30 January 2009

Optics 1 : Electronics 0, tale of a partial failure

Here follows the report of a partial failure of an optical one-way contact over a distance of about 9 km (5.5 mi).

TX location: my parent's flat in the suburbs of Torino, NW Italy.
RX location: parking lot of Superga hill, 9km East of TX

In between us, right downhill the river Po and then 8 km of city with its heating fumes, car pollution, humid air that created the usual urban haze. Lights up to 10km were visible and the moon was shining on far away mountains.

TX equipment: 10mm hi-bri red led MCW driven at an average of 80mA
RX equipment: OPT201 followed by 488Hz bandpass mounted on a 90mm telescope

On the RX side we also had an EEE netbook running Argo for QRSS work.

Aligning the RX was tricky. Street lights helped to identify the area but not the actual spot. I used a pair of binoculars and cellphone with earphone to talk to the TX site (no HAM there!). No high power lamps on TX side. Then an idea: TX site is right under the landing path to our airport, and an airplane was approaching. When it was right on top of the building we got a "now!" signal.

After was just a matter of aiming the telescope and search for the blinking LED. It was sending 6 dashes, pause, cycle in slow MCW morse. Car stop lights traveling around were catching out attention all over.

Simone, the optics owner and expert, could spot the TX in a couple of minutes. Hooray! In the telescope the 10mm LED was clearly visible but as an average star would look at the naked eye! Buried in yellow street lights and reflections.

We mounted the receiver, adjusted beaming, ..., but nothing. Not even the 50 Hz hum was audible. We replaced earphones with the computer DSP software but nothing. Sometimes 50 Hz & friends were visible in the normal scroll speed, but no 488 Hz line.

Now I've got to work on improvements. I need a larger and/or more powerful source. Those 50dB over background noise we had over the 200m path were not enough for this DX. I will add more LEDs, and maybe put them in the focus of a beam expander (Fresnel lens or parabolic mirror).
I will build an aiming system for the TX, even if it has a rather broad beam.

I will build a k3pgp receiver. I have 5x BPW34 detectors. I'll probably follow Yves's circuit. Any idea for a 2N2482 replacement?

We will wait for a windy day to repeat the experiment.