Just to add a little to the mix: I did a few 5W QSOs on 40 and 30 m today. With a random wire in a tree, there was a huge variance in SNR according to RBN - some 30+ dB in ES and ON land from Munich, as well as many poor 2 - 4 dB spots where I typically get 10 dB or so with a similar setting.
I have to admit this was all CW, and QSB was heavy.
What surprised me was that on 20 m I got very good WSPR reports from several German stations less than 300 miles from my QTH.
And in general, the variation was really huge, as this excerpt from RBN documents:
ES5PC DK3IT 10117.7 CW CQ 3 dB 17 wpm 1220z 30 Aug
ON5KQ DK3IT 10117.9 CW CQ 6 dB 16 wpm 1220z 30 Aug
ON5KQ DK3IT 10117.9 CW CQ 7 dB 16 wpm 1215z 30 Aug
ES5PC DK3IT 10117.7 CW CQ 3 dB 16 wpm 1214z 30 Aug
ES5PC DK3IT 10118.1 CW CQ 35 dB 14 wpm 1159z 30 Aug
ON5KQ DK3IT 10118.3 CW CQ 36 dB 14 wpm 1159z 30 Aug
Same rig, same antenna, same power!