I have been playing with WSPR for a while over the last several months using my Zachtek 80-10 meter transmitter into a 30 ft high OCF dipole.
I have been slowly adding in attenuation and watching the monitor spots reduce, but some remained.

Sunday night I inserted a total of 36 dBm of attenuation (.050118723363 uW) For 3 days no monitors heard me. Thursday morning a station 406 miles west captured one of my 40-meter transmissions.