Monday, April 21, 2008

VOACAP vs WSPR reports - part2

My previous post on the topic concluded by "I leave you judge...". In fact, there was a trap: VOACAP provides SNR in 1Hz bandwidth, while WSPR provides SNR in 2500Hz bandwidth, thus a difference of 10Log2500=34dB. Quite a difference ! Following are a few more plots, in order to try determining the correct "VOACAP" calibration for WSPR. For this purpose I have used the data collected on my 30m beacon, by W1BW (near to 400 spots). All reports have been normalized to 1W ERP (I used up to 100W at band closure / opening), which explains why a few reports are below -30dB. The first plot shows, on top all the spots collected by W1BW between 1st and 20 April (the dark blue dots). The light blue curve is a simple average on 4 spots and the blue curve the polynomial order 6 function. The other curves represent the VOACAP output for the parameter SNRxx (SNR at required reliability), adjusted for 2500Hz BW, for a circuit reliability of 90%, 50% and 10%. Other parameters: Power 1W, Isotropic Antennas, min angle 3deg, Ant gain 0dBi (both TX/RX assuming omni antennas are used at each end) , Noise -152dBW (CCIR quiet) , Method 30, Absorption "Normal", Coefficients CCIR (monthly) and SSN =15 (value suggested by G4ILO's VOAprop). The "required SNR" was set to 4dB (-30dB min required signal for WSPR to decode, plus 34dB for the bandwidth relation) but this parameter has no impact on the SNRxx output, and the Fprob. parameters were set to default. Last, but not least, the values near the edge (band opening/closure) and during the period 13 to 16Z should considered with care due to the limited number of spots available. Observations: 1) the SNR and SNRxx @50% provide identical results.2)Setting the required reliability to 10% generated earlier predicted band opening and a later band closure. 3) there is a litle time offset between the observed peaks and the predicted peaks which may be explained by the fact that VOACAP provides the value for the 9th hour, which is in fact 9z to 10z.
This has been corrected on the above plot, which shows basicaly the same things, but where all spots collected in the 9th hour were averaged at 9:00z. This time the required reliability for VOACAP was set to 5%, 10%, 25%, 50% and 75%. We can see the impact of the required reliability parameter: Reducing the xx below 20% has for consequence to predict an earlier band opening a later band closure and a higher level. In fact 50%or higher seems to provide more matching results (except in terms of level, but I'll come back on this). I also wanted to see the impact of the other parameters. First the coefficients URSI88 (daily) vs CCIR (monthly), then the absorption model (IONCAP vs Normal). as you can see, those parameters have little impact on this particular path and none on the maximum levels. I also tried the 3 different methods available: method 20 (complete System performance) method 22 (forced short path model) and method 30 (short long smoothing), but on this particular circuit, the 3 methods provided absolutely identical results.
Finaly I made the SSN to vary from 15 (suggested G4ILO's VOAprop value) to 10 (predicted monthly smoothed SSN) down to 4 (VE3NEA's Hamcap suggested value, from the latest table available on his site). At this stage we can see that, using the recommended / defaults values (method 30, absorption Normal, CCIR monthly coefficients) and a SSN=4 for the SNR parameter output, seems to provide the best match in terms of band opening/closure with the observed values. Remain the 16~20dB difference observed near the 2 maximums... Is VOACAP prediction too conservative, is noise level lower than estimated at W1BW location, does he have some RX antenna gain, is the WSPR reported level too optimistic? Might be a combination of all these factors, or did I do a big mistake somewhere... more to follow and of course comments and suggestions are welcome !

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