For the purpose of this study, the primary interest is in the Washington DC area including adjacent states (Virginia and Maryland in particular.)
From the 1940s through the 1980s, the AM broadcast band extended from 540-1600 kHz. (It was 550-1500 kHz prior to 1941, and was expanded to 540-1700 kHz in 1993.) The large number of AM stations northeast US provides an almost continuous interference band up to 1.6 MHz on ionograms, especially at night, providing a useful frequency reference at the low end of ionograms.
Shortwave bands were more complicated, especially in the Washington DC area where numerous government and military headquarters were based. High levels of HF traffic were likely to originate outside of the broadcast bands with the Cold War and various Asian conflicts occurring in the 1950s-1960s. With that in mind, the primary shortwave broadcast bands were as follows (1952 convention, frequencies in MHz):
The most useful frequency checks should come from the standard time/frequency station WWV. In addition to working on the standard ionosonde design, the NBS CRPL also inherited operation of WWV (which started out near Washington DC in 1923) and was responsible for the new WWVH (Hawaii, 1948).
Boulder became the primary NBS radio research site after 1954, with WWVB starting LF operations in 1956 and WWVL operating at VLF in 1960 from northeast Colorado. WWV HF operations were transferred from Washington, DC (actually, Greenbelt MD at that time) to Ft. Collins CO on December 1, 1966.
The following summarizes WWV operations during the period of interest for this study.
One interesting note is that WWV and WWVH made time announcements in local time prior to 28 April 1967, when they switched to GMT. The switch to UTC came in December 1968.
Snyder, W. F. and C. L. Bragaw (1986), Achievement in radio: seventy years of radio science, technology, standards, and measurement at the National Bureau of Standards, pp. 260-292, Boulder: National Bureau of Standards.
[updated 2013-04-06 03:00 UT]
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