Using WSPR to compare antennas – part 3
Next-Gen WSPR System

WSPR Spots & Propagation

I've spent some time experimenting with VOAAREA to generate prediction maps for my 200mw 30m WSPR transmissions. VOAAREA is one of the tools in the NTIA/ITS HF Propagation Prediction suite originally developed for Voice of America. VOAAREA generates maps for any of the prediction parameters generated from the underlying VOACAP prediction engine.

The tool suite is a professional HF propagation prediction package and you can download it from the VOACAP web site.

This web site also provides links to how to use the different programs in the package as well as some interesting articles about HF propagation prediction.

If you have EZNEC+ or EZNEC Pro you can model your antenna in EZNEC and then save it in a format that is readable by the VOACAP suite.

I started using the VOAAREA tools to try and explain why Larry WB3ANQ gets such great signal reports (even at 5mw power levels) from David VK6DI near Perth in Western Australia. I'll write up those results in the next post.

Using VOAAREA I generated a full days worth of propagation predictions for my 200 mW WSPR transmissions on 30m using a 160m inverted L antenna loaded up with a remote antenna tuner. I modeled the antenna in EZNEC+, saved as a model for the VOACAP engine and then did a batch run for each hour of the day (in UTC of course) for April 2009 with a predicted smoothed sunspot number (SSN) of 8 (the current forecast from NOAA).

Using Photoshop I took the map for each hour and generated a short video of the propagation progression through the day. You can watch the video by clicking on the video below (its hosted on YouTube).

The video views best in HD format by clicking through to the YouTube web page.

VOACAP uses a signal to noise ratio (SNR) normalized to dB/Hz. The WSPR spot database at records the SNR levels from WSPR which are in dB in a 2500 Hz bandwidth. To convert from the VOACAP SNR levels to WSPR you have to add approximately 33 dB. So a WSPR spot of -27 dB corresponds to a VOACAP SNR of -60dB/Hz. The map is color coded with a spot of -27 dB as the lowest level.

You can see spots in the WSPR database down to -32dB – but in practice most stations seem to bottom out around -29 dB on receive so I modeled -27 dB as the lowest level of interest.

The model runs assume a "residential" noise level of -145 dBW/Hz – I suspect that most urban noise levels are higher than this by perhaps 10 dB (-135 dBW/Hz) so its worth taking that into consideration when comparing the predictions against spots in the database.

I haven't worked out a way to automate taking spots from the database and placing them on the map – I'll add that if I ever get around to it – however, comparing spots to predictions looks pretty accurate.





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