[Lf] Narrow Bandwidth reception]

[Andre' Kesteloot]

Mike Dennison wrote:

> >I would like to know other users experiences using the Spectrogram programs.
> >I have frequently found that I am only able to detect the weakest stations
> >using a sample rate of 5.5k with a FFT size of 16384. This gives an
> >on-screen bandwidth of 86 Hz. If I attempt to see more of the band, either
> >by increasing the sample rate, or decreasing the FFT size, I am unable to
> >detect the signal that I know is there.  This seems fairly obvious, in that
> >decreasing bandwidth should improve signal to noise, but I wonder if all
> >users have the same results?
> >73 de Graham B. Phillips.  G3XTZ.
>
> This is exactly my experience which, as you say, is what theory
> suggests. I am sometimes surprised to see spectrograms with wider
> bandwidths (though I understand some people have computer problems with
> the higher sample rate). Another very important control is the averaging
> which must be set to the highest that will allow you to see dots
> properly - note that this will vary with dot length. Spectrogram 5.09
> calls this Spectrum Average and is best set at just above the dot length
> (I set it at 4). Earlier versions needed a much bigger figure - say 10
> times the dot length. This makes a huge difference to the S/N and
> especially the rejection of burst interference such as QRN. Note that if
> this control is set correctly, the full advantages of the ON7YD VCW
> system are realised because the dots can be longer for the same duration
> of message.
> --
> Mike, G3XDV
> IO91VT
> http://www.dennison.demon.co.uk/activity.htm