Hi All, > > What I want to see is someone design a tuneable preselector with many > > adjustable parameters. At these frequencies and with a load impedance of 50 ohms you can come a long way with a good 1 mH inductor and a tuneable capacitor in series with the antenna. The 1 mH inductor is in the order of 3 k ohm at 500 kHz in series with about 100 pF it will resonate at the desired frequency. Q should be about 60 for a 3 dB bandwidth of about 8 kHz.
With just a single (air-dielectric) capacitor to tune this solution is pretty efficient but it is necessary that the output impedance of the antenna is low. Otherwise it will not work. Use a source or emitter follower for example. For narrower bandwidth, use a bigger inductor and a smaller capacitor. The tuning range becomes fairly big and you can easily investigate whether any false signals are present. Connect the normally grounded side of the capacitor to the low impedance point of your antenna. (The output of your small loop amplifier or small wire source follower.) The impedance at the junction between the inductor and the capacitor becomes vey high. It probably needs screening to not pick up all the switch power and computer noise in your schack. 73 Leif / SM5BSZ _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flexradio.com/ Homepage: http://www.flexradio.com/
