are more complex than solid-state PAs but still preferred for very high power and bandwidth.||
Circulators||A circulator is a non-reciprocal 3-port or 4-port passive device in which power entering one port exits predominantly from the next port in sequence (1→2, 2→3, 3→4, etc.).||It is usually implemented using ferrite material biased by a DC magnetic field, exploiting gyromagnetic properties to break reciprocity at microwave frequencies.||Typical 3-port use: port 1 = transmitter, port 2 = antenna, port 3 = receiver or matched load. Transmitted power goes 1→2; any echo or reflection from antenna goes 2→3, protecting the transmitter and allowing duplex operation.||Circulators may cover narrow or moderate bandwidths and require careful design of ferrite, bias magnet and waveguide/microstrip structure.
||Applications:
T/R duplexers in radar, combining/splitting signals, antenna feed networks, protection of sensitive components and creating isolators by terminating one port
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Isolators||An isolator is a two-port non-reciprocal device that allows microwave power to pass with low loss in one direction and presents high attenuation in the reverse direction.||It is often realised from a 3-port circulator with the third port terminated in a matched load: power from port 1 → port 2, but any reflection from port 2 is sent to the load (not back to port 1).||Isolators protect oscillators, amplifiers and other sensitive sources from reflections due to mismatch, preventing instability, oscillations and changes in output power or frequency.||They are widely used in measurement setups, transmitter chains, low-noise receiver front ends and high-power microwave systems where good VSWR protection is essential.||Ferrite isolators are common in waveguide and microstrip form, designed for a specific frequency band with specified forward loss (e.G., < 1 dB) and reverse isolation (e.G., > 20–30 dB).