Microwave oven
A 700 W magnetron at 2.45 GHz mounted at one corner of the cavity. A waveguide section couples the magnetron to the cavity through a slot. A rotating "mode stirrer" (a metal fan blade turning slowly) randomizes the field distribution so the standing-wave maxima sweep around and food heats more uniformly. The cavity is overmoded: dozens of resonant modes overlap in the band the magnetron drives, and the field pattern is essentially a complicated standing-wave landscape. The door has a perforated metal mesh with hole spacing much less than cm, so microwaves cannot leak out (Faraday cage), but visible light passes freely. Safety interlocks ensure the magnetron is off whenever the door is open. The whole assembly is a 1940s tube, a 1950s cavity, and a 1960s control loop: the technology that wins on cost.
X-band weather radar
Standard NEXRAD: 750 kW peak pulsed at 2.7–3 GHz (S-band, actually) or X-band variants at 9.4 GHz. Pulse width 1.5 µs at 1 kHz repetition rate. Klystron transmitter, WR-90 waveguide feeding a 8.5-meter parabolic dish. Circulator routes echoes to a HEMT LNA, then a heterodyne receiver. Doppler processing to extract velocity from phase shifts of consecutive returns. The whole system is a textbook microwave-engineering exercise: tube transmitter, waveguide piping, ferrite circulator, solid-state receiver, antenna feed, and digital backend.
Satellite TV downlink
Geostationary satellite at 36,000 km transmits 100–200 W at 12 GHz Ku-band into a footprint covering a continent. Your 60-cm dish receives at the focal point, where an LNB (low-noise block) integrates a corrugated horn feed, polarization switch, HEMT LNA chain (NF ~0.7 dB), local oscillator (now usually a PLL synthesizer, was a Gunn diode in the 1990s), mixer, and IF amplifier. The LNB outputs L-band (1–2 GHz) on a coax to your receiver, where a tuner picks one channel and demodulates QPSK or 8-PSK. Per-user bitrates of ~30 Mbps after FEC are routine.
5G mm-wave base station
Phased array of 64 to 256 elements, each a microstrip patch antenna fed by a GaN HEMT power amp and protected by a circulator. Each element has its own digitally controlled phase shifter and amplitude weight. Beamforming in software steers the beam (or multiple beams) to individual users in milliseconds. Carrier at 28 or 39 GHz; channel bandwidth up to 400 MHz; per-user throughput multi-Gbps. The whole thing is a substrate-integrated-waveguide and microstrip festival on Rogers RT/duroid, with phase calibration achieved by VNA during factory test.
Police speed radar
Gunn-diode oscillator at 24 GHz (K-band) with a small horn antenna and a coupler that diverts a fraction of the transmit signal to a mixer. The reflected wave from a moving car comes back Doppler-shifted by Hz/(km/h). The mixer extracts this difference. A small DSP counts cycles and reports speed. Total component count: a Gunn diode, a horn, a coupler, a Schottky mixer, a microcontroller. Total cost: a few hundred dollars.
Cassini deep-space link
Eight-watt X-band TWT transmitter on the spacecraft, 4-meter parabolic dish on the ground, cryogenic HEMT LNA at 8.4 GHz with NF 0.2 dB at 12 K. Free-space path loss over 1.4 billion km is around 280 dB. With antenna gain on both sides and the cryogenic LNA, the link closes at a few bits per second after Reed-Solomon and convolutional coding. Without microwave engineering at every step, no images of Saturn.