Frequency Conversion — Hz · kHz · MHz · GHz · THz

Heinrich Hertz (1857-1894) was a Privatdozent at the University of Karlsruhe when Hermann von Helmholtz, his doctoral advisor, challenged him to confirm Maxwell's 1865 electromagnetic-wave equations. Between 1886 and 1888 in his cellar lab, Hertz built a spark-gap transmitter that radiated at ~50 MHz and a loop-resonator detector that registered visible sparks across the room — the first artificial radio transmission. He measured wavelength, polarization and reflection coefficients, proving light and radio were the same phenomenon at different frequencies.

When asked about practical applications, Hertz reportedly answered "Nothing, I guess" — yet within a decade Marconi was bridging the Atlantic with continuous-wave transmissions. The cycle-per-second unit was named "hertz" by the International Electrotechnical Commission (IEC) in 1930, formally adopted by the 11th General Conference on Weights and Measures (CGPM) in 1960 as an SI-derived unit. Before 1960, engineers wrote "cycles per second" or "c/s" on schematics — Allen Bradley capacitors and Hammarlund radios from that era still bear the older notation.

The radio spectrum was first regulated at the 1903 Berlin Preliminary Wireless Conference (Germany, UK, USA, France and 5 others) after the SS Princess Beatrice collision caused by overlapping Marconi and Telefunken signals. The ITU Radio Regulations now bind 193 member states; ITU-R Recommendation V.431 defines the band names — ELF (3-30 Hz), VLF, LF, MF, HF, VHF, UHF, SHF, EHF, THz. IEEE Standard 521-2002 adds the radar-band letters (L, S, C, X, Ku, K, Ka), still used in radar and satellite engineering.

The 60 Hz vs 50 Hz split came from a 1891 engineering dispute. William Stanley's 1885 Great Barrington transformer (the voltage-conversion heritage piece) ran at 133 Hz. Tesla's induction motor patents settled on 60 Hz; Westinghouse adopted it for the 1893 Niagara Falls plant. Meanwhile AEG/Helios in Berlin standardized 50 Hz under the IEC 60038 voltage-and-frequency umbrella that still governs European mains today.

The cesium-133 atomic frequency standard fixed the SI second in 1967 (13th CGPM): 1 s = 9,192,631,770 cycles of the Cs-133 hyperfine transition. Every modern frequency counter, GPS satellite, mobile-network base station and stock-market clock is traceable through NIST F2 (Boulder), PTB CSF-2 (Braunschweig), or NPL NPL-CsF2 (Teddington) to this transition. The 2019 SI redefinition kept the cesium definition unchanged, making frequency the most precisely measured physical quantity in science.

Mobile generations climb the spectrum: 2G GSM 900/1800 MHz (1992), 3G UMTS 2100 MHz (2001), 4G LTE 700-2600 MHz (2009), 5G FR1 410 MHz-7.125 GHz, 5G FR2 24-71 GHz (2018). 6G research targets sub-THz at 100-300 GHz — visible on the spectrum strip just below the optical band. Each step trades cell range for capacity; mmWave penetration drops to tens of meters but channel bandwidth grows from 20 MHz (LTE) to 400 MHz (FR2) per 3GPP Release 17.

Optical and electrical frequencies converge in the THz band. The 2009 EU Tera-MIR initiative and DARPA HiFIVE program demonstrated 0.3-1 THz wireless links and chip-scale THz combs. At 545 THz (visible green), λ = 550 nm — small enough to define photolithography pitch in semiconductor fabs. The spectrum strip on this page spans 15 decades from ELF mains to UV — every named frequency in modern engineering one click apart, anchored to the Hertz 1888 experiment and the 1960 CGPM SI definition.