Since that time we have kicked around the possibility of going down to 1 uW and last November started a new campaign to do so.
By early May of 2014 we had reached 15 uW on 30m after months of trying but could go no lower. We need long periods of time free from interference in order to obtain a sufficient number of 10 minute grabs to use image stacking to extract the signal from the noise. Activity has increased since 2011 both from QRSS and other digital modes. The notorious OTHR from Akritori, Cyprus is a regular visitor also. In addition, this was a most unusual Winter here in North Florida with frequent thunder storms related to the infamous Polar Vortex which plagued the East Coast this year.
At this point we decided to try 20m to escape the QRM and hopefully see less QRN. The difference was amazing. Not only did we have the spectrum all to ourselves but the QRN dropped almost to nothing. We started at 100 uW and came down rapidly in 3 dB steps to 5.8 uW. After that the difficulty with each 3 dB step increased exponentially with the final step from 2.5 uW to 1 uW requiring 12 days until all the variables lined up in our favor. Here's a 3-day stitched image showing our approach to the 5.8 uW level:
Note that the WA5DJJ signal was still drifting a bit until Dave installed the MEPT in a thick-walled styrofoam box.
On the night of May 17 the 1 uW signal was visible in and out on the 8 hour grabber from 0430z to 1040z. Thirty-eight 10 minute grabs over this time period were processed with the stacking software Rot'n'Stack to produce a barely discernible image in which I could read all the letters of Dave's call. That's 1 uW over a distance of 1164 miles/1873 km. If you do the math that's over one billion miles per Watt for a signal propagated via the ionosphere.
Here is the results of the image stacking:
Note the use of a second signal from WA5DJJ from an older MEPT feeding a vertical. It was not stable in time thus preventing stacking. We used this to give some idea of propagation. KB5R also joined in for the same purpose. The cw signal from IK6ZEW was strong but not stackable. The 1 uW signal eminated from a QRP Labs U2 mept feeding a Cushcraft A3 triband Yagi. The receiving antenna at W4HBK was a 30/40m inverted V with the apex at 18 meters.
For comparison here is one of the better 10 minute grabs:
It's fun to compare our miles/Watt to that of the NASA's deep space probe Voyager 1 which at a distance of 11.8 billion miles from Earth running a 20 Watt transmitter which gives 590 million miles per Watt. Of course they are sending HiDef images and telemetry compared to our 6 scratchy letters but on the other hand they are using a 70m dish at the receiving site.
Here is the story from Dave's point of view: