Posts by PY2SDR


    Hmmmmmmm... You are giving me ideas!


    Would you like to do some tests with WSPR?

    I have exchanged a couple of emails with Joe K1JT about the QRPp experiment and the -24 dB limit for the FT8 decoder. The following are Joe's remarks.


    ###########################################################################

    By convention all SNRs reported by the software package WSJT-X are

    measured in a standard reference noise bandwidth of 2500 Hz.

    Detailed measurements of FT8 sensitivity in various simulated

    propagation conditions are summarized in a graph posted on the WSJT web site here:


    http://physics.princeton.edu/p…T8_Decode_Probability.png


    This plot was made by generating 1000 simulated FT8 signals at each

    specified SNR, subjecting them to simulated propagation by nine standard

    ITU propagation models, and counting the number of resulting decodes for

    each case.


    Propagation to the geostationary satellite QO-100 ahould closely

    approximate the AWGN (additive white Gaussian noise) path, for which the

    leftmost curve in the plot shows a 50% probability of decoding success

    at SNR = -20.8 dB and a very small (less than 1%) probability at SNR =

    -23 dB. The probability of decoding an FT8 signal with true SNR less

    than -22 dB is extremely small.


    Why is it, then, that the decoder sometimes reports SNRs as low -24 dB?

    And why do we never see reported SNR -25 dB or lower?


    All physical measurements have associated uncertainties. In principle

    we could estimate and report an uncertainty along with each measured

    value -- for example, something like SNR = -18 ± 2 dB. If WSJT-X were

    intended as a measurement tool, we would do it this way; but for its

    intended purpose such reports would be cumbersome, distracting, and

    irrelevant.


    Reported SNRs lower than -22 dB are nearly always underestimates of the

    true value. To avoid unwarranted focus of attention on very small

    reported SNRs, we arbitrarily clamp reported FT8 SNRs at -24 dB.

    ###########################################################################



    Hello Michael,


    I am very puzzled about why some days are different than others. There are many variables involved and not having a way to instrument them, it may be hard to know for sure what could be the cause for the differences we have been observing.


    It would be good not know the antenna RF input power levels and antenna gains that the participating stations are using.


    73, Edson PY2SDR

    It is difficult to know for sure what could cause the differences in the SNR. Variantions in the uplink and downlink radio paths would probably the the main candidates.


    The setup at the AMSAT-BR RX is the following:


    As refletor I have a 1.2m offset dish. I use SDR Console to correct the LNBF frequency drift, demodulate and route the demodulated audio samples (16-bit @ 48 kHz) over the network to a Linux PC using UDP datagrams. At the Linux PC I receive the samples using netcat and route the samples via a shell pipe to a command-line process (ft8d) I have developed to write a WAV file in sync with the 15 seconds FT8 time windows. Once the WAV file is complete, the process calls the FT8 decoder (the original wsjt-x jt9 command-line process). The decoder returns the spots, if any, to ft8d which then filters the CQ spots and forwards them to the AMSAT-BR web site.


    73, Edson PY2SDR

    Until yesterday only one station was able to reach -23 dB (EbN0 = 3 dB), but today one station was able to reach -24 dB and several stations are being able to reach -23 and -22 dB. I am puzzled to what could cause this. No changes have been made to the RX hardware or software setup.

    On a Twitter thread about the FT8 RB experiment, Phil Karn, KA9Q, and Daniel Esteves, EA4GPZ have suggested using EbN0 for measing the SNR instead of using the way the FT8 decoder displays SNR. EbN0 is a more standardized (and perhaps more meaningful) way to measure SNR in digital modes. I have added EbN0 to the RB page.


    http://amsat-br.org/QO-100/qo100ft8.php


    The Twitter post is:

    https://twitter.com/ea4gpz/status/1241377717674745856


    Some background info about EbN0:


    http://www.sss-mag.com/ebn0.html

    Hello Pat,


    The RB receive station does not change the received message. It only filters out non CQ messages. It seems that you station is transmitting CQ.


    73, Edson PY2SDR

    The problem is that JT modes like FT8 can only support a very small set of structured messages (CQ in one of them) or up to 13 alphanumeric characters. Something like RB PY2SDR GG56 would not be possible.


    I have been thinking about using only the callsign (6 characters max) and a six-character locator. For example PY2SDR GG56TV. A different frequency could be used so that the experiment would not interfere with FT8 QSOs. What do people think?

    The experiment is a reverse beacon (receive only).


    73, Edson PY2SDR

    Hello John,


    This looks VERY VERY nice!


    73, Edson PY2SDR