Anteena Design
This handy, pocket-size mobile transmission detector or sniffer can sense the presence of an activated mobile cellphone from a distance of one and-a-half metres. So it can be used to prevent use of mobile phones in examination halls, confidential rooms, etc. It is also useful for detecting the use of mobile phone for spying and unauthorised video transmission.
The circuit can detect both the incoming and outgoing calls, SMS and video transmission even if the mobile phone is kept in the silent mode. The moment the bug detects RF transmission signal from an activated mobile phone, it starts sounding a beep alarm and the LED blinks. The alarm continues until the signal transmission ceases.
Mobile phone bug detector diagram
An ordinary RF detector using tuned LC circuits is not suitable for detecting signals in the GHz frequency band used in mobile phones. The transmission frequency of mobile phones ranges from 0.9 to 3 GHz with a wavelength of 3.3 to 10 cm. So a circuit detecting gigahertz signals is required for a mobile bug.
Here the circuit uses a 0.22?F disk capacitor (C3) to capture the RF signals from the mobile phone. The lead length of the capacitor is fixed as 18 mm with a spacing of 8 mm between the leads to get the desired frequency. The disk capacitor along with the leads acts as a small gigahertz loop antenna to collect the RF signals from the mobile phone.
Op-amp IC CA3130 (IC1) is used in the circuit as a current-to-voltage converter with capacitor C3 connected between its inverting and non-inverting inputs. It is a CMOS version using gate-protected p-channel MOSFET transistors in the input to provide very high input impedance, very low input current and very high speed of performance. The output CMOS transistor is capable of swinging the output voltage to within 10 mV of either supply voltage terminal.
Capacitor C3 in conjunction with the lead inductance acts as a transmission line that intercepts the signals from the mobile phone. This capacitor creates a field, stores energy and transfers the stored energy in the form of minute current to the inputs of IC1. This will upset the balanced input of IC1 and convert the current into the corresponding output voltage.
Capacitor C4 along with high-value resistor R1 keeps the non-inverting input stable for easy swing of the output to high state. Resistor R2 provides the discharge path for capacitor C4. Feedback resistor R3 makes the inverting input high when the output becomes high. Capacitor C5 (47pF) is connected across ‘strobe’ (pin and ‘null’ inputs (pin 1) of IC1 for phase compensation and gain control to optimise the frequency response.
When the mobile phone signal is detected by C3, the output of IC1 becomes high and low alternately according to the frequency of the signal as indicated by LED1. This triggers monostable timer IC2 through capacitor C7. Capacitor C6 maintains the base bias of transistor T1 for fast switching action. The low-value timing components R6 and C9 produce very short time delay to avoid audio nuisance.
Assemble the circuit on a general purpose PCB as compact as possible and enclose in a small box like junk mobile case. As mentioned earlier, capacitor C3 should have a lead length of 18 mm with lead spacing of 8 mm. Carefully solder the capacitor in standing position with equal spacing of the leads. The response can be optimised by trimming the lead length of C3 for the desired frequency. You may use a short telescopic type antenna.
Use the miniature 12V battery of a remote control and a small buzzer to make the gadget pocket-size. The unit will give the warning indication if someone uses mobile phone within a radius of 1.5 meters.
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Antenna Description – Size and Usage Zone
SMALL MULTI-DIRECTIONAL
- DESCRIPTION: The smallest of TV-Antennas; reception of signals from all directions.
- APPEARANCE: Attractive design; including novel shaped disk and patch antennas, and antennas that attach to satellite systems.
- USE: In yellow color code areas where signal strength is highest and away from reflecting structures or low areas.
MEDIUM MULTI-DIRECTIONAL
- DESCRIPTION: Somewhat larger and slightly more powerful.
- APPEARANCE: A variety of novel shaped antennas can be found in this group ... including: stick, wing shaped and disk antennas with long elements.
- USE: Green color code areas. An amplified antenna is recommended in the green area anytime a long (20 feet or more) cable run from the antenna is required, or when more than one device (TV or VCR) is to be used with an antenna. They work best away from reflecting structures or low areas.
LARGE MULTI-DIRECTIONAL
- DESCRIPTION: Bigger in size, these antennas receive more signal power. Better for greater distances from signal source and areas with low signal strength.
- APPEARANCE: These antennas can be used to reject simple ghost situations.
- USE: When mounted at rooftop heights (30 feet or higher) outdoors, amplified antennas can be used in light green color code areas away from reflecting structures or low areas.
SMALL DIRECTIONAL
- DESCRIPTION: Antennas that act like large multidirectional on channels 2-6 but on higher channels these antennas start to have useful ghost reducing effects. Picture quality is excellent when no signal reflecting structures are around.
- APPEARANCE: Multi-element rooftop antennas.
- USE: Suitable for far edge of light green color code areas.
Amplified antennas with rooftop mounting can be used in these areas if the area is free of signal reflecting structures and is not in a low area.
MEDIUM DIRECTIONAL
- DESCRIPTION: Most popular rooftop antenna because of its modest size and ghost reducing characteristics.
- APPEARANCE: Multi-element rooftop antennas.
- USE: If there are ghost producing reflective structures near TV receiver antenna location, this kind of antenna is best for yellow, green, light green and red color code areas. Amplified antennas with rooftop mounting can be used with the blue color code.
LARGE DIRECTIONAL
- DESCRIPTION: Large antennas used in weak signal areas for maximum possible TV reception.
- APPEARANCE: Multi-element rooftop antenna.
- USE: Can be used in any color code area, but requires an amplifier and roof mounting for blue, violet and red color codes. Amplifiers are not recommended for yellow color codes.
Note: There is also a CEA-Certified label for "Indoor Antennae;" however, this label merely certifies that the indoor antenna is operational in a geographic area, considered appropriate for indoor antennae
