Saturday, July 25, 2009

Successes and failures

It has been a bit of a mixed day in G4ILO's Shack. I decided to have another try with the DC20B QRP transceiver. Ever since Chuck, W5USB sent me a spare keyer chip it has been working perfectly except for the fact that it is on 14.062MHz - too far from the QRP frequency of 14.060 for me to work other rockbound QPers. I thought that the problem might be the crystal, so I sent off for another one. Unfortunately after installing it the rig was still on exactly the same frequency!

I recalled that someone suggested the way to lower the frequency of a crystal was to use some series inductance. I don't possess any inductors, but I do have some ferrite beads, so I cut the circuit trace connecting the crystal to the trimmer and inserted an inductor consisting of one turn through a ferrite bead. That made no difference. I then tried four turns of thin magnet wire through the bead, and the crystal would not oscillate at all. Two turns didn't work either. By this point I was so annoyed by the time and money I have wasted on this thing that I threw it in the junk box. Unless someone can tell me EXACTLY how to get this radio down onto 14.060 - short of ordering a custom crystal, which would be a waste of money - I'm giving up on it. I already have the MFJ Cub, so I only got this kit because I thought building it would be fun, which it hasn't been at all.

The second failure didn't waste too much of my time, but coming on top of the first one it didn't help my mood any. I had often wondered if it was possible to make a miniature paddle directly on the back of a 3.5mm stereo jack plug, using a piece of double sided PCB as the paddle. I soldered the tip and ring terminals to opposite sides of the PCB and fashioned contacts out of the part of the ground connector that normally clamps to the cable. The result was a failure - the contacts were not solid enough so the feel was awful and they moved about so one side or the other ended up making permanent contact and you sent a stream of dits and dahs. Well it was a nice idea, but I was disappointed that it didn't work.

The success was the changes I made to the G4ILO Wonder Loop. I fabricated a tripod mount in the base of the tuner so I could mount it on a photographic tripod. This was quite a challenge for my limited engineering skills and the result - involving a lot of epoxy adhesive, some of which hasn't set yet because I didn't mix it very well - will never be revealed in any photograph. But it seems to work, and allows an additional option for deployment if no suitable surface is available.

I changed the fixing for the coupling loop to use Velcro, so no screwdrivers are needed for assembly. I also reduced the size of the coupling loop! I am a complete dunce at anything to do with numbers and when trying to work out in my head what a fifth of the main loop would be I actually came up with a value that was nearer a third. The amazing thing was that I was still able to get a perfect SWR with it. With the new smaller loop the tuning seems that much sharper so efficiency is probably better.

The antenna works so well that when just a few feet away from either my Samsung NC10 netbook or the old Eee PC it makes the computer's trackpad pointer go crazy when transmitting. This has so far prevented me from trying any digimode contacts and is going to be a bit of a disadvantage to operating "picnic table portable" unless it's a very big picnic table. But it's a good sign that the antenna really works - the only other portable antenna that can do this is the Superantennas MP-1.
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