| Low power 628nm (red) light
beacon - co-sited with GB3CAM
beacons at Wyton |
As of 19/06/13 there is also a
850nm IR beacon on the site - see link
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.
A +-10 degree circular beam pattern is currently being radiated, with a 2 degree elevation bias. This means that there many be some illumination of lower cloud layers, with the possibility of some propagation via cloud scatter, which would be an interesting mode to try. [I now accept that this was
an extremely optimistic comment, and
not one that has any realistic
chance of success!]
The
beacon runs 400mW dc input to ten 10mm
pre-focussed LEDs wired in series.
For anyone interested in the beacon drive circuitry, the circuit can be found here. |
 |
 Tx with optional fresnell lens (currently not fitted) 
10 x 10mm LEDs running 400mW dc |
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 receive head used here was
a KA7OEI design with a BPW34 photo-diode, mounted
behind an A4 size Fresnell lens. It fed a lap-top PC
running either 'Spectrum Lab' (below) or 'Winrad'
(right). View either picture in it's own window for better resolution.  |
 |
The beacon is also audible at Nine Mile Hill (point 1 on the map), as below. This is a distance of 32 km.
    The audio wav file for this is HERE. There seems always to be twinkle on the signal at this location, with the level fluctuating between 5 and 30 dB above noise at this bandwidth (6 Hz). You can see this if you play back the wav file through an SDR programme such as Winrad above. This recording was taken using an A4 size Fresnell lens preceding the BPW34 detector. Phil G8MLA was also copying the beacon on his receiver that uses a credit card size Fresnell lens. This pretty much equates to a 4" lens in terms of capture area, both of which result in a 10 dB lower sensitivity than when using an A4 size Fresnell lens version. I now have a 4" lens receiver, and so can confirm this last statement. The beacon is weak though un-missible with such a receiver . Despite the lower sensitivity, these small receivers are much easier and convenient to use, and can be hand-held. |
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 excellent 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.
But my Favorite site by far is adjacent the Woodland cemetery just off the road linking Six mile Bottom and Brinkley, at a distance from the beacon of 38km. This is very quiet with little traffic passing by:
Traveling east from Six mile Bottom
up towards Brinkley, turn right at the Westley/
Balsham cross-roads. Count the trees on your right
as you travel towards the Woodland cemetery - you
are only talking several hundred yards At the fourth tree, there is a ten foot gap in the hedge - you can park either side of the road and get a good view of the beacon. |
 |
For anyone wanting a real challenge, you can try searching for the beacon up on Fulbourn hill on the Worts' Causeway (from the field entrance opposite the local contest site). At this location, you are well off the beacon's main beam - plus you have all the light polution from Cambridge to contend with, since you will be aiming right over the centre of the town.
A tripod is really be 'a must', though after 5 minutes of hand-holding a typical 4" optics receiver, Joe M0ZRN and I could both hear the odd burst of tone - occasionally!. A very good direction marker to use is the multi-height lit broadcast mast to the West of the A1 at Morborne. The Wyton beacon is to be found a fraction of a degree to the left of the Morborne lights. At the time of writing, there is a very bright white, mains-hum modulated light just below and to one side the beacon, so Good luck!. This path provides a good test of your telescope's minor lobes due for example reflections off the inner wall of the telescope housing. I need to take the A4 size frenel lens receiver up to this site at some point, and make a recording of what it can here.
********** REPORTS OF BEACON RECEPTION GRATEFULLY
RECEIVED!************