Comparing 500kHz and 136kHz WSPR over a 3km path

This evening I've been running about 5W PA output (not ERP)  from IRF510 PAs on both 136 and 500kHz WSPR and getting reports from G6ALB 3km away. I'm using the same ATU (just different capacitor settings) into the same loop antenna. On 136kHz I get-20dB S/N whereas on 500kHz +10dB S/N. This is a difference of 30dB!! 136kHz seems a heck of a lot harder than 500kHz.

LATER: Just got this from Steve Olney - VK2XV:

G'day Roger,

I read with interest your observation of the behaviour of the "in the air" loop via WSPR.

"On 136kHz the best report was about -20dB S/N whereas on 500kHz it was +10dB. This is a difference of 30dB!!"

I am not surprised at the 30dB difference as the radiation resistance varies with the 4th power of frequency for the same dimensions giving 23dB difference (i.e. 10 log ((136/500)^4)). In addition WSPR reports a S/N against in-band noise.    Atmospheric noise is inversely proportional to frequency and there is about 36dB difference (500kHz being quieter) between 2200M and 600M. This difference is effectively reduced by man-made noise which is higher wrt to atmospheric noise at 500kHz compared to 136kHz. This reduces the difference in noise levels from 36dB to about 10dB to 15dB (depending on location). Therefore I would expect about (23 + 10) = 33dB to about (23 + 15) = 38db difference between WSPR reporting for 2200M and 600M.

Your measurements are reasonably close to this - given there would be other differential effects such as ground loss.

Keep the interesting stuff coming !!!

73 Steve Olney - VK2XV