LF: RE: DLF passive received with earth antenna

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

Dear Stefan, LF Group,

Taking a guess at ERP of DHO38 as 100kW, the FS at 411km distance would be 
about 5mV/m (if ERP were only 10kW, FS would be 1.7mV/m, if it were 1MW, 
17mV/m, so 5mV is probably not too many dB wrong). The EMF induced in a 
1-turn, 1m^2 loop would be about 2.5uV.  From your description yesterday, 
this should give a level on the SpecLab display of -9dB -(44mV/2.5uV)dB 
which is -94dB, so below your background noise level, as you discovered.
The noise voltage of a TLC271 is typically 25nV/sqrtHz - this is a poor 
match for a low impedance source such as a single-turn loop, but even so the 
signal level should be well above the noise. With 3Hz FFT resolution, op-amp 
noise might be 50 or 60nV, which would be about -127dB on the SpecLab scale 
according to your calibration. Therefore the op-amp is probably not the 
limiting factor here, it is more likely the sound card input is determining 
the noise floor. So you need to increase the gain in front of the sound card 
by 30dB  - 40dB to get a good SNR.
You could do this by:

-Increasing the number of turns in the loop - e.g. a 30 turn loop like I 
used for 9kHz reception, which would increase EMF by nearly 30dB (it would 
also increase Z of the loop by a factor of about 900, but at VLF this would 
still be small compared to the op-amp Zin)
-Put a step-up transformer between loop and op-amp - a 1:30 ratio would 
achieve a similar result to having a 30 turn loop.
-Increase the gain of the op-amp stage. To get 30dB gain at 23.4kHz, a 
TLC271 has marginal gain-bandwidth product; an OP-27 would be better in this 
respect as well as lower noise.
Or some combination of these things. But remember this will also increase 
the level of unwanted signals at the soundcard input - if you have any 
nearby broadcast stations, you might need to include low-pass filtering.
BTW, if my guess of 5mV/m is correct, and the loop EMF is 44mV, the 
effective loop area would be about 18000m^2...
Cheers, Jim Moritz
73 de M0BMU