South Coast Repeater and Beacon Group

GB3SCF - 3.4GHz beacon

Overview - by Andy G4JNT

This beacon started life in a very embryonic low power form on the Purbeck site, but only lived there for a few weeks. On moving to the present site is was completely rebuilt by Chris G8BKE. The line-up consists of an early version of the DDK009 module, starting with the now ubiquitous Butler oscillator at 94.47 MHz and followed by several multiplication stages giving X36 in total, with the final output at 3400.905MHz.Initially it used frequency shift keying for the CW message.

The output from this feeds a PA consisting of an Ionica module with the final devices removed and the output taken from the penultimate, driver, stage. Delivering a little over 1.5 Watts when saturated, at the end of the run of coax up the mast, 0.8 Watts feeds to the Alford slot antenna generating around 6 Watts EIRP.

GB3SCF is the second unit of the complex to employ frequency locking to a 10MHz reference.The same scheme as for GB3SCX was adopted, using a DDS 'in reverse' clocked by the VCXO to phase-lock a synthesized signal at the reference frequency, using an NE612 mixer chip plus op-amp filter.The DDK009 module was modified to accept an external 94MHz input.

For this beacon, the later generation AD9852 DDS chip was used.This has a 48 bit phase accumulator and ensures the programmed mark frequency is set just 40 microHz low (yes, really, but assumesreference that is 'spot on'). Again an RTTY message was added to the normal beacon ident, but this time instead of using a separate keyer module, the PIC controlling the DDS contains all the CW and RTTY generation code in one piece of software. RTTY shift for this beacon was set to the standard 170Hz as rain scatter and other bandwidth widening phenomena are expected to be a lot lower on the 3.4GHz band than at 10GHz

In the interest of conformity with all the other beacons on site, the CW was changed to on-off keying, necessitating the usual modifications to switch power to several driver stages within the DDK009 module.

NEW 3 June 2012
The Controller has now been modified so it transmits JT65C modulation. This data mode uses 65 tones, spaced by 10.69Hz which is only possible because the DDS controlling the frequency locking is the 48 bit type. The 32 bit ones used on the other beacons do not allow sufficient frequency setting resolution. Additionally, a new GPS timing unit was installed on site delivering NMEA format data. This will be more versaile than the Motorola Binary format timing data supplied by the existing Jupiter-T GPS receiver which does the frequency locking.

The tuning for JT65c is slightly different from that for JT4G, in that the JT65 reference tone is a not-so-nice-for-listening-to, 1270Hz instead of the (very close to) 800Hz used with JT4G. The tones / tuning have therefore been defined so the same tuning method as used for JT4G can still be adopted. The reference carrier / CW is sent on 3400.905MHz exactly. This should be tuned in so as to give an 800Hz tone - as you would for JT4G. The JT65C transmission will then appear in the range 1270 to 1964Hz. The result is a wider total transmission, covering 800Hz to 1954Hz, a little over 1kHz in total.

JT65 uses a signalling bandwidth / baud rate, of 2.69Hz which gives 2dB S/N advantage over JT4's 4.375Hz. Also the Reed-Solomon error correction allows another 0.5dB, so to total signalling advantage sould be in the region of 2.5dB over JT4G under clean propagation. The narrower tone spacing makes this mode a lot more susceptible to scattering effects, and it is for this reason the mode has been used as an experiment - to see what real off air decoding is capable of. 3400MHz is probably the highest band where JT65 is practical