Abstract / introduction:
In olden days while we were using analog signal for communication, the jamming circuit was very easy just by producing the high frequency noise signals, but today trend is completely changed in such a way that use of digital devices taken place from analog devices. High frequency signals are not capable to block those signals from reaching the devices, so we need very high frequency signals to block the actual signals from reaching the devices, so jammers are used for blocking the signals. Jammer circuit produces the high frequency signal which will confuse the receiver of particular system from receiving the signal, even though circuit is working properly, user of the system feels that circuit is not working properly. This type of high frequency signal generation through jammer is called noise and the circuit is called jammer because it will mix the main signal with noise signal.
HISTORY ASSOCIATED WITH JAMMING:
During World War II ground radio operators would attempt to mislead pilots by false instructions in their own language, in what was more precisely a spoofing attack than jamming. Radar jamming is also important to disrupt use of radar used to guide an enemy's missiles or aircraft. Modern secure communication techniques use such methods as spread spectrum modulation to resist the deleterious effects of jamming. Jamming of foreign radio broadcast stations has often been used in wartime (and during periods of tense international relations) to prevent or deter citizens from listening to broadcasts from enemy countries. However, such jamming is usually of limited effectiveness because the affected stations usually change frequencies, put on additional frequencies and/or increase transmission power. Jamming has also occasionally been used by the Governments of Germany (during WW2), Israel, Cuba, Iraq, Iran (Iraq and Iran war, 1980-1988), China, North and South Korea and several Latin-American countries, as well as by Ireland against pirate radio stations such as Radio Nova. The United Kingdom government used two coordinated, separately located transmitters to jam the offshore radio ship, Radio North Sea International off the coast of Britain in 1970.The bombs that blew up commuter trains in Spain in March 2004, as well as blasts in Bali in October2002 and Jakarta in August 2003, all relied on cell phones to trigger explosives. It has been widely reported that a cell-phone jammer thwarted an assassination attempt on Pakistani President Musharraf in December 2003. When President Bush visited London in November 2004, it was reported that British police considered using jammers to protect the president's motorcade through London.
Working principle:
The variable capacitor C4and L1 will constitute the tank circuit which will produce the high frequency signal, the capacitor C4 is variable so that we can produce different frequency signal by adjusting the variable capacitor. When the Q1 is turned ON, the tank circuit will start its operation and produce the VHF signal (very high frequency signal) which will jam or create the noise in the original signal so that receiver cannot receive the signal. Even if it is received also, the signal cannot be used by the receiver circuit.
The resistors R1 and R2 will act as the biasing circuit and R3 is used for limiting the emitter current in the circuit.
The 2N2222 is a common NPN bipolar junction transistor (BJT) used for general purpose low-power amplifying or switching applications. It is designed for low to medium current, low power, medium voltage, and can operate at moderately high speeds.so this is positive point for us because we want 88MHz-108MHz frequency. The 2N2222 is considered a very common transistor and is used as an exemplar of an NPN transistor. It is frequently used as a small-signal transistor and it remains a small general purpose transistor of enduring popularity. Different frequencies can be achieved by changing the values of capacitor and inductor. The formula is
FM jammer schematic
The parts are: R1=15k, R2=3.9K, R3=220 C1=10uF/25V, C2=0.01uF, C3=6pF, B=Battery 9V.
The coil L should be closely wound 6 turns (start tunning with the closely wound distance of turns and then play with it and the C4 capacitor, to find the desired frequency) of enameled copper wire 1.5mm and internal coil diameter of 1cm.
The variable capacitor C4 should be air trimmer capacitor rating 4 to 30 pf or the closest to this value.
Pros and cons:
· Pros:
§ Normally jammers are used in government offices in some countries, but jammers are mainly used by military, navy, air force and entire defense systems. People will not use jammers everywhere, but they use in selected places and highly confidential meetings, gathering etc.
§ FM jammers can be used in areas where radio transmissions are dangerous (areas with a potentially explosive atmosphere), such as chemical storage facilities or grain elevators. Moreover, it can be used in places like Hospitals, restaurants, movie theatres, concerts, shopping malls and churches where silence is required
· Cons
§ This type of circuit is banned in many countries. Use it by your own risk.
§ Capacitor C1 value should be changed for every station for getting the different frequency. So this process is time consuming.
Future scope:
While the law clearly prohibits using a device to actively disrupt a cell-phone signal, there are no rules against passive cell-phone blocking. Companies are working on devices that control a cell phone but do not "jam the signal.
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