Andy Talbot pisze:
Looking at the WSPR database, F6CNI is decoding me consistently in the
0dB S/N region.
At 391km this can't be realistic, and most likely comes from using a
narrow filter - sub 200Hz - before feeding audio to the decoding
software. The WSPR software does need a flat audio response in the
1400 - 1600Hz range, and preferably over the complete SSB bandwidth.
Andy
www.g4jnt.com <http://www.g4jnt.com/>
Andy, LF group
i do not have at hand the wspr source code at this moment but as i
remember it is a 'simple' addition/substraction of 1500
in the decoding/encoding scheme as the 'real' band in the program is
between (-100 Hz; +100Hz)
As i do not TX (yet! :) ) on 137 khz and use WSPR only for
RX'ing it is , say, 'convenient' , to put your TRX into CW mode
and use narrow filters to get rid of qrm otherwise being hard to kill
when using wide (ssb) passband
the issue of feedeing the wspr database with correct freq. is solved by
typing in the rx window the difference between your bfo offset
and the rf carrier frequency
suppose, you are using ssb mode and the rf carrier is on 137.500 kHz so
in order to rx this, you type in the rx window 136.000 khz
tx window 137.500 ; your trx is in usb mode and in the range 1400-1600
you look for the wspr signal
now , you set your trx in CW mode and your BFO offset is 600 Hz , so you
type 136.900 in rx window instead of nominal 136.000
and the report yields correct freq. spot /136900 Hz + 600Hz = 137500Hz/
(probably, in some rx hardware the left/right injection of bfo may be an
'algebraic' problem... )
Finally,
all the wspr S/N reports are presumably reffered to a 2500Hz wide
passband. of course, the s/n report when using 500/250 cw filter is
useless...
but.... all the modern, black, trx boxes have variable SSB filters so
it is hard to believe that everybody is using a unified 2.4 or 2.7 khz
filter
also, those modern boxes have another feature named PBT ( pass band
tuning) Using it you may cut-off a good part of your 2500 Hz filter
either from below or above
the desired freqency so again, the reported s/n in such cases is
questionable...
in my view , the wspr is a weak signal mode - the real challenge is ,
therefore, to get positive decode of a transmitted signal using all
accessible measures
(filters,amplifiers, etc...) thus making the reported s/n rather of
secondary importance ..
the above statement was reffered to 137/500 khz bands.
on classical HF bands things look probably better , being more unified ..
73, Piotr,sq7mpj
qth: Lodz /jo91rs/
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