The above plot shows the 80m WSPR-data collected from W1XP (FN42fo) and my portable station (IN87nn) during 4 nights, from May3 to May7 (about 250 spots). 4 to 10W were used on both sides but all plotted levels are normalized to 1W.
The general trend, over the 4 nights is a 6 to 7dB offset in favor of W1XP transmitter, with a 10dB peak around 01:30. This may indicate a higher TX antenna performance or a higher noise floor (or both) at W1XP. It is interesting to note that at W1XP sunset (when the band opens for him), he seems to hear me better. As suggested by ON4UN in "low band DXíng", the explanation may be linked to a lower noise level while the D-layer is not yet fully dissipated. This is confirmed by the below VOACAP Noise-power plot for Boston area, where we can see that on 3.5Mhz, the noise is minimal 3 hours before sunset and maximal in the middle of the night (The below graph is for a very quiet site, the variation may be partly or totaly cancelled or changed if the main source of noise is local). Those variations, although the power and antennas gain remain constant, do not mean that the propagation is not bilateral,just than the noise is not distributed equaly at both locations. The 7 dB offset trend clearly means, that my TX antenna does not work as well as W1XP's one (unless some problem on W1XP's Receiver, signal treatment, QRM,QRN, birdies, etc...). The only way to know would be to compair with a third station used as reference.