850nm Infra-red beacon

Installed on 19th June 2013

On a clear night, this beacon is easy to find using 4" optics at the Fulbourn Hill site (regularly used by the CDARC and Camb-hams)


There were two reasons for installing an IR beacon - firstly to provide an alternative beacon, this time aimed at Fulbourn hill, which is way off the red beacon main beam, and secondly to have one running at a sufficiently different wavelength that the effectiveness of receive filters could be investigated. It was also felt that an IR beacon might be more tolerant of day-time light levels.

The nominal beacon beams are:

a)  Red - circular +-10 degrees centred on 122 degrees from Wyton

b)  IR - elliptical beam, +-10 deg azi, +-2 deg ele, centred on 135 degrees

The elevation beamwidth was reduced because it was felt that cloud/forward scatter propagation at the power level involved was not a very useful mode to use.
Target erp is further increased by running the 10 LEDs at fives the current of the red beacon. LED efficiency figures are only available for the IR emitters, but assuming that they are roughly equal, the on-beam signal level some be about 15 dB greater for the IR beacon. It will be interesting to discover if this is so in practice!

This beacon is baseband modulated with a simple two-tone square wave drive source, the two two tones being nominally 1 kHz and 1.05 kHz. The
repetition rate is approximately 250 msec.

beacon mechanics       


Beacon circuit:
  IR beacon circuit