Wideband FM Jammer

Posted on Feb 5, 2014

This wideband jammer simultaneously blocks all transmissions within a desired band typically by sweeping  across the desired band to be blocked, starting at the lowest frequency and ending at the highest frequency of the band. For this purpose a linear sawtooth signal is applied to the modulator in the FM transmitter described here. The transmit

Wideband FM Jammer
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ter uses varactor diodes in the tuning circuit to determine the transmitters frequency of operation. Applying a sawtooth signal to the varactor diodes results in the transmission frequency changing with respect to the changing voltage of the sawtooth. As the voltage of the sawtooth signal increases so does the frequency at which the transmitter transmits. This cycle is repeated many times a second. A sawtooth signal generator is shown in Below figure. To ensure the jamming signal sounds natural, white noise is added to the sawtooth signal. To improve stability, consider feeding power to the Operational Amplifiers seperately be means of a voltage divider for each (use the same component values as shown in the schematic). For experimentation purposes and to prevent unwanted interference, connect the output of the transmitter to a 50 Ohm/10 W resistor dummy load.  In practice the transmitter would be connected to a wideband VHF antenna (but that may be illegal). To match the impedance between the transmitter and antenna/ dummy load  adjust the two trimmer capacitors in the last stage until the best match is obtained. To measure the SWR use a SWR meter. A SWR of 1:1. 5 or better can easily be achieved when connecting the transmitter to a 30 MHz to 800 MHz discone antenna (a wideband antenna such as a discone or similar is recommended for best results). The adjustment of the trimmer capacitors is not described here,...

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