Dear Stefan, LF Group,
I checked the position of my portable location, it is in IO91WR, at 51.7104
degrees N, 0.12549 degrees W, if my GPS receiver is to be believed, making
the distance 658km.
Jim, what about the effect of your band filter? Gives this a significant
S/N improvement on 8,97? Can you show us 2 pics of an overview, say from
0...20kHz, with and without the filter? Would be interesting!
I will try to make some comparison spectrograms when the man-made noise
level is lower than it is tonight!
The apparent SNR increase results from the limiting applied to the signal.
This eliminates a large part of the energy of the high-level QRN impulses,
while having little effect on the signal and "white" noise background. But
this only works if all the signals are below the noise level in the
bandwidth applied to the limiter, and if this bandwidth is much wider than
the signal bandwidth. If there is a strong signal present, the limiter
output is mostly just this signal, with other signals and noise suppressed
(similar to an FM IF limiting amp). For this reason, the filter was added to
reduce the level of the powerful VLF utilities, and the lower frequency
mains noise, and TV line timebases etc., in the sound card input signal. I
think it should be possible to configure Spectrum Lab to do the necessary
filtering, along with limiting and FFT, but I have not yet been able to get
it to do all this at the same time as a high resolution complex FFT centered
on 9kHz - perhaps Wolf could comment?
What do you think about active E-field antennas? I think i will try such
one the next time using my much liked BF862 and a simple band pass filter,
just like in your preamp. I got the idea that i can watch my own signal
receiving here in Heidelberg.
From results others are getting, clearly E-field active antennas can give
good results with a very small antenna. However, the directional nulls in a
loop antenna pattern are often useful in reducing QRN or QRM, if the
directions are favourable. At my QTH, broadband active antennas are not very
practical, due to the Brookmans Park AM broadcast transmitters - there are 4
transmitters within 1km of my location, with 400kW total, and the FS peaks
at 20V/m or more here. This will overload even a good broadband active
antenna. So my antennas must include some sort of integrated filtering,
which is easily achieved with a loop antenna.
I plan to key the PA via a laser beam from the roof of my flat, just over
the city ;-) Maybe going to infrared ;-)
Oh, I think definitely a high-power visible laser - green would look nice...
Cheers, Jim Moritz
73 de M0BMU
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