Why Analog fast-scan TV in the US uses the NTSC standard, which builds each complete frame from 525 horizontal scan lines. The frame is sent as two interlaced fields of about 262.5 lines each, with the fields alternating at 60 per second so that complete frames appear at 30 per second. Amateur FSTV on 70 cm and higher bands follows these same NTSC timings so ordinary analog TV receivers can display the signal.
Watch out The numbers 30 and 60 are the frame rate and field rate in hertz, not line counts, and 1080 belongs to modern HDTV digital formats rather than NTSC.
NTSC: 525 lines, 30 frames, 60 fields. Only 525 is a line count; 30 and 60 are rates.