OBSERVATIONS FROM SCOTLAND

OBSERVATIONS FROM SCOTLAND 7th July 2008 GM1SXX

GB3COA (Part 1).

This OBS concerns a long-standing project, the proposed 23Cms radio beacon GB3COA. I first sought permission for this beacon many years ago. It was originally planned to operate from the Coats Observatory, Paisley but council politics got in the way and it was re-submitted for operation from my last home. I've since moved to a new home and have no plans to move home again... ever! Local 'radio politics' also got in the way. The original designated frequency of 1296.920Mhz overlaps that of a German beacon, now used by our local weak-signal people as a propagation indicator. Despite strong support from several individuals on the Microwave Committee, I therefore *very* reluctantly withdrew the application (again!) and reapplied for a different frequency slot. This all took a lot of negotiation and a lot of time . Thanks to the work of the RSGB Microwave Committee, and especially the work of Murray Niman, Brian Coleman, and Graham Murchie, I've resubmitted the license application and it's now 'pending'...... again! This time, I will not withdraw the application for *anyone* for *any* reason.... period! You can see the beacon vetting info at www.microwavers.org/vetting.htm

The beacon allocation on 23Cms is not particularly wide and this results in frequent 're-use' of spot frequencies. 1296.920 is a case in point. It just so happens that our locals use the German beacon DB0VC on 1296.920 as a propagation indicator into the continent, so for them, the RSGB's choice of the same frequency for GB3COA was not a good one. Of course, that's why the planning process exists!... so those unhappy with a proposal CAN lodge a complaint in a timely manner. No-one did... that was until GB3COA's application was approved. Frequency re-use is common-place on 23Cms beacons with up to four beacons in each IARU Region sharing common frequencies, but geographically separated. My application for GB3COA which was designated the 1296.920Mhz frequency stood for many years, with the data freely available to all, yet no-one complained until the beacon application was actually approved. No-one goes to all the effort of planning a beacon and building hardware unless they seriously want to operate one!

IMHO The month or so before a new beacon is due to go active, is not such a good time to lodge such a complaint :-) Despite suggestions made on the microwave reflector that poor planning or no planning was to blame for this 'problem' the aforesaid planning DID happen and happened before anyone in this area even was involved or interested in 23Cms.

GB3COA was therefore re-submitted with the frequency chosen yet again by the planning committee ... and not by myself!) being 1296.755Mhz.

Still, I digress. Back to the hardware.

I now have much of the hardware working but with some loose ends to tidy up. I've ordered solid plastic rod (DELRIN) on which a new set of quagis will be constructed and I'm close to finishing building a new PA unit.

The beacon is constructed around a 16C74 PIC microprocessor controlling a crystal-controlled transmit strip. The crystal is ovened at 50 degrees C for thermal stability. The crystal oscillator is 'pulled' slightly by a varicap circuit to produce the 400Hz shift required for the Mark/Space tones. By slightly, I mean around 33 Hertz! This approximate 33Hz shift becomes 33*12 (a nominal 400Hz) at the output of the exciter. Any drift present in the signal is also multiplied by twelve. The crystal used is a custom manufactured 5th overtone type and operates on 108.062916Mhz. I must thank Quartslab for the great service they have given me.

The crystal oscillator feeds a double-tuned *2 multiplier stage producing a strong carrier on 216.12583Mhz. This signal then goes to two more multiplier stages, *3 to 648.3775 and *2 to the final frequency of 1296.755Mhz. The power level at this stage is around 30mw. A three stage printed microstrip filter follows which cleans up the signal and provides a clean 10mw signal for the power amplifier.

The power amp is a three stage microstrip tuned bipolar linear amplifier using a BFQ34 PA device. It was designed by G4DDK. The power amplifier is disabled under computer control of a small relay at switch on until the oscillator has had time to come up to temperature.

The PIC microcontroller uses an embedded language interpreter/compiler and an extermal FLASH memory to hold the control code. This allows the PIC unit to be quickly reprogrammed, should the need arise. The PIC 'computer' has an onboard serial link that provides data feedback (of the message being sent) from the beacon as well as a means of programming the PIC. In this respect, it is quite a flexible system. There is 8K bytes of FLASH program memory space available to the builder. In addition, The PIC chip also offers some RAM memory. While this doesn't sound like much memory (remember the good old days... etc), it's actually quite a generous chunk of space. PIC code is compact and 8K is plenty for the job required.

The Beacon Unit is a complete system in a sealed box. It consists of a plastic housing, a thick heat-sinking aluminium baseplate, the PIC 'computer' module, the transmit exciter module and the PA module. The power supply unit is external. The beacon unit has power in, keylock, serial connector, power LED, status LED and aerial output. It will be mounted indoors feed the aerials via the shortest run possible of Westflex 103 semi-airspaced co-axial cable. The signal will be divided at the top of the mast to feed the two Quagi aerials. All outdoor connections will be epoxy potted for long-term reliability.

A key-switch mounted outdoors allows the beacon to be shut down by designated shut-down operators. Once disabled, it can only be switched back on from indoors. This is done to meet the license conditions.

Quartslab are makers of custom and stock quartz crystals. Should you need a specialist crystal, contact Susan Court with your requirements. Anyone who can 'massage' a slab of quartz to the accuracy these guys can, gets my vote of confidence. Quartslab used to be based in Erith, Kent but have now moved to Ireland. Their website is at .... www.connogue.com/quartslab

73 Al.
GM1SXX