When I first started using my Raspberry Pi as a WSPR beacon transmitter I was surprised at how quickly reports from across the Pond started showing up. I was only using 100 mW of power, solar conditions are the worst they've been in ages and yet, I was easily spotted by DX stations in numerous countries.
|20dB attenuator = 1 milliwatt out|
A 6dB attenuator cuts the output down to 25 mW. A couple of N-BNC adapters and a length of RG58 reduced that to 20 mW as registered on my WM-2 QRP wattmeter. It would be a miracle if anyone heard me at that level and in these conditions, right?
In just a few transmit cycles at 1/50th of a watt, I was heard by G4ZFQ, ON7KO, DK8JP, DK6UG, F5SN, HB9ADJ and VK3WHO. My antenna is a 2-element Yagi up 15m and poited at EU.
The next day I replaced the 6 dB attenuator with a 10 dB pad cutting my output down to 10 mW. Many US and VE stations were copying me - and then came reports from ON7KO and VK3WHO.
The report from VK3WHO was a surprise - the antenna was pointed at Europe, so he was copying me either off the back corner of my antenna or via long path. Amazing either way given the power level of 1/100th of a watt. On both days, he copied me at almost the exact same time (2216Z and 2238Z) at these two power levels and then no more - as if the band opened briefly to the extent communications was possible and then closed again until the same time the next day.
I am now WSPRing at 1 milliwatt and have yet to be heard outside of NA.