004: Noise canceller for HF low band reception


This is my experience while I lived in Hachinohe of JA7. The inverter system of the apartment house radiated radio noise so strongly, that the reception of 7MHz weak signal was almost difficult. One day, I found the circuit diagram of noise canceller manufactured by MFJ in the net. It made me feel the possibility of the problem solving, and soon I made same kind of noise canceller by myself. The device and components needed are as follows. Noise antenna, noise amplifier, phase shifter, phase selector of the noise signal polarity, combiner consisted of a couple of FETs. Noise of the same amplitude which becomes reversed phase each other is canceled, and the purpose signal from a far place is output to a receiver just as it is. This is the principle. Actually, a noise antenna was a point of success. I found a good way by trial and error. Impedance is too low as a noise antenna for the rather short vinyl wire in the room, and it does not work well. The capacitive probe like system which connects several meters of wire to the hot end of the parallel resonant LC circuit through a small value of capacitor was the best. I had the noise reduction effect more than 15 dB, and finally the QRP work became possible by this. The noise circumstance of radio amateur is getting worse year by year. Doesn't the clear sky return which does not need such kind of noise canceller? (Making was in 2014.)





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