WSPR: Signal strength is not the whole story

Yet another warm, sunny afternoon led me to think I might get 500 microwatts from the coast to somewhere today.

When you know the tide very well, you can do this (and beat the tourists to a car parking space!)

 

Sadly, propagation was such that it looked like I might get nowhere!

So, I took the 10dB attenuator off the WSPRlite unit, and sent 5mW instead.  Signals with this power clearly showed that 500 microwatts lay just below the detection limit at the time.  I probably would have achieved one or two spots, but I wasn't inclined to wait for several cycles of WSPR to find out if I ever did; each 2 minutes is a long time when you are standing in a strong wind at the beach!

The results with this extremely low power are very interesting.  I was also running the delta loop back home, with a second WSPRlite at 5mW.

The important message is that when we compare antennas, the plots below clearly show that simply comparing signal strength reports is not enough.  We also have to consider how many spots there were for each antenna.  

For example, if I used the median for EA8BFK's spots of my 5mW from each antenna, I could conclude that the coastal position was only 2dB better than the delta at home - although I would have to add about 3dB to that figure, to allow for the better 'inherent gain' that the delta has over the 1/4 wave vertical.

But this would be a very misleading conclusion, because the coastal antenna was heard by EA8BFK 12 times in just under an hour, whereas the delta was heard only three times.  The same was true with pretty much all other stations.

EA8BFK WSPR RX spots, 14MHz

OH8GKP RX hears the coastal antenna 13 times, but the delta only three times.

SM2HJJ's 7 spots for the coastal antenna, compared with only 1 spot for the delta.

The situation was even more critical for K9AN, R2ARX, LY3EC and WA9FIO, in that they heard the coastal antenna (even though the US stations were landward, though seawater was almost surrounding the antenna locally), but not the delta at all.  In the case of the US stations, the signal levels were all about -29dB, but LY3EC's reception was at -15dB, revealing a very good enhancement of about 20dB in that case, all factors considered.

So compare your antennas carefully!