South Coast Repeater and Beacon Group

GB3SCC - 5.7GHz beacon

Overview - by Andy G4JNT

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 DFCWi modulation

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