I have an 148' square loop on the ground. It is a "shielded" coaxial loop design. Why shielded? I have lots of RG-6 coax available, so I figured why not? Likely does not matter much, if at all, but anyway.
Interestingly enough, it is my primary LW, MW, and HF receiving antenna. I sometimes use a preamp, though it is rarely needed for frequencies under 15MHz.
I have attached my extraordinarily basic - as in Paint.NET

- antenna details below. I went for a loop size slightly larger than a wavelength at the 40M band, but being on the ground, the actual resonance could be (and probably is) much lower.
There used to be a dedicated balun at the feedpoint, but it offered little to no discernible difference, so it was replaced with a direct coaxial feed as depicted and several snap-on ferrites on the feedline. I do use a common mode choke (~3000-ohms rating at HF) back near the attached receiver, though it is not really needed either IMO.