As you may know, I have no external antenna for 10m at present. Instead I use the 50cm loop on the shack windowsill.
It is extremely hard to measure the difference in performance of, say, a dipole and my loop. Even over quite small distances signal levels can vary greatly. There are many factors: location, time, directivity, polarisation, radiation angle etc.
The best I could do is compare signal levels at similar times in the best and worst directions of the loop with that received by others locally. The best I can work out is that the loop is up to 6dB down. There are quite wide errors bars on this.
Overall, the loop is performing better than I expected.
If there is F2 DX from the USA or South America later, I shall try again. With lower angle signals the differences might be greater.
G0LRD (26km from me) copies far more, although if he can copy signals that are 6dB weaker, I would expect this to make a considerable difference on FT8.
It will be interesting to see my results later with an external antenna. The impression I get is that the small loop is not too bad on Es signals from Europe, but it performs less well with low angle DX signals.
UPDATE 1915z: Well there's a surprise! CX2CC (11128km) was coming through just now. I was getting him 5dB S/N better than G0LRD (26km). This may just be the momentary polarisation being optimum. Still it proves the loop cannot be too bad.
UPDATE 2101z: At the moment I am confused: many times G0LRD is copying better than me and he certainly is spotting far more stations than me. Nonetheless the loop is not at all bad considering its small size and being indoors!