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 Relics: FM transmission: Wideband FM broadcast band jammer
Research Paper: wideband FM broadcast band jammer
Preface: Disclaimer: Build and/or use this device strictly at your own discretion as it is illegal to operate a radio frequency jammer in most countries. The design in this research paper is intended for controlled experimentation and to describe the principle of RF jamming only. The owner of this web site does not endorse any unlawful use of this device. You are strongly encouraged to observe the regulations of your countries national telecommunications authority.
Principle of operation
Sawtooth and white-noise signal generator:
A sawtooth signal generator is shown in figure 1. 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).
 Figure 1: FM jammer: sawtooth and white-noise signal generator schematic (click on the image for a larger version)
FM transmitter: 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, trial-and-error was used. Start with the first and end with the last making small adjustments each time until the desired effect is achieved (be patient, it will eventually work correctly). It is important to note that the trimmer capacitor connected to the emitter and collector of transistor TR1 is adjusted such that the modulator oscillates correctly as it is the source of the jamming signal. Using a FM radio, listen to the transmitted signal, you should only hear white-noise when the trimmer capacitors are correctly adjusted. With the trimmer capacitors adjusted correctly the transmitter output power is approximately +30 dB to +40 dB across the band, which is theoretically sufficient to block all stations within a radius of about twenty to fifty metres from the antenna. A transmitter with lower output power is shown in figure 2 (b). Standard RF shielding techniques should be used, i.e. RF shielding between transmitter, power supply, sawtooth and white-noise signal generator, connecting ground of all circuits to chassis, etc.
 Figure 2 (a): FM jammer: FM transmitter schematic (click on the image for a larger version)
 Figure 2 (b): FM jammer: Low power FM transmitter schematic (click on the image for a larger version)
Power supply:
 Figure 3: FM jammer: power supply schematic (click on the image for a larger version)
Construction:
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