Failure to Use WSPR to Compare Antennas
September 24, 2010 2 Comments
Earlier this morning I decided to try to compare my the performance of my three loop antennas using WSPR. Because the WSPR protocol allows you to transmit and receive beacon transmissions and since it records the signal-to-noise ratio (SNR) of each received transmission, I thought it would be easy to compare antennas. I was wrong. It is not easy, even with WSPR.
The plan was to collect around an hour of spotting reports with one antenna, switch to another for an hour, then a third. By comparing the SNR of my signal at some specific remote stations, I would be able to compare the effectiveness of my transmitting loops. By comparing the SNR of some specific remote stations here, I would be able to compare the receive effectiveness.
I ran the experiment and started to look at the data. What I discovered very quickly was that the SNR reports varied considerably even with the same antenna. Each station beacons only every 10 minutes or so, so there is usually 10 or even 20 minute difference between signals of the same station. During this time, propagation conditions along the path change. Here is a graph of the SNR of several stations I received over a couple of hours (the graph only show the stations I received more than 5 times).
It’s easy to see that the SNR changes quickly and dramatically. DK8FT went up by about 10dB in 40 minutes. UA3ARC took a dive of about 10dB at the same time that the signal of OH2GAX was getting stronger. To make sure that this was not something caused by something going wrong at my end, I plotted the SNR of signals received by another station, UA3ARC, over several hours (covering the time in the graph above). The results are similar.
The signal of G4IHZ degraded by 15dB in less than an hour. The correlation between the SNR of the two English stations, G4IHZ and G4CUI, shows that the variations are caused by changes in the propagation along the path (the two transmitter are close so the paths to the receiver are similar).
Clearly, you can’t compare antennas using WSPR using the naive technique that I was using. The differences I saw between antennas were on the same scale as the variation of the reports from a single antenna, so it’s impossible to compare the antennas this way, at least not without filtering the huge noise in SNR that the changing propagation creates.
Other people who took the time to run similar experiments for long periods of time and to average the reports got more robust results. Patrick Destrem switched between two transmitting antenna and averaged the results. He used a computer-controlled antenna switch to switched between the two antennas, so he could use a different one in every WSPR transmission cycle. It is not exactly clear how he got the smooth curves that suggest that his vertical antenna is better than the dipole, but the curve does seem to show this. His plot of all the reports is very noisy, like mine, so the averaging is crucial. I think he fitted a low-degree (probably degree 4) polynomial to all the data points. This fitting is a bit strange, because it suggests reasonable propagation even between 2m and 6am, when in fact there was no propagation at all. Patrick used a similar methodology to compare two receiving antennas, although with a relatively small number of data points.
Stu Phillips compared two transmitting antennas by beaconing from each one for about a week and comparing the reports. This is not as good as Patrick’s rapid-switching method, because propagation in the two weeks is not necessarily the same. He did draw some interesting conclusions from the results, but they the differences were not as dramatic as he hoped they would be.
Charles Preston compared two transmitting antennas by beaconing from them simultaneously, using two separate transmitters. This is even better than Patrick’s rapid-switching method, but this requires more equipment, of course. Unfortunately, he did not collect a lot of data, so it’s hard to draw conclusions from his results.
In hindsight, using WSPR to compare the two transmitting loops was a lousy idea. What I wanted to know was which antenna was more efficient. That is, which one converts more of the input power to electromagnetic waves and less to heat. It probably makes more sense to figure this out with a field-strength meter than with SNR reports from thousands of kilometers away. The WSPR comparisons are probably useful for evaluating the usefulness of radiation patterns, but all my loops should have pretty much the same patterns. I’ll try this again with a field-strength meter to see if I can see a difference.