While browsing around the net I found the following documents which might be of
"Technique for building and calibrating VLF/LF receive loop antennas"
Unfortunately the photos are a bit dark.
"Dipole and monopole antenna gain and effective area for communication formulas"
"A comparison involving antenna measurements and performance predictions has sometimes revealed a 6-dB discrepancy between the ground-wave transmission measurements and the correspondning calculations"
That 6 dB sounds familiar to me.