Antenna Input & Audio Lineout Adaptor For Portable Radios

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 conductor of the headphone cable
doubling as an antenna.

The problem:

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!


Circuit diagram

The challenge:

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 connector (because not all FM
radios have one) and without having to mess with the radio’s internal
works. The accompanying circuit diagram shows how this can be achieved.
The radio-frequency choke (L1) has a low impedance at audio frequencies,
thereby making an audio ground path to the line output sockets from the
radio’s antenna input (“common” terminal).

Conversely, the RFC presents a high
impedance to the RF antenna signal, so preventing it from being shorted
to ground. The antenna signal is coupled to the radio via two 220pF
polystyrene (or ceramic) capacitors, which also block low-frequency
interference (eg, mains hum). Note that the design relies on the
capacitance in the audio cable to couple the antenna “ground” (cable
shield) to the radio’s internal “ground”.

Building it:

To build the adaptor, simply mount the parts in a small plastic box
and wire up as shown. A suitable choke is available from Jaycar (Cat.
No. LF-1534). The leads going to the 3.5mm plug should be no longer than
about 100mm and need not be shielded. With a good TV/FM antenna, the
author’s unit performed remarkably well, even in a poor FM reception
area. The audio frequency response and signal-to-noise ratio were
surprisingly good considering the low cost of the radio (about $40).

Author: Michael Bauer – Copyright: Silicon Chip Electronics

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