The beacon hardware for this unit was provided by Neil Underwood, G4LDR and
consists of a DB6NT 5.76GHz source module to generate the RF at a low level.
Starting off with a 120MHz crystal this is progressively multiplied by 48 to
give a filtered output at the wanted frequency of a few milliwatts. The
module was again adapted for on/off keying by switching the final multiplier
stages and the G0IAY/G4JNT keyer, already used for most of the beacons,
generates the CW message.
GB3SCC is the lowest frequency beacon of the complex to have any hardware
other than the antenna at the head end. As the feeder loss at 5.7GHz would
result in hard earned power being lost, the PA module which was a surplus
1W unit supplied by Neil, was mounted at the head unit and 12V DC power
sent to it up the coax. DC extraction and decoupling units had to be built
in at base and head unit to separate the RF and power supplies.
The antenna consists of a 8+8 slotted waveguide antenna with approximately
10dBi gain and around 10 Watts EIRP.
The large metal mass of this antenna forms part of of the heatsinking for
the PA. Initially, bench testing of the PA suggested that with continuous
operation it may run excessively hot in summer temperatures. So initially
we operated the beacon on a switched transmit basis. The keyer module
can be programmed for a transmitter control function as well as the periods
of plain carrier, so this was used to switch power to the head unit on a 45
seconds on, 45 seconds off cycle. Several users complained that this was
confusing, and as a compromise the off period was later reduced to 20
seconds. Recently, after the promise from Neil that a replacement 5.7GHz
PA would be forthcoming if the original got destroyed by overheating, the
beacon was switched to continuous operation which is its current status.
Now GPS Locked
Dual Frequency CW Idle
Latest Upgrade, October 2013. The beacon has been updated to send JT4G. The RDDS
system originally used for DFCWi was modified with new PIC code and addition
of NMEA timing data from a GPS receiver.
As with all the BH beacons,
Tone 0 of the JT4G is placed on the beacon's nominal carrier, 5760.905MHz
so correct tuning is achieved with an SSB carrier reference 800Hz lower at 5760.9042MHz