RadCom April 2024, Vol. 100, No. 4

T he 10m band is in great shape as we approach the peak of solar cycle 25. For years, I have used a 500mW 10m WSPR beacon, so the possibility of using even - lower power came to mind. It would be fun to turn off the PA and try WSPR at even-lower powers, having been spotted the world over at 500mW. Figure 1 shows my WSPR beacon, with a switch to turn on and off the final-stage power amplifier. The red button is for synchronising the beacon with the start of the universal-time (UT) minute. The experiment These days, 500mW seems like really high power! Remembering that a typical LED light bulb is 10W or so, even 500mW is miniscule, so 500 µ W seems like nothing at all. Using the tinySA spectrum analyser, the output power of my beacon was verified as being just 500 µ W (0.5mW). To my delight, even at this tiny power level, the signal was spotted by six stations on three continents including the USA in my first experiment. Of course, there are fewer spots than when I am using 500mW, yet it seems amazing that, even at this tiny power level, it is copied at all. Unfortunately, I did not save the WSPR map which showed these spots, so I ran the experiment again, and this time my signals were spotted in four places (See Figure 2 ). The antenna I am using is just an end-fed long wire, so nothing special. Reports received suggest that, if the receiver noise level is low enough, signals even weaker would be spotted. Maybe the next test will be at 100 µ W (0.1mW). By the way, please email me for details of my equipment, and my method of measuring the power, if you are interested. Of course, it is the signal-to-noise ratio (SNR) at the receiver which determines whether or not a signal can be decoded, not the actual power level of the transmitter. WSPR provides a processing gain enabling a signal with an SNR of about -30dB to be decoded in a noise bandwidth of 2.5kHz, and 500 µ W is 30dB lower than 500mW, so that any station which can decode my 500mW signal with an SNR of 0dB or better should be able to decode my 500 µ W signals, all other things being equal. However, the variability of the 10m band ensures that all other things are not equal, and so we should not be too surprised by these results. The beauty of the 10m band is that it is very wide, antennas are small, and noise levels are much lower than at lower frequencies, so even low power can go a very long way. It is wide enough to accommodate all modes, not that WSPR needs much. A WSPR signal has a bandwidth of about 6Hz. This is 6/2500 of the standard bandwidth adopted by radio amateurs for noise calculations. This fraction, expressed in decibels, is about -26dB. If ‘good copy’ on an SSB signal 2.5kHz wide requires, say, +10dB SNR, then the lower bandwidth of WSPR already achieves a gain of 16dB over an SSB signal, ie the WSPR signal can be 16dB, or 2-3 S points weaker for ‘good copy’. This explains part of WSPR’s huge processing gain compared to SSB. The rest can be ascribed to the way in which WSPR messages are constructed, and the length of time over which they are transmitted. The popular FT8 mode is a bit less sensitive than WSPR, but is still much more sensitive than CW. Using this mode, very-low power can result in two- way contacts across the planet. When the 10m band is really active, even the USA may be worked using amplitude modulation (AM) with just a few watts and a simple antenna. So, get on to 10m and enjoy it! Even with the old Foundation power limits, working the world is possible with 10W pep of SSB and a simple antenna. In the past, antennas have been just a CB vertical or wire dipoles. I worked QRP DXCC on 10m SSB back in the 1980s. There is no need for a linear amplifier or a big beam. With 1.7MHz bandwidth available, it is extremely rare to find the band busy, with the FT8 ‘window’ being just a few kHz wide, and the primary WSPR ‘window’ just 200Hz wide. The sort of experiment described here could be replicated by adding suitably-rated external attenuators to any WSPR transmitter, but it is very important that any attenuation is put right on the output to avoid leakage from the coaxial cable. Feature April 2024 67 Roger Lapthorn, G3XBM rogerlapthorn@gmail.com A QRPP experiment with 10m WSPR FIGURE 1: My WSPR beacon, showing the switch to turn on and off the final-stage power amplifier. FIGURE 2: Four WSPR spots with 500 µ W transmitter power.