Loop antenna on 500kHz

My vertical loop antenna (about 80sq m area) made with 1mm diameter wire (tuned with a capacitance decade box) performs quite well on 500kHz judging by reports. Yesterday I lowered the bottom wire to run along the ground, increasing the loop area from about 70 to 80sq m.  Reports from M0LMH (223km) and M0BMU (69km) are pretty good with my 1mW ERP signal at M0BMU up to -5dB S/N on WSPR, which is as good as I've ever had and he's not in the best direction for the loop. Also, PA3EGO was up to -2dB S/N on receive. Results on 500kHz are much easier to obtain than on 137kHz although this is explainable by the much lower ERP (probably 20dB less than on 500kHz).