South Coast Repeater and Beacon Group

GB3SCX - 10.368GHz beacon

Overview - by Andy G4JNT

This was the first beacon of the complex, and started off life at the two previous sites on top of a tower block in Bournemouth, then on the Purbecks overlooking Swanage before being moved to Bell Hill.

Initially GB3SCX consisted of a DDK004 module modified for on-off keying in the same way as for all the other beacon hardware, and with an additional DC injection block at its output.This module contains a crystal oscillator at 108MHz, then progressive multiplication to give an output at X24. The few milliwatts of 2556MHz is combined with 12V DC power andfed up the coax to the head unit where a WDG001 module multiplies this to the final output frequency of 10368.905MHz. Initially this was followed by a WDG006 PA module using an MGF1802 GaAs fet delivering around 200mW but the device proved unreliable over the years. After it had failed a second time the entire head unit was rebuilt using a 2 Watt PA donated by Bren G4GHP.

The antenna is a 7+7 slotted waveguide giving around 10dBi gain and hence an EIRP around 18 Watts results.

The free running Butler oscillator in the DDK004, even with a clip on crystal heater, gave poor frequency stability when exposed to the temperature variations of the Bell Hill equipment cabin. So the source was replaced with one locked to a 10MHz reference input.

A separate crystal oscillator is operated as a Voltage Controlled Oscillator (VCXO) delivering an output at 108MHz. This goes two ways - one output feeds the now-modified DDK004 to generate the final RF signal.The other port goes to form the clock of an AD9851 Direct Digital Synthesizer. A PIC controlling this sets the output from this device to be 10MHz when clocked with EXACTLY the right frequency to give 10368.905MHz (when multiplied up). The resulting 10MHz output from the DDS is compared in an NE612 mixer IC with the reference input and after amplification of the error signal in an op-amp plus filtering, is used to control the VCXO frequency.Hence the VCXO is phase locked to the 10MHz reference at whatever fine-tuned frequency the DDS is set to produce.

The PLL bandwidth proved wide enough (approx 200Hz bandwidth due to the high comparison frequency of 10MHz) to allow Frequency Shift keyed RTTY to be added to the final transmission. Consequently the PIC software was changed to programme the DDS in real-time with two frequencies corresponding to mark and space of the RTTY. A shift of 850Hz was used here, this being the widest available in standard RTTY decoding software.

The Keyer module was modified to add a RTTY message and the resulting data fed to the PIC controlling the DDS.A speed of 50 baud was selected, in common with most amateur RTTY use at VHF(HF is usually at a speed of 45.45 baud).

The 32 bit frequency setting resolution of the AD9851 DDS means that the programmed mark frequency of GB3SCX is actually 6.85Hz High at 10368.90500685 MHz (assuming the 10MHz reference is 'spot on')!

GB3SCX Beacon now sends DFCWi

On 19 September 2007 the RTTY data was removed and the Frequency Shifting capability of the PLL used, instead, to generate Dual Frequency CW, idle (DFCWi). The parameters of the transmission are :-
~ Symbol interval - 0.5 second
~ Idle frequency 10368.905MHz,
~ Dot Tone +400Hz, ~ Dash Tone +600Hz

All details of DFCWi, including software to transmit the mode using a soundcard to generate the audio tones and links to suitable decoding software can be found here :

In August 2009 the DFCWi transmission was replaced with JT4G, one of the WSJT weak signal modes consisting of four tones separated by 315Hz sending a heavily error-corrected digital message.
More details of the JT4G signal and generation can be found at JT4_GB3SCX.pdf