Accessible FM Radio Case Study - Part 2 - PCB Antenna

[Part 1] [Code and Design Files]

In the first part of this series, I looked at the overall requirements for the project. A key one of these was to avoid the need for the classic 1/4 wave telescopic antenna, which is, something of a hazard.

Cars haven't had telescopic antenna for some time now. It turns out they are generally a conductive trace built into the car glass. (Not much use for this project). However, I had noticed the 'shark-fin' antenna on the top of cars. It seems, that according to this research paper. It is possible to make a compact PCB antenna suitable for receiving FM broadcast transmissions in the normal range of 88-108 MHz.

PCB antenna design

The research paper gives a detailed design for the PCB, including all dimensions. I have recreated this in KiCAD and you can find the design files and even generated CAM files for production in the github repository for this project.

The PCBs came back from JLCPCB and looked good, so I made a couple of the boards up using the surface mount components specified in the paper. You can see the values in the picture above.

I actually made up 2 boards, one with the intended SMA co-axial connector, and one using a chopped off end of a 3.5mm audio connector, to make it easy to connect the antenna to the TEA5767 board that I was using for testing.

I used a hot glue gun to stick down the audio lead to prevent the wires pulling off.

TEA5767 Module

As a first test of the feasibility of using this type of PCB coil antenna, I used the TEA5767 module shown here.

There are two 3.5mm headphone style sockets on the board. One is for stereo output, and can directly drive headphones and the other is for attaching an antenna. 4 header pins provide power and the 2 I2C bus lines.

The module has an I2C interface that I will control using an Arduino Uno. The I2C interface allows you to set the tuning and volume, but also, crucially allows you to obtain the signal strength.

I can use this to compare the performance of the antenna against other antenna options, by tuning to a known radio station and then switching antennas.

The TEA5767 module is also 5V operation so I can connect it directly to the Arduino Uno and use the Arduino to power it.

The wiring is as follows:

• 5V on the Arduino (red) to Vcc on the radio module.
• GND on the Arduino (blue) to GND on the radio module.
• SCL on the Arduino (yellow) to SCL on the radio module.
• SDA on the Arduino (orange) to SDA on the radio module.

You can find the Arduino test sketch here.

/*

 * Modified from original library and examples by big12boy 2017

 * https://github.com/big12boy/TEA5767

 * 

 * Mostly reformat and recomment and simplification.

 * 

 * Signal strength only updates if you do a reset. Use Arduino Reset button

 * 

 * Tested on Arduino Uno connected to TEA5767 module. 

 * 

 * This used to test TEA5767 basic operation and measure signal strength for various antenna.

 * 

 * See blog post here for more detail: 

 * https://www.doctormonk.com/2025/04/accessible-fm-radio-case-study-part-1.html

 * 

 */

#include <TEA5767.h>

TEA5767 radio = TEA5767();

float frequency = 93.0; // Enter your own Frequency in MHz. Look up the frequency of a station near you. 

long baud = 9600;       //Enter your own Baudrate. I always use 9600.

void setup() {

  Serial.begin(baud);

  Wire.begin();

}

void loop() {

  radio.setFrequency(frequency);

  printReady();

  printStereo();

  printSignalLevel();

  

  Serial.println();

  delay(1000); 

}

void printReady(){

  int rdy = radio.getReady();

  Serial.print("Ready: ");

  Serial.println(rdy);

}

void printStereo(){

  bool stereo = radio.isStereo(); 

  Serial.print("Stereo: ");

  Serial.println(stereo);

}

void printSignalLevel(){

  short level = radio.getSignalLevel(); 

  Serial.print("Signal (0-15): ");

  Serial.println(level);

}

Before running the program on your Arduino, change the line: 

float frequency = 93.0;

to a frequency of a station that you can receive strongly with an FM radio. In my case, this is BBC Radio 4.

When you open the serial monitor in the Arduino IDE and set the baud rate to 9600, you should see something like this:

You can, of course, plug some headphones in to the radio module to hear how much noise is accompanying the signal.

Test Results

Trying to keep all other things constant, the antenna options I measured, were:

• No antenna, sig strength 0/15
• 23cm telescopic antenna, sig strength 4/15
• PCB antenna, sig strength 8/15

On listening to the audio on headphones, the quality was quite reasonable with only a little hissy noise.

This is really good news, and I'm hoping we can do without the telescopic antenna.

In Part 3 of this series, I'm going to start looking at audio amplifier and speaker options, as this will inform decisions about the power supply.