Flg might be of interest to the SM boys thinking of going on 8 kHz. I found
it in the memoirs of Dr Jack Pierce (MIT, USA) who had a hand in the design
of Omega and Loran-C :
"On the occasion I found most interesting, the boys in Hawaii had pieced
together all the loading coils they could find and managed to get the small
antenna at Haiku tuned to 6250 hertz. At this frequency the power radiated
could not have been more than a very few milliwatts. I observed the signal
at Cambridge (Massachusetts, USA) with our photographic technique, using
triggering pulses at a submultiple frequency, so that a cycle or two stood
still on an oscilloscope screen and the photographic record (which plotted
the oscilloscope picture against time of day) appeared as black and white
stripes. During the night the signal was marvelously clear but at sunrise
it declined so rapidly that at first I thought that the transmitter had
been turned off. This was because that frequency of the signal came close
to or below the cutoff frequency in the waveguide-like space between the
earth and the lowest ionized layer. The height of the layer dropped
suddenly when the first rays of sunlight reached it in the morning, and the
attenuation of the signal increased rapidly. This and one or two other
experiments between six and ten kilohertz convinced us that reliable
longdistance operation was not to be expected at all hours at a frequency
much below ten kilohertz."
Date? January 1946.
The "small antenna at Haiku" was a 300 ft vertical! These experiments led
to the development of Omega, 10.2, 11.666, and 13.3 kHz.