Today, in daylight again, I did a repeat of my recent tests with the baseband pre-focused red LED TX (20mA), looking for the signal 0.2km along the road. This time I also took my PC and looked for the signal with Spectran and the SAQ SDR receiver. The audio tone was around 40-50dB over noise on both systems. At this range I would have expected far better S/N. The noise floor looks high. When I blocked off the lens the noise floor went UP about 15dB. Why I wonder? The picture shows the test set-up. If you click on the image you can see
the pre-focused red LED pointing out of the house window and the
receiving kit in the bottom foreground. Based on these results speech communication, even with just the same LED and no external lens on TX, should be possible to at least 5km. With 100mm lenses at the TX end, considerably further should be possible. Below are the test results.
UPDATE: After dark I repeated the range test, just using the crystal earpiece to listen to the signal detected by the PIN diode head. At 0.4km, the greatest distance I could go line-of-sight from my home QTH, the audible signal was STRONG, which is very encouraging.