Anonymous quotes:
"---but the real puzzle is why only these frequencies?"

Shortwave propagation has always been highly variable. Selective fading
is the rule. So much so, that frequency diversity has proved effective
to improve reliability even though 2 or more frequencies must be used
for only one channel in but a single direction.

Hit or miss communications makes ionospheric radio obsolete for
practical purposes in an age of satellites and optical fiber cables.

For amateurs, hit or miss is part of the fun.

Best regards, Richard Harrison, KB5WZI

Crazy George wrote:
"Gobau?"

Amazingly close according to MSN`s Wikipedia which says------
Goubou linne:
"A single-wire transmission line or waveguide."

Best regards, Richard Harrison, KB5WZI

The folks who keep measuring those signals do so at the same place year
after year.
Others keep building skyscrapers all around their hamshack, thus blocking
the signals.
The signals fading from the shortwave bands is from the perspective of a
receiver being slowly shielded.