From: "Mike Dennison" <mike.dennison@rsgb.org.uk>
Organization: Radio Society of Great Britain
To: rsgb_lf_group@blacksheep.org
Date: Wed, 16 Jun 1999 09:28:20 +0100
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Subject: Re: LF: DSP?
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In-reply-to: <01BEB7CF.082E06E0@pc033hk.hk.cro.cz>
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Reply-To: rsgb_lf_group@blacksheep.org

OK1FIG wrote:
> We are probably damned to fight agains QRN and QRM. Recently I had 
> opportunity to buy an external DSP device. I wonder what the experience 
> of DSP users are. Certainly it is great to have a very narrow non-ringing filter.
> I mean rather use of  noise-reduction techniques. Do they really provide
> a possibility of making very weak signals (normal CW) more readable?
> Or is it only a toy and I had better to rely on my well-trained ear?
> 

My experience is that with only one weak signal down in the noise, 
an external DSP filter does not give better readability than a "well 
trained ear". As the signal gets stronger, or QRM is audible, it is 
more comfortable listening to the DSP filtered signal. I have not 
found a DSP filter that has an effect on QRN. I think it is worth 
having such a filter, if only to reduce the strong close-in QRM that 
GW4ALG wrote about (not a problem in OK - yet), but don't expect 
to be able to read weaker stations than you can now.

I suspect that the way forward with LF reception is with a very 
narrow-band, highly directional rotatable receive antenna with broad 
front and steep null. This will allow the reduction of amateur QRM, 
electrical noise, and blocking by commercial signals. Interestingly, 
when I built an amplified ferrite rod receive antenna for 73kHz it had 
a much narrower bandwidth (few hundred Hz) and sharper null than 
my 1m loop antenna - but the loop received better!

Mike, G3XDV (IO91VT)
http://www.dennison.demon.co.uk/activity.htm