|
Dear Jim, Stefan, LF group,
BTW, if my guess of 5mV/m is correct, and the loop EMF is 44mV, the effective loop area would be about 18000m^2...
that sounds impressive, based on the antenna length (280m) the "hight" of the loop would be 64m.
At 9kHz the radiation resistance of a 18000 m^2 loop is a mere 8 micro-Ohm, thus a current of 11A is needed to radiate 1mW.
But as the penetration depth into the earth is inverse proportional to the square root of the frequency the effective loop area at 9kHz might be 29000 m^2, increasing the radiation resistance to over 20 micro-Ohm. What
means that 7A antenna current is need to get 1mW radiated.
But even that would require many kW.
73, Rik ON7YD
________________________________________
Van: [email protected] [[email protected]] namens James Moritz [[email protected]]
Verzonden: donderdag 15 juli 2010 1:40
Aan: [email protected]
Onderwerp: LF: RE: DLF passive received with earth antenna
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
|
