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ANTENNA_NOISE_BRIDGE_DETECTOR

2016-05-15 06:54  
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ANTENNA_NOISE_BRIDGE_DETECTOR
The circuit is illustrated in the figure. The noise output from your antenna noise bridge is applied though a coax jumper cable to J1, an SO-239. This noise, which usually will peak slightly below 1.0 V, is broadband white noise and is fed though C1, a 100-pF capacitor, to a pair of small-signal diodes connected as a rectifier/voltage doubler. The rectified dc voltage, filtered by C3, an 0.1-μF disk capacitor, is then applied to the base of a small-signal NPN transistor (Q1), which serves as a meter amplifier. Meter M1 is a 200-μA meter. It monitors collector current through Q1. On/off switch S2 is mounted on the GAIN control, which, in series with R3, forms a voltage divider across battery BT1, a 9-V battery, which powers this instrument. The GAIN control is wired so that the wiper travels from the end of R3 to ground as the krtob is rotated clockwise. This sets the emitter bias and the point at which Q1 will go into conduction as rectified noise voltage is applied to the base of Q1. The current drain from the battery is approximately 8 pA with no input, increasing to slightly over 200 μA with the meter at full scale. With such low current drain, an alkaline battery should last for years, even if you forget to turn the instrument off! This instrument covers the range from below 40 m to above 10 min two bands: 40 and 30 m, and 20-17-15-12-10 m. Bandswitch S1, an SPDT toggle or slide switch, selects the frequency range. The tuning capacitor C2 is a small 150-pF air variable.