Some years ago (in the days when I was fit and well) I tried these compact receive only antennas on 137kHz and higher bands. They work on the principle that S/N matters more than absolute sensitivity, especially on lower bands when external noise is the limiting factor rather than rig noise figure. I recall using an EFP mag mount on the car and driving around picking up my QRSS3 signal on 137kHz at 24km (still a decent signal even with very very low ERP from my earth-electrode "antenna" in the ground. Having great success, I gave up and came home! Just a small whip, very very low TX ERP and 137kHz. These really work!