A real life data point here. I live in a low density urban area, fairly close to a bunch of high power AM transmitters. Low input impedance of the loop preamp is absolutely critical here, because it effectively shorts the common mode source. The loop is a relatively high impedance whip for the common mode. I tried M0AYF (exact same topology as PA0FRI, don't see how it can be different from the point of view of CMRR), and it was unusable with a 1m diameter loop. AM intermod was all over the low HF band. The null was also not as good as with the LZ1AQ clone.
LZ1AQ works ok if the current through transistors is sufficient.
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73, Mike AF7KR
