Why Does the Ronja Optical Transceiver work so well in the daylight?

The BPW43 Radiant sensitive area is 0.78 mm^2  - Used on the Ronja Receiver ( 0 dB)
The BPW34 Radiant sensitive area is 7.5 mm^2  - Used on the KA7OEI  Receiver  ( +9.8 dB in area)  
The OPT101 Radiant sensitive area is 5.24 mm^2 - Used on the K0THZ Receiver ( +8.3 dB in area)

So the BPW43 is MUCH LESS sensitive to light (and much faster) than either the BPW34 or OPT101. Which may be one of the reasons they are able to operate in daylight. They only transmit with 17 Mw of output power but the range was limited to 1.4 km. So they were able to achieve an adequate SNR in daylight over that short distance.

So the "Secret" to successful daylight operation for our system might be to use a Bandpass Optical Filter on the receiver (which Ronja did not use), but also use a restricted aperture on the receiver (or a smaller lens) for daylight to help avoid saturation of the receiver. We can also use a negative bias voltage on the OPT101 to reduce the offset voltage at the input of the amplifier and thus extend the ambient light range. 

AND use a high power transmitter. I don't know what the radiated output power is for our LED flashlights, but the input power dissipated by the LED is around 3 Watts. Multiply that by the radiometric efficiency of the LED and you still get WAY more than 17 Mw.

If the efficiency of the LED is 50% (hypothetical - I have seen numbers between 25% and 80% for Red LED's) then our transmitter would be about 20 db more powerful. Each 6db doubles the range, so 20dB would theoretically increase our range by 2^(20/6) or 2^3.33 or 10 times the distance. 10 x 1.4 km = 14 km (8+ miles). We only need audio bandwidth instead of Mhz bandwidth so that should help as well.

These are VERY ROUGH calculations but they give us a VERY ROUGH estimate of what might be achievable with our current transmitter and receiver and give us a direction for further experiments. We are struggling to find 5 mile LOS paths here in central MN so 8+ miles is probably good enough.

Rob - K0XL / K0THZ