What is it? This is a narrow +-10 degree beam , modulated red LED beacon pointed towards Cambridge/Newmarked from Wyton, as below. This map also shows where the beacon can be seen with the naked eye on a clear night.
The beacon runs 400mW dc input to ten 10mm pre-focussed LEDs wired in series. FSK AM modulation of the standing dc bias is applied using tones at 1 and 15 kHz, enabling both baseband and carrier (hetrodyne) receiver formats to be used. CW signaling of the FSK signal with the 1 kHz tone representing the 'mark' state is used. Tone harmonics are all at least 40 dB down on the fundamental tone level. Three 'GB3CAM' words are sent followed by 30 seconds of '5's (as near as I could get to a 50% FSK duty cycle). A recording of the 1 kHz tone, taken at Holywell (point 6 on map above) at a distance of 5.25 km, can be heard here. As can be seen below, signals are 55 to 60 dB above noise.
The beacon is also audible at Nine Mile Hill (point 1 on the map), as below. This is a distance of 32 km.
Another good/convenient locatation is directly opposite the BT tower on the Longstanton to Over road (point 5 on map). There is a tarmaced field entrance with an excelent view of the beacon, and plenty of space to set up tripods etc. It is gated, but again there is no need to go beyond the gate. The distance from the beacon is 11.25 km. A +-10 degree circular beam pattern is currently being radiated, with a 2 degree elevation bias. This means that there will be illumination of lower cloud layers, when present, with the posibility of some propagation via cloud scatter, which may be an interesting mode for non line-of-sight paths. For anyone interested in the beacon drive circuitry, the circuit can be found here. REPORTS OF BEACON RECEPTION GRATEFULLY RECEIVED! |
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