26 November 2008

Terminator's eye

Finally we've made it! Simone and I could arrange a first complete equipment test.

The OPT201 receiver had been boxed (plastic) and prepared with a 31.8mm standard mount for telescopes. The MCW transmitter was simply mounted on a CD player cover (no picture available yet).

Since I could easily provide a circa 200m path, instead of looking for a dark grass field at <0°c, we decided to stay near the warmth of my place. When Simone looked out of the window was about to ask where the transmitter was. Then he said: "Oh, gosh, there it is! It looks like Terminator's eye!" A 10mm red LED looks pretty bright at 200m. I would compare it to a red light on the back of a car (5W@12V).

Simone brought the "antenna": a tripod and a modified half binocular (dia 50mm, F 300mm). Being experienced at looking for stars he pointed it in the right direction in a couple of minutes, then replaced the ocular with the receiver. Given the strength of the signal we tried staying inside and reducing ambient light (produced by a CFL, by the way) to minimize reflections on the window glass.
The 488 Hz tone was extremely strong and steady, effectively masking out any AC line harmonic. The screenshot below shows more than 50dB over background noise (look at the upper part). The screenshot was taken while my daughter was playing with the laptop keyboard and touchpad sitting on her Ikea drawing desk.

Then I recorded few seconds of signal for postprocess. Since the transmitter was square-wave modulated, many other harmonics were visible:

Note how weak even harmonics are, while odd ones are rather strong, in all the available recorded bandwidth.

Since Simone still had some time available we tried aiming at cars and stars (actually planets, Jupiter and Venus), but nothing could be heard.

Next test will be on a longer distance with some modulated information. Either one way in beacon mode or two-way, if a second CW operator is found. The quest for a proper field is open!