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Message-ID: <3CFE3CAC98D24985BB1087776B749958@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <810000.37529.qm@web28510.mail.ukl.yahoo.com> <DB3B98175C484C6BB58EE9B76B3D3610@JimPC> <420507.88232.qm@web28104.mail.ukl.yahoo.com>
In-Reply-To: <420507.88232.qm@web28104.mail.ukl.yahoo.com>
Date: Mon, 3 Jan 2011 21:07:50 -0000
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Subject: LF: Re: G3XIZ TX Monday 3 Jan
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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" <m0fmt@yahoo.co.uk>
To: <rsgb_lf_group@blacksheep.org>
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