With the risk of being tedious I just want to inform that I made some more FS 
measurements Saturday morning.
I have replaced the untuned loop with a tuned ferrite antenna + FET preamp.
The new meter is checked with a par of Helmholtz coils with radius 0,44 m.
The reason for rebuilding the instrument is A) To make it more portable  B) To 
get more references C) It fits in a normal sized pair of Helmholtz coils.
The new meter shows 10 dB increased sensitivity compared to the untuned loop.
Quite useful in measurements at larger distance.

This new setup indicates larger values than the previous.
The difference between calculated and measured values is now approx 1,5 dB.
More details here:
http://home5.swipnet.se/~w-54761/fs.htm

/sm6pxj
Christer