Microwave, Radar, and Ancillary Ship and Aircraft Radio Systems
Microwave hardware
Above roughly 1 GHz, lumped LC circuits give way to cavities, striplines, and waveguides. A waveguide is a high-pass structure: below cutoff frequency the wave will not propagate. Klystrons, traveling-wave tubes, and magnetrons generate or amplify microwave power; the magnetron is the classic marine radar tube, pulsed by a modulator and protected by a circulator or duplexer so the receiver survives the transmit pulse.
Radar fundamentals
Radar range depends on peak power, antenna gain, target cross-section, and wavelength, with a fourth-root dependence on power. Short pulses give fine range resolution; narrow beams give fine bearing resolution. PRF sets the maximum unambiguous range: a longer listening interval reaches farther but samples less often. X-band (about 9 GHz, 3 cm) gives good resolution with small antennas; S-band (about 3 GHz, 10 cm) penetrates rain better. The radar horizon exceeds the optical one, again roughly 1.23 x square root of height in feet, in nautical miles.
Ancillary systems
A SART responds on 9 GHz and paints a line of blips on a radar screen. An EPIRB transmits 406 MHz to COSPAS-SARSAT with a 121.5 MHz homing signal and must be registered. NAVTEX broadcasts internationally on 518 kHz in English. AIS uses VHF channels 87B and 88B (161.975 and 162.025 MHz) and supplements, never replaces, radar and lookout. VHF DSC uses Channel 70 (156.525 MHz) for alerting only.
Traps
Channel 70 is data only; 121.5 MHz is a homing and aircraft emergency frequency, not the satellite alert channel; a SART is not a voice transmitter.
Rule of thumb: shorter wavelength gives better resolution but worse rain penetration.