When television experiments began in the 1920s, the use of the Nipkow disk limited the number of lines that could be used to create a picture. Originally disks produced a 24 line picture, which was gradually increased, but although a 200-line disk had been produced, 100 or even fewer lines were more typical.
The progression in resolution of regular broadcasts had reached 48 lines by August 1928 (Hugo Gernsback), and 60 in March 1929 (RCA), but using electronic scanning made much better resolutions practical. The Don Lee Broadcasting company was sending 240-line images in June 1936, and RCA had increased the resolution to 343 lines shortly thereafter, and to 441 in 1939. But with no standard for all broadcasters, sets could not receive different companies' signals.
And though the Radio Manufacturers' Association (see below) had recommended the adoption of a 441-line standard, some other manufacturers were supporting even higher definitions, such as 605 (Philco) and 625 (DuMont) lines, but with other frame repetition rates as well.
In 1941, the FCC adopted the standard that has been ever since common in the USA: 525 lines.
The early low-definition transmissions could be transmitted in a radio channel of a few kHz of bandwidth. However, as definition increased, bandwidth had to increase. The 240-line signal used by RCA in 1933 (with 24 frames per second) required a 2 MHz bandwidth, and the 343-line transmissions of 1936, with the 30 frames per second that was adopted (based on the 60 Hz frequency of standard AC power transmission) used 5.75 MHz bandwidth. In 1936, the Radio Manufacturers Association recommended a standard of 441 lines, requiring a 6 MHz bandwidth; this was at the time beyond any manufacturer's capability, but soon thereafter Philco produced equipment capable of supporting this standard. The 525-line standard subsequently adopted by the FCC was capable of using the same 6 MHz bandwidth..