long wave
Technical term in physics and telecommunications for radio waves with wavelengths longer than 1 km (frequencies below 300 kHz).
Long waves can propagate relatively easily even underwater.
短波 refers to radio waves with short wavelengths (about 10–100m), while 長波 refers to radio waves with much longer wavelengths, making them opposite ends of the spectrum.
very low frequency wave; VLF wave
Compound of 長 (long) and 波 (wave), a straightforward calque from English 'long wave'.