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!

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).

Michael Bauer,
Prahran, Vic.