The US Radio Act of 1912 limited amateur ("ham") radio communication to "short waves": wavelengths less than 200 meters, or frequencies of greater than 1.5 MHz.
No private or commercial station not engaged in the transaction of bona fide commercial business by radio communication or in experimentation in connection with the development and manufacture of radio apparatus for commercial purposes shall use a transmitting wave length exceeding two hundred meters...
At the time, these frequencies were thought to be not useful for communication.
Currently in the US, use of frequencies over 300 GHz are unlicensed, but this is likely to change in the next few years.
The International Amateur Radio Union (IARU) August 2006 paper on "Spectrum requirements of the amateur and amateur-satellite services" states that
The following are the bands preferred [above 275 GHz] for the Amateur and Amateur-Satellite Services because they are within the better bands identified above and are free of other radio-frequency lines of the greatest importance to the radio astronomy service. Primary allocations within these bands appears feasible, and the bands from 510 GHz and below are the most ideal for the amateur services, based on atmospheric attenuation, and where the bulk of the allocations within these bands is preferred.
280 - 294 358 - 363 365 - 371 389 - 400 493 - 496 506 - 510 692 - 710 810 - 850
However, David Britz of AT&T wrote in an August 2007 presentation to IEEE 802.11-07 "Proposed ITU/WRC Spectrum and Usage Allocation For Terahertz Frequencies" that
The best Terahertz spectrum is being carved up among science, satellite and amateur radio interests.
Copyright 1999-2007 Mark Wahl. All rights reserved.