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Posted at - 3rd Jan 2018
Finally, I made my very first antenna matching unit. It was intended to be used with my BITX80 & 50W setup.
Features:
Taking reactive power (VAR) into consideration, the thin terminals of the variable capacitor were replaced by CuAg 1mm solid wire, also connections were made with 2.5mm2 stranded Cu. Coils were wound on T130-2 iron powder toroids with 1mm CuAg coated with that blue shrink-tubing. They stays cool under several minutes of CW while feeding a load of 120R -j60 at 3.7MHz with 50W.
Wrong Wrong Wrong!!! First time I used a big 43-material toroid for coarse tuning coil and during tests I've found that it wasted about 10-12dB of power getting very hot after several seconds. So it was replaced by another T130-2 iron powder toroid which is specially designed for low loss - high-Q - inductors. Also, note that, iron powder toroid contain organic chemicals which are sensible to hot environment, permanently degrading core performance if it is exposed too long.
Now the only worry about losses are the rotative contacts, switches and voltage rating of the variable air capacitor. Arcing between capacitor plates could happen beginning with 200V. For this antenna tuner I didn't calculate anything about VAR. So, for the next one I should calculate the currents/voltages through coils and capacitors, flux in core, and matching range by frequency!!!
As it can be seen, the schematic is super simple, two series adjustable coils and a variable capacitor placed before or after the coils.
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Actually, is not a lossless system as I thougth. After 5 minutes of 50W CW on 3700KHz the core of coarse tuning coil heated about 5°C. That's not too bad, that's less than 1dB loss. As I said, the next antenna tuner system should have all calculus in place from the start.
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