There has been a de-facto HF standard for 60 years.
The USA military first used it in specifications of radio equipment when
placing contracts with manufacturers around the end of WW2. There may have
been some restrictions on publicity at the time.
The Standard is 6 dB per S-unit and 50 micro-volts into 50 ohms at S=9.
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I have read somewhere that it was Art Collins , of Collins Radio fame ,who
first mooted/established the above standard for up to 30 MHz. Much later
,among radio amateurs, the S9 signal level for freqs above 30 MHz was set
at 5 microvolts into 50 Ohms .

Frank GM0CSZ / KN6WH