Antenna Input & Audio Lineout Adaptor For Portable Radios

  
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Here is an idea for a simple low-cost adaptor that allows a portable FM radio (or MP3 player with FM tuner) to be connected to an external antenna and to audio equipment such as a hifi system or PC sound card. Portable FM radios and some MP3 players typically provide a 3. 5mm stereo jack socket for the headphone connection, with the shield conducto
Antenna Input & Audio Lineout Adaptor For Portable Radios - schematic

r of the headphone cable doubling as an antenna. Recently, the author bought a cheap FM radio with a USB connector, designed to be operated with a PC. The package included an audio cable with a 3. 5mm stereo phone plug at each end. The plug that goes into the radio has an additional wire (about 2m long) hanging out of it, which is meant to serve as an indoor antenna. When using the supplied cable, the system suffered from poor radio reception (too much interference), and poor audio quality (lack of bass). The first problem was easily explained, as the radio was used in a marginal TV/FM reception area. When the cable was "buzzed out", the reason for the second problem became apparent. There was no audio ground connection, as the cable screen is not connected to anything at the radio end! As mentioned, the antenna wire in these units is connected to the "common" terminal of the 3. 5mm socket, which normally doubles as the audio signal return path. If this terminal were to be connected to the ground of external audio equipment, the antenna signal would be clobbered. Perhaps the designer of this cable assumed that an adequate audio ground connection would be made indirectly via the USB cable a poor assumption! The challenge then was to provide a good antenna signal for the radio while at the same time making a good audio ground connection to external equipment. Preferably, this was to be achieved without relying on the USB...



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