Article by G8JSM on Nov 21 2011 at 14:11

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Peaceful pager free 2 metre operation


I regularly operate from Billinge Hill on 2 metres using a variety of handhelds, it\'s my local “hill”, it takes me 10 minutes to get there and a further 15 to 20 minutes to get from the car to the trig point, some people claim to do it in 5. Not being in the first flush of youth I have realised that some tasks are not as easy as others, but with a bit of perseverance, research, and talking to others, many things become possible. You could complain to the site management that they are running too much power, you could complain to the equipment manufacturers that their receivers don\\\'t work properly, you could complain to …...... OR you could do something constructive, beg, buy or build a filter. Begging is not really an option, but you may be able to scrounge some bits, buying ( wash mouth out with soap then copious amounts of alcohol to dull pain) is expensive. Building is therefore the preferred option unless you can find some surplus bit of PMR kit. I started off using an FT290 with a mutek frontend and had minimal desensing problems even using a homebrew end fed half wave atop a 6 metre fishing pole. However I wanted to explore different bands, 6m , 70cm, 70MHz. Not even being able to see some of the components that would be required to build a piece of kit that would do this I reluctantly had to buy (soap alcohol etc.) the radios. Being physically small, light and with wideband receivers compromises were inevitably made. I started off trying multiple ¼ wave stubs (big and cumbersome and difficult to tune but worked after a fashion) then progressed to series tuned circuits and conventional band pass filters, still not enough rejection and hard to get bits, at this point a conversation with Richard G3CWI started me thinking about helical resonators. The ARRL handbook proved to be a good source of information, with a comprehensive nomograph for helical filter design, but even better they have a free software program called Helical that does the design work for you. This can also be downloaded free from www.tonnesoftware.com. I settled on a design that was big enough for me to work on, but light enough and small enough to carry. I then made up a kit of parts, 4 coils 1.6mm dia copper wound on a 15mm diameter former and allowed to spring out to the required ¾ inch, resonator cavities from pcb material cut and predrilled for the coils and connectors, miscellaneous nuts and bolts The wire came from the secondary of a defunct mains transformer, the bnc sockets were reclaimed from some junk that I had purchased for a very reasonable price at a car boot sale. I had enough single sided pcb to make the sides and top and bottom, but had to acquire some scraps of double sided material for the ends and the dividers, but any rigid conductive solderable material should work reasonably well. During my rummage through my “junkbox” I came across a light blue/green rectangular box with n-types and lots of bolts sticking out of it, the coils were silver plated and wound on ptfe formers. I had got it at a rally many moons ago in a bulk purchase and thought that it was uhf, however plugging the dimensions into the Helical software revealed that it was in fact vhf and a little tweaking got it on to two metres, the only problem was that it weighed 1.5 kg, I took it up the hill twice to prove the point. Thus I had the incentive to build something a lot lighter and a little smaller. Building the filter took about 4 hours, it took about an hour to get it somewhere near on frequency, the tuning was very critical(,as an example pruning ¼ inch off a coil moved the resonant frequency several Megahertz), coarse adjustment was by stretching/compressing the coils,fine tuning by threaded rod, a previous iteration used 6mm dia. tuning screws, this was reduced to 3mm dia. on the final version. Pictures here http://www.flickr.com/photos/g8jsm/sets/72157627853826369/ Discussion here http://www.sotawatch.org/reflector.php?topic=6251#51432 If any one wants any further info about how I built and set it up or maybe borrow the filter on their next excursion up there contact me at G8JSM@fsmail.net Hope this is helpful, 73 Colin