It would be nice if there were some standards that we could use to evaluate how well a loop antenna performs. So with that in mind, allow me to propose some ideas here:
1. Using a ground wave signal, how deep is the null (in dB)? Because the S-Meters of most radios are not particularly linear, I suggest using the meter reading at the null and then peak the signal. Then set an attenuator to get the same S-meter reading as the null was. Note how much attenuation it was and then use that as a peak to null ratio.
2. Using a reference E-Field source (perhaps a sheet of aluminum at least the size of the loop itself, excited with about 0 dBm at less than 1/10 wavelength away from the loop) How much signal leaks in?
3. If the loop has an amplifier, couple two reference tones in to it and measure the 3rd order IMD Intercept.
4. Overall performance: how well does the loop work? Are there EMI sources that tend to give more trouble with this loop than others? Does the null work well in practice? How durable is the loop?
I know, this is excessively geeky. But having read a few loop antenna reviews, I have to wonder if perhaps there might be a better way to compare them.