Dear Pete, LF Group,
Good to see you are getting some results... The equipment in use here was
nothing unusual - a "budget" Acer laptop abt 4 years old with internal sound
card, and the VLF loop and preamp, the schematic for which I posted some
time ago. Probably the main difference between my spectrogram and the one
from M0FMT is you appear to be using settings for relatively fast QRSS
speeds; QRSS10 or thereabouts. The pixels appear to have a resolution of
about 100mHz, compared to the 1.5mHz I used. Using the narrower resolution
increases the SNR by about 18dB, at least when averaged over the 600s period
of the FFT. I am also using the SpecLab hard limiter in conjunction with a
9kHz bandpass filter (which could alternatively be implemented in SpecLab
instead of in the preamp, as I have it), which reduces the contribution of
QRN to the overall noise level by perhaps several more dB. It looks like the
noise level in both spectrograms is dominated by external noise, so there is
little that can be done beyond this in terms of equipment design to improve
SNR.
In some cases, using a loop may give some advantage over a vertical, if the
source of QRN is in a different direction from the wanted signal, but often
QRN seems to originate from a wide area, so the sharp null of a loop is not
particularly effective. A definite advantage of small whip or loop antennas
over bigger fixed wire antennas is that they can be moved around easily to
find a location with minimum local QRM, whether this is different places
around your home QTH, or going /P in the middle of a field well away from
mains wiring This has brought a few more dBs improvement when I have done it
in the past. In both cases, having a portable PC is also very helpful, since
it can be operated independently from the mains power, which helps to
determine and/or eliminate this as a noise source.
Trying to receive weaker signals would require longer integration periods
with even higher FFT resolution. The Acer laptop and its sound card are
adequate for QRSS600/millihertz resolution as in this case, but for
microhertz resolution, higher short and long term frequency stability would
be needed for both TX and RX.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "M0FMT" <[email protected]>
To: <[email protected]>
Sent: Monday, January 03, 2011 4:11 PM
Subject: Re: LF: Re: VLF - G3XIZ TX Monday 3 Jan
Hi LF and Jim what pc are you running? Like processor speed and memory?
Please
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