How to use radio microphone as an anti bug and as a device for eavesdropping

Wireless microphone VHF range with radio transmission of a modulated signal is used not only in concert venues, but also at home, for example, for karaoke. Range of the MIC-140 up to 15 m. Therefore, receiving the signal from the microphone on a regular FM receiver allows you to use the device also as a radio station for communication between the rooms. The power of the MIC-140 is so low that it is not dangerous for the airwaves. This device does not require registration with Gosvyaznadzor. If you want, you can buy the radio microphone for a symbolic price of up to 500 Dollars and modernize it so that your soul was pleasant.

Photo 1. Exterior view of the MIC-140 radio microphone

The practical applications of radio microphone are truly diverse

1. The radio microphone showed good results in the author’s version as a listening device. And radio signal from this microphone can be received by any portable transceiver or omni-band receiver, such as Kenwood TH-F7 photo 2 , tuned to the frequency of radio microphone in this case – to a frequency of 98.1 MHz . Considering the low cost of a radio microphone, in some cases it is not even necessary to assemble the circuitry themselves. Just disassemble the microphone and gently move his “stuffing” in another, more compact housing, for example, the size of a matchbox.

Photo 2. Portable transceiver Kenwood TH-F7

2. An “anti bugging” device. In the vicinity of the radio microphone all other transmitters even the powerful FM radio stations, if they were operating on this frequency are blocked. There is a faint rustle on the air from the working radio microphone generator. Thus, if the frequency of 98.1 MHz is working someone’s bug, then by turning on the radio microphone can be blocked eavesdropping device.

3. In the long term the carrier frequency of the radio microphone can be retuned to match the frequency of the “bug” and thus neutralize it.

The wiring diagram and operating principle of the MIC-140 radio microphone is presented to the folk experts to understand the principle of its operation, and will undoubtedly be useful in the case of the modernization of the device or its repair.

This is the technical language of radio amateurs

Radio microphone transmitter is an electronic device, which can be assembled even with discrete components giving it only 1-2 free evenings , and the receiver is any radio, which can be tuned to a frequency of 88-98 MHz.

As a rule, any modern VHF receiver includes an automatic frequency tuning unit AFC , which allows you to tune to the frequency of the radio as accurately as possible. Radio signal receiver, which is included in the MIC-140 radio microphone package is shown in pics 3 and 4.

Photo 3 and 4. VHF FM signal receiver at 98.1 MHz: exterior and sectional view

The receiver is equipped with a telescopic antenna with a maximum length of 45 cm. As experience has shown, the receiver normally catches the signal from the microphone within a 3-room apartment, even if the antenna is extended only 8-10 cm. In the future the antenna can be replaced with a piece of flexible insulated wire, such as MHTF-0,8.

Output of radio receiver if necessary connected through special plug on the housing to the headphones or through another plug to the power amplifier. The frequency of the oscillator is modulated by the audio frequency signal from the microphone, which converts the sound into electrical impulses.

Photo 5. View of the radio microphone with the top cover removed

Here you can see the dynamic microphone itself – with the coil resistance of 200 ohms, the edge of the circuit board, rigidly glued inside the cylindrical body of the device, the battery compartment, the mode switch radio microphone, wired microphone and off and other details of the body.

Fig. 6. Circuit Diagram of radio microphone

Working principle

When the microphone is turned on the generator unit applied voltage from the cell GB1. The master oscillator is made on the VT3 transistor. Operating point of this transistor is set with the divider R10, R11. Capacitors C7 and C8 are blocking capacitors.

The load of transistor VT3 is the coil circuit L1. Capacitors C4 and C5 form a capacitive divider. On the ratio of the capacitance of these capacitors depend the amplitude and shape of the generator signals.

The frequency modulation is carried out by a varicap BB105B. The voltage to the varicap is fed from the divider on resistors R7 and R8, with which the factory set the initial offset varicap.

The modulating voltage of the audio frequency is fed to the varicap from the collector of transistor VT2. The frequency deviation can be changed by adjusting the resistor R9.

The low frequency amplifier is made on the transistors VT1 and VT2 with direct amplification circuit.

The necessary gain for the transistor stage is set with the help of the resistor R1 and R2. The gain of this cascade on two transistors is enough for normal work on the whole dynamic range of the microphone.

Specifications Radio Microphone

Radio transmitter at 98.1 MHz

Range up to 15 meters

Transmitter power of 10 mW

Frequency range 100 Hz-13 kHz

Impedance output impedance when wired to the amplifier 600 ohms

Antenna features

The WA1 antenna is a spiral silver-plated wire with a diameter of 0.6 mm the diameter of the spiral is 7 mm, the number of spiral turns is 38 , “hidden” in a rubber pin. This pin, in turn, is attached to the microphone with a 3-pin connector in the end of the microphone body see. Figure. 5 . A longer antenna is not needed, because it would disrupt the oscillation of the master oscillator on the transistor VT1.

Why is the output jack on the body exactly 3 pins?

The makers made the MIC-140 a universal microphone it can work as a VHF-FM transmitter as well as a regular microphone. In the latter case, two of the three pins of the connector are necessary to connect the output signal of the microphone amplifier to the input of the power amplifier by wire .

The power supply for the wireless microphone is one AA-type battery with a rated voltage of 1.5 V. Only 7 mA, which is enough power for 3 hours of continuous operation.

But, even in this case, the device works reliably and is ready for overloads, because the transmitter power is small, and the power supply voltage of only 1.5 V, which guarantees the operation of the output transistor VT3 in the transmission mode without overloading.

Most of the elements are mounted vertically on the circuit board to save space in the microphone body . The L1 coil is frameless. It contains 5 turns of copper silver-plated wire with a diameter of 0.8 mm, the inner diameter of 7 mm, winded with a pitch of 1.25 mm. The leads from the first and second coils.