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RF noise bridge for HF.
Finished project.

This rf noise bridge will provide about 40dB of noise over the 160m to 6m amateur bands. The unit uses 3 x BC547 transistors for amplification of noise produced by a 6.8 volt zener diode. Drawing a current of around 17mA, makes the unit suitable for portable battery operation. The device has provision for providing a null on the receiver output, using both adjustable resistive and reactive components.
 
 
Noise Bridge photo
 
 
Inside look.

Here is an inside view. A bit of veroboard, a handwound transformer and some commonly available electronic components. The zener was biased for maximum noise. The capacitor is a miniature broadcast variety. The potentiometer is a 500 ohm carbon linear. The transformer was wound with 7 turns (quad) 0.125mm ECC wire. Two windings wired in series are used as the primary supply to the bridge circuit. I used parallel wiring for the bridge components, to reduce the effect of hand capacitance (as opposed to series wiring) to the antenna and receiver sockets.
 
 
Inside the noise bridge
 
 
The schematic.

Here's the schematic. I've also included an alternative detail for C7 so one can determine if the antenna's reactance is more capacitive or inductive. The is also provision for an audio modulation input (J5) for, say, a tone input.
 
 
Noise bridge schematic
 
       
    2006 - 2009 S.Spirat 
Another SPIRAT initiative