The Radio Act of 1912 covered all the wireless telegraph activity and in 1913 a Radio Call Program was published. Each amateur station had a three characters Call Sign. The first figure was for the district plus two letters. For instance:  1WH was the call sign of Hiram Percy Maxim, a most famous founder of the ARRL.

 

To avoid the interference caused by radio amateurs to the commercial and maritime telegraph stations, radio amateurs were allowed to operate at wavelengths shorter than 200 meters (1,500 KHz). At that time, shorter wavelengths (higher frequencies) were considered as inefficient for long range wireless communication. Amateur radio activity on the short waves spread all over the world and soon it was proved that short waves are ideal for very long range communication, even with the low power transmitters in use by amateurs. Radio amateurs discovered the reflection effect of the ionosphere layers on the propagation of shot waves around the globe.

 

Radio amateurs were involved in the development of radio communication since the days of spark-gap transmitters, through the vacuum tubes era to the existing semiconductors, integrated circuits and digital era. 

 

Starting with Morse Code mode at the Wireless Telegraph Era, radio amateurs operated AM (Amplitude Modulation) mode at the beginning of the Vacuum Tubes Era. The Morse Code is called CW (Continious Waves). The AM mode was replaced by the more efficient SSB mode in the 1960's. FM mode is used on the VHF UHF bands.