Regular reports of my grabber activity and that of others, plus information on QRSS software, hardware and technique that comes my way

Tuesday, September 24, 2013

A Comparison of QRSS and WSPR for Measurement of Signal Strengths

This post describes measurements I made with the help of Keith, G6NHU, to compare Signal-to-Noise measurements displayed by WSPR with those I make using QRSS using the Spectrum Lab spectrogram technique described in a previous post

The versatile Ultimate 2 from QRP Labs has the capability of transmitting both QRSS and WSPR and can do so in the same message frame.  G6NHU has been doing this with his U2 for some time now by designing a message that sends QRSS for 8 minutes followed by a 2 minute WSPR frame, making it possible to monitor both modes on a routine basis.  Here's what it looks like on 20m:

Figure 1.  G6NHU QRSS/WSPR message

Keith usually transmits WSPR on the WSPR frequencies but for the purpose of this experiment it was on the same frequency as the QRSS otherwise it could easily disappear in the crowded WSPR band.  Note also that the WSPR message has the same bandwidth as the 5 Hz FSK so doesn't cause problems to other QRSS stations.

For our tests I ran the WSPR program and recorded G6NHU while simultaneously making grabs of the QRSS part with this screen:

Figure 2.  QRSS Screen Used to Measure SNR

The QRSS measurement is based on the SL "long term average" function which in this case is the 2 minute period at the end of the QRSS message shown in Figure 2.   The WSPR measurement immediately followed the 2 minute-averaged grab.  Refer to he link give above for more details.

Over the time frame from 1900z to 0100z on 20m I recorded about 30 instances where G6NHU appeared on WSPR and had a clean grab on the Pensacola Snapper near the same time.  Not having much to do an a very rainy day I read all the QRSS captures and made a spreadsheet of QRSS/WSPR SNR determinations. From this I plotted the SNR's measured via QRSS and WSPR:

Figure 4.  Plot of SNR's Determined by QRSS and WSPR

The big difference in scale is caused by the bandwidths used in WSPR and QRSS.  WSPR relates the SNR to a BW of 2500 Hz while my Spectrum Lab settings are for a noise BW of 0.25 Hz. SNR is inversely proportional to BW because noise power varies with BW.  Therefore the change in SNR expressed in dB is,

delta(SNR)  = 10log((S1/N1)/(S2/N2)) = 10log(BW2/BW1), dB

     note that S1=S2 since the signal is monochromatic and does not change with BW

Thus the change in SNR for 2500 Hz and 0.25 Hz bandwidth's is:

         10log(2500/0.25) = 40 dB

which is added to the WSPR data in Figure 3 to obtain that inFigure 4.

Figure 4.  Correction for Bandwidth Differences

One thing in favor of the QRSS method is that it's possible to actually view the QRN/QRM situation at each determination of SNR...see Figure 2.  Compare this to the busy WSPR spectrum in Figure 1 where interference seems to be a distinct possibility.  As described above, the QRSS and WSPR measurements are 2 minutes apart which may account for some of the differences in agreement.

The large fluctuations in measured SNR are due mainly to QSB and not system noise.

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