LF: RE: DLF passive received with earth antenna

["James Moritz" <james.moritz@btopenworld.com>]

Dear Stefan, LF Group,

The EMF induced in a small loop is:

V = 2.1e-8 * fNAE

f = frequency
N = no. of turns
A = area
E = Electric field strength V/m (this assumes E/H = 377ohms, i.e. not near 
field)
This will be the voltage you measure, assuming negligible loading. As Rik 
says, the ratio of voltages at a given frequency will be equal to the ratio 
of loop areas - this holds provided the loops are a small fraction of a 
wavelength in perimeter. With multi-turn loops, there may be errors caused 
by distributed capacitance, etc., but probably not a significant problem at 
VLF.
Depending on the ground resistance and so on, this "earth antenna" might 
also act as an E-field antenna, or combined E-field / H-field antenna, like 
the K9AY and other "resistively terminated" loops. In this case, one would 
expect a different signal voltage at each end of the loop.
Cheers, Jim Moritz
73 de M0BMU