"why not emit at a higher and thus more useful frequency?" Because they tend not to follow the Earth's curvature.
"The signal was long believed to be that of an over-the-horizon radar (OTH) system. This theory was publicly confirmed after the fall of the Soviet Union, and is now known to be the Duga-3[1] system, part of the Soviet ABM early-warning network. NATO military intelligence had photographed the system and given it the NATO reporting name Steel Yard."
en.wikipedia.org/wiki/Russian_Woodpecker...so the Russians themselves confirmed it.
The Germans used it on their U-boats, and it was first used/discovered by the French...
"French scientists started researching of underwater radio communications in 1917. Head of researching was French scientist Broji. They found that some radio waves partly penetrate into water. They used radio frequencies from 15 kHz to 33,3 kHz. (Very Low Frequency band, VLF). With the strong powered radio signals they succeeded to communicate from a distance of 16 nautical miles, depth of submarine's antenna was 10 metres. "
Americans stqarted their ELF system in 1984 the Russians in 1990 [or so] prior to that it was all experimental between those two, BUT that's not to say that other countries did not have their own communications systems. The Woodpecker was on shortwave, the DUGA system is the longwave.
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"Radio amateurs conducted the first successful transatlantic tests[3] in December 1921, operating in the 200 meter mediumwave band (1500 kHz)—the shortest wavelength then available to amateurs. In 1922 hundreds of North American amateurs were heard in Europe at 200 meters and at least 20 North American amateurs heard amateur signals from Europe. The first two-way communications between North American and Hawaiian amateurs began in 1922 at 200 meters. Although operation on wavelengths shorter than 200 meters was technically illegal (but tolerated as the authorities mistakenly believed at first that such frequencies were useless for commercial or military use), amateurs began to experiment with those wavelengths using newly available vacuum tubes shortly after World War I."
"1912 LT (later RADM) Stanford C. Hooper, USN, becomes the first U.S. naval officer
to be assigned the title “Fleet Wireless Officer.”
Naval aircraft equipped with radio set succeeds in transmitting a message from a
height of 300 feet to the USS STRINGHAM over a distance of 3 nautical miles.
Contact is also made by the same aircraft with the USS BAILEY and Radio
Station at Annapolis, Md.
Congress passes legislation providing for the regulation of radiotelegraphy.
Navy opens its radio facilities to commercial traffic.
Navy modernizes its coastal radio stations.
Navy establishes the Office of the Superintendent of Radio under the Bureau of
Navigation. Technical aspects of radio communications are assigned to the Naval
Bureau of Steam Engineering.
A Naval General Order establishes the Naval Radio Service, predecessor of the
modern Naval Communications System. CAPT W.H.G. Bullard, USN, is first
Superintendent.
Navy is first to change the name of its “wireless stations” to the new term, “radio
stations,” a term to be adapted by the entire communications industry.
Navy submarines, equipped with radio signaling equipment, receive and transmit
signals off Newport, R.I., at a range of four miles.
The U.S. Navy establishes a “transmitter” laboratory at the New York Navy Yard
and a “receiver” laboratory at the Navy Yard Washington, D.C.
1913 The cruiser USS SALEM tests naval radio communications by maintaining
continuous contact with the U.S. mainland during a voyage across the Atlantic
Ocean."
" 1918 Submerged Navy submarine receives and sends radio signals. Reception is found
possible from overseas stations in a submarine whose periscope is 21 feet below
the surface."
StuartG
www.radioman.org/history.pdf