Bob, VK2YQA wrote:
"What is the RX level on earth etc.?"

J.D. Kraus was a great radio astronomer. On page 403 of the 3rd. ed of
"Antennas" which Cecil touts is found eqn. 5 for the Source Flux Density
in watts per sq. mtr. per Hz bandwidth.
Kraus notes that:
"In radio astronomy observations, flux densities are very small, and the
unit of convenience is the jansky (jy) = 10 to the minus 26 W/m squared/
Hz bandwidth, after Jansky who made the first radio astronomy
observations in 1933.

I`ve worked with wide-band terrestrial microwave receivers (2 or 3 MHz
bandwidth) having sensitivities of about -90 dbm, and 2-way FM radios
having sensitivities of about 1/4 microvolt for a bandwidth of about 16
KHz. Both examples were about state of the art at the time.

I`d bet on Kraus anytime.

Best regards, Richard Harrison, KB5WZI