A low noise floor is a near requirement for transatlantic longwave listening. My "shielded" loop made from 148' of RG-6 rests horizontally on the groound. It has massive ground losses, but it also tends to help with noise. Most of my longwave listening is done via S1 or less signals, which usually put those stations right above my local noise floor. I also use the loop-on-ground up to well above 15MHz with decent results, thought usually with a 20dB preamp at such higher frequencies.
Before the loop, I was using a simple unterminated beverage-on-ground antenna built from RG-6 for my low-frequency antenna. Positive to the center conductor, which was shorted to the far end to the shield. Negative to just a ground rod. Horrid losses, but again, very low noise. If I had the space, I would deploy a true terminated beverage at several hundreds of feet in length, aimed towards Europe.