During the earlier days of radio communication it was believed that short waves ranging from 3 to 30 MHz can not span the globe, so not useful for long distance wireless communication. Till 1912, radio frequencies used for communication were restricted to the frequencies below 2 MHz (2000 KHz or a wavelength of 150 metres and more) which found utility in short distance communication only. By 1923, as many as seven hundred broadcasting stations came into existence in the USA. Incidentally, these stations were operated by a group of radio experimenters who were amateurs.

The 1906 Berlin Radio Conference allocated medium wave frequencies like 500 and 1000 kHz (600 and 300 metres respectively) to the maritime services and put on restrictions in amateur radio operation in these bands. As a consequence of the forcing out of the radio amateurs from medium wave frequencies, they had to jump to the higher side of the radio spectrum. The government restriction not allowing these ham radio stations to operate in the short waves indirectly brought a revolution in the field of worldwide radio communication. Specialists were non-plussed to see that with the aid of short wave amateur radio stations, it was possible to set up communication with any point on earth! Little transmitters which consumed no more power than a twenty watts bulb sent signals farther than long wave stations with a capacity of several thousand watts. Most of their long distance two-way radio contacts were incidental.

The beginning of World War I even put an end to the amateur radio operations. Recognising the plight of the radio amateurs and considering amateur radio as an useful activity, International Telecommunication Union (ITU) retained different parts of the short wave spectrum for their exclusive use and ham radio operators once again started their transmission from October 1, 1919.
The main reason of preventing the experimental stations from invading the long wave bands of the radio frequency spectrum was to avoid overcrowding of the long waves, which were used extensively by government professional and commercial stations.

Hiram Percy Maxim, who was the founder of American Amateur Radio Relay League (ARRL) made radio contact between Schnell, USA and Delom, French in November 1923 using about 2.7 MHz frequency, which was in the lower side of the short wave band. In the month of March, 1914, Hiram Percy Maxim tried in vain to contact Springfield, Mass from his station in Hartford, Conn, some 37 Kms away. His equipment could not transmit over this distance, but a station between the two points relayed his message and also relayed back a prompt reply. This incident led Maxim to conceive of an organization across the country. He and Clarence Tuska, a college student founded the American Radio (Amateur) Relay League-ARRL in May, 1914 and they started publishing an amateur radio journal named "QST" in 1915.

As you have seen that during the 1920s transmissions in shorter wavelength were achieved and found to be more efficient for long distance communication, by 1924, many government and commercial wireless stations started operating on the short wave, below 200 meter wavelength, which led to interference and great confusion. Then the International Radio Conference of the International Telecommunication Union (ITU) was held in 1927 in Washington. Eighty countries were represented in the conference. In addition to frequencies allocated to different services in the 1906 Berlin Radio Conference, in which maritime services were were alloted 500 and 1000 KHz (600 and 300 meters respectively), while frequencies below 180 KHz were alloted to long distance communication by coastal stations; the band between 188 and 500 KHz was allocated to military and naval stations and closed for everyone else, the 1927 Washington Conference alloted different frequency bands to new media like broadcasting and aeronautical services. In this conference the radio frequency spectrum was then fixed to range from 10 KHz to 60,000 KHz, i.e. 60 MHz and also included the short wave range from 3 to 30 MHz.

The problem of variable propagation conditions can be partially overcome by using frequency diversity (as we told above), in which an alloted wireless communication network is provided with several frequency assignments spanning the high frequency (short wave) band of frequencies, so that the radio operator can choose the channel that gives the best results at any given time.

The amateur or ham radio stations are presently alloted the following bands in the radio frequency spectrum of which the 10, 18 and 24 MHz bands are the latest approved by the World Administrative Radio Conference (WARC) at its International Telecommunication Union (ITU) Conference in 1979.ITU
 

FREQUENCY ALLOCATION TO HAM RADIO STATIONS
1800-2000 KHz	160m	Radio Region 2 & 3
1810-1850 KHz		Region 1
3500-3800 KHz	80m	In Region 2 to 4000 KHz
7000-7100 KHz	40m	In Region 2 to 7300 KHz
10,100-10,150 KHz	30m	On secondary shared basis
14,000-14,350 KHz	20m	The most popular short wave ham band
18,068-18,168 KHz	16m
21,000-21,450 KHz	15m
24,890-24,990 KHz	13m
28,000-29,700 KHz	10m
50 MHz	6m	Very High Frequency (for short distance line of sight contact)
144-146 MHz	2m	Very High Frequency (for short distance line of sight contact) In Region 2 to 148 MHz
434-438 MHz		Ultra High Frequency
1260-1300 MHz		For Earth to Space communication
3300-3400 MHz		For Ham Satellite Communication
5725-5840 MHz		For Ham Satellite Communication