Message-ID: <009d01c965ce$5af083f0$0301a8c0@mal769a60aa920>
From: "mal hamilton" <g3kevmal@talktalk.net>
To: <rsgb_lf_group@blacksheep.org>
References: <008c01c9652f$5e5d4950$0301a8c0@mal769a60aa920> <4951C1F1.3040101@telus.net> <008601c965ba$cc55fe80$0301a8c0@mal769a60aa920> <B14F838735DB426DB6581310573DDE37@p4>
Date: Wed, 24 Dec 2008 13:48:51 -0000
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Subject: Re: LF: wspr
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Hi Mark
A sensible response to my own observations, others seem to miss the poiint 
and go off at a tangent if it differs from their own opinion.
In this context about WSPR I am able to see the other mans point of view as 
long as they stick to the topic under discussion.
No one mode suits all environments and this is where experimentation plays 
its part.

----- Original Message ----- 
From: "Mark" <markhughes1@blueyonder.co.uk>
To: <rsgb_lf_group@blacksheep.org>
Sent: Wednesday, December 24, 2008 1:20 PM
Subject: Re: LF: wspr


> Mal
> The answer is simple in many cases.
> Why not?
> I will explain.
> You can use a narrower filter if you like, it would have to be 200Hz wide 
> to
> allow the reception of all WSPR signals in the range of the software. CW
> filters tend to be too narrow for that, SSB  filters are standard on most
> rigs, so it is easy to use this mode. It is also mandatory to use SSB mode
> for transmitting, and on some rigs it is difficult to transmit on SSB and
> have the CW filters in. A good CW 50Hz filter would probably work OK, 
> given
> the very narrow bandwidth of the individual WSPR signals, with a small
> number of WSPR transmissions on the band you could use it. It simply isn't
> needed though, and it is desirable to allow the software to see the entire
> 200Hz segment, even if it is just to allow for drift or decoding other
> signals.
> For some modes the use of narrow conventional filters is bad. They
> intrinsically have poor group delay response and cause distortions to the
> signal not often directly audible, but that can degrade data is a way that
> reduces decoding margins.  This effect is worst when the wanted signal is
> relatively wideband (occupying the whole bandwidth of the filter) or near
> the edge of the passband. DRM, DAB and DTT signals that are OFDM based are
> vulnerable.
>
> All the WSPR operation that the MF band would need, could be squeezed in a
> segment of the band less than 200Hz wide, probably 100Hz would be more 
> than
> ever needed.  WSPR also time division multiplexes, so more than one 
> station
> can occupy the same frequency. No one is proposing WSPR signals all over 
> the
> band, that is self defeating as the software cannot analyse more than a
> 200Hz chunk of bandwidth.
>
> The key point is that just because the receivers have a wider passband 
> than
> actually needed does not usually render the mode ineffective in the 
> presence
> of signals that are outside the decode passband.
>
> Like any system, reducing the bandwidth of any part of the system, means
> that subsequent stages are less affected by adjacent signals.
> The antenna itself acts as a filter because it is tuned, your low pass
> filter removes many big broadcast signals, but no one is advocating a
> tuneable 50Hz wide bandpass filter implemented at 500KHz, or anything
> approaching this, for CW or any other mode. It is simply not necessary in
> most cases, and a disadvantage in most cases.
> Almost all modern conventional receivers have roofing filters that are 
> quite
> wide in early stages, followed by narrow mode specific filters. Just 
> because
> the filter before is wider does not unduly degrade performance of the
> receiver after the more narrowband filtering.
> Interestingly, my grabber receiver has only very simple roofing filters,
> many hundreds of KHz wide and very low Q, they stop almost nothing 'out of
> band'
> All the rest of the filtering is done in software after an A to D 
> converter
> (soundcard) which has a remarkably high dynamic range. There is no AGC, it
> is not needed, and the software filters which operate on the same 
> principals
> as the WSPR decoding system, allow me to listen to a weak signal with an
> unwanted signal over 70dB higher, less than 100Hz away. I can actually 
> still
> read the stronger stations on the band whilst transmitting low power on 
> the
> main antenna, only a few tens of feet away, and a few hundred Hz away.
> Ironically I get more problem when I transmit on 6m, though the RX antenna
> is only about 2 feet away from the 6m beam.
> I hope this helps your understanding of the principals behind WSPR and 
> other
> FFT based systems. They are not so different. If that is not what you are
> looking for then please clarify.
>
> I have to say that the immunity of WSPR to interference is not as good as 
> I
> would expected, if an unwanted signal comes up in the 6Hz bandwidth even 
> for
> a short time, it can corrupt the decode. It is pretty good at ignoring
> strong stable signals that are reasonably spaced away.  last night and 
> this
> morning a small number of decodes of Jim's beacon were lost due to my
> 'wandering sprog'.  The presence of the big CW and QRSS signals in my
> receive passband (pre WSPR processing) does not affect decode. BHZ and 
> DI2AM
> are often 40dB above noise, and they are only a few hundred Hz away from
> Jims WSPR signal, and presented to the WSPR decoder unattenuated by any
> filtering.
>
> I find the levels required to decode WSPR to screen about the same as my
> ability to read QRSS comfortably, and significantly better than my ability
> to read weak CW.
> as an EME operator, I can read weak CW quite well.  The slow QSB on 600m 
> is
> a problem for both WSPR and QRSS. and indeed there may be instances where 
> a
> quick burst of normal CW would be better than the slow modes. WSPRs big
> advantage is that it automatically gathers propagation data. The ability 
> to
> observe and accurately quantify the varying signal strength over time is
> extremely useful.
> Mark GM4ISM


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