Topband: long daytime propagation

[k9la at frontier.com]

Arthur KB3FJO said:  . . . and achieved long-distance propagation by way of a signal that was reflected or refracted by the bottom of the D layer, and so didn't suffer the absorption that low-frequency signals usually suffer in that region.
That's the principle upon which LORAN-C worked (LORAN-C is/was down around 100 KHz). Its long-range capability was due to enough refraction to keep it from getting too high into the lower ionosphere – where absorption occurs. This also made is somewhat less susceptible to solar disturbances, as most disturbances affect the ionosphere above the D region.
But the amount of refraction is inversely proportional to the square of the frequency. Thus 1.8 MHz RF will necessarily refract (be bent) less for the same electron density profile, and thus it will get higher into the ionosphere than lower frequencies like 100 KHz.
Using a readily-available tool (Proplab Pro), we see that 1.8 MHz RF during the day still goes through the absorbing D region before being bent and returning to Earth. Thus "normal" absorption combined with transmit power, antennas, and noise environment determines how far 1.8 MHz will go during the day. This exercise goes a long way in debunking the myth that 160m is only good for short local contacts during the day – but it depends on your station capability.
As a side note, if one plays with PropLab Pro enough one will recognize from the ray traces that the extremely low elevation angles on 1.8 MHz at night appear to offer an advantage over the "normal" higher elevation angles. I believe this is why VE1ZZ does so well – his location allows a good amount of energy at the extremely low angles. Whether land-locked guys can take advantage of this is what's being discussed right now on this reflector.
Carl K9LA