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FM Transmitter Plans

FM transmitters have gained popularity in the circles of hobby electronics enthusiasts. An FM transmitter can listen in on a phone or room conversation and transmit the audio to a standard FM radio.

There are two common types of room transmitters; the room transmitter and the telephone transmitter. The room bug works through an 'electret' sensitive microphone and the telephone coutnerpart takes the audio directly off the phone line. Room bugs are not the easy to use devices they are usually described to be, this is because the audio level of the subjects in the room cannot be controlled and can cause something called overmodulation which causes bad reception on the receiver side, though this can be fixed by covering the transmitter with a soft cloth or tissue paper telephone transmitters are preferred because they can transmit the telephone signal of the subject without such complications resulting in a much better output on the receiver side.

Using a standard FM receiver or radio does have its severe disadvantages. Of course the first being that anyone can listen in on the conversation being monitored. Another is that radio stations can interfere with the signal coming from the room or telephone bug. To work past this issue a VHF receiver is used with a capability to receive in transmission bands alternative to the public FM range of 88 to 108 Megahertz. Similarly the FM transmitter will have to transmit on the alternative band, this can be done either by changing the properties of the 'oscillator' on the FM transmitter or quite simply by buying a VHF transmitter which transmits in the alternative band.

VHF transmitters can transmit in a range of 300 to 600 Meters typically, there are special high power designs which are reputed to transmit at much larger distances but the majority of standard bugs do not due to limited power supply and constrictions in size.

The room bug requires a battery usually 9 Volts which can last typically upto 48 Hours but the telephone transmitter can operate on the power of phone line giving it an indefinite time of operation. Also important to consider is that there are two types of telephone transmitters available, serial and parallel. The serial is very easy to build but has the major disadvantage that it can be easily detected on a phone line, the parallel does not have this disadvantage and can also be connected to the line without interrupting the wiring.

To download a high quality plan for a Telephone Transmitter click HERE.