Pushing the QCX (T1 question) #qcx


I'm interested in trying to push my QCX into being usable on both the 20m and 30m bands, with some obvious reductions in efficiency.  The parts that would affect the performance seem to be the LPF, the BPF, and the number of turns on the T1 transformer.  The LPF will just be what it is, though I might try to use a slightly different configuration to get a steeper attenuation curve above 14 MHz, the BPF I can adjust by switching in some parallel capacitance the way that Steve G4EDG has done (see builders' gallery), but the primary windings on T1 are something I can only speculate about. 

The manual calls for 3 turns each on the primary and secondary T1 windings for 20m, and 4 turns each for 30m.  Considering this is a transformer and that lower frequencies call for even more turns I'm guessing that the extra windings are a matter of efficiency.  If I were to use 4 windings on T1 (not counting the 30 BPF turns) instead of 3, would this adversely impact the performance on 20m?  Conversely, how much efficiency would I drop if I were to try and operate on 30m with 3 windings, as per the 20m configuration?  In either case my guess is not much, but if anyone has any insight or experience trying this then I'd be keen to hear it. 

My kit is still in the mail and I'll be building it almost exclusively for portable SOTA work so having an extra band would really help for those days when the atmospheric conditions are challenging.  Steve has reported good results from his 17m/20m build, but I suspect with propagation the way it is these days I'd get more use out of a 20m/30m split. 


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