I don't have references handy. ?But I think the reason for the step-up transformer, is because the small loop has an exceedingly low radiation resistance, and one gets better power transfer with the step-up XFMR. ?You get the most power transfer from antenna to receiver when you have a conjugate impedance match, and the step-up comes closer to matching the real component. ?If there is also a tuning capacitor in there, that might also help, but maybe not necessary for an RX-only antenna.
On LF and MF, maximum power transfer (a perfect match) is not needed because background noise tends to dominate. ?(i.e., more signal from the antenna does not give you more S/N.) ?But if the mismatch is too great, the received signal may be so low that the S/N suffers. ?So the XFMR helps with that.
