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From: James Moritz <[email protected]>
To: [email protected]
Sent: Mon, 3 January, 2011 21:07:50
Subject: LF: Re: G3XIZ TX Monday 3 Jan

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