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.

Accessible FM Radio Case Study - Part 1

[Part 2] [Code and Design Files]

In this series of blog posts, I am going to document my journey (which I am about to start) in designing a FM Radio receiver for which I have the very specific requirement of being usable by my brother, who suffered a stroke a few years ago. This has left him with cognitive impairment and use of just his left arm.

This blog will effectively form my notes, and follow my usual design process which is:

1. Investigate technologies
2. Create 'spike' examples to de-risk various aspects of the design
3. Breadboard or strip-board prototype
4. Testing
5. Prototype PCB creation
6. Enclosure and user testing

As I hope this will end up in a design that will help others, I am going to release all the CAD and firmware files under an open license. 

User interface

• Volume control - rotary, a pot, therefore with absolute positioning and no turning down to silent.
• On/Off switch - probably a stylish toggle switch, but low physical resistance and ok for one handed operation without having to hold the radio.
• Three preset buttons. Real switches with an indicator to see which is selected. Obvious icons (probably on paper) to select the channel. Talk (BBC Radio 4), Classical (BBC Radio 3), Pop (BBC Radio 2).
• A separate interface (behind a panel - or using a BLE phone app) to configure stations

Other requirements

• Large capacity LiPo battery power
• USB charge and pass through
• Decent power amplifier and speaker
• FM broadcast receiver (*)
• Bluetooth audio for wireless headphones
• No eye-pokey antenna
• Stable enclosure (one-hand friendly)

* Why FM rather than DAB or Internet Radio. Well DAB is power hungry and offers no real advantage (except perhaps future-proofing) over FM. 

As for internet radio? Well, this was my first thought as it solves the antenna problem. However, when I have looked at this before, it's quite difficult to get stable URIs for station streams, especially for BBC stations, which is likely all my brother will listen to.

Looking at what was commercially available, I found this: 

Which is actually pretty close to what I was planning. And if all else fails, I could easily see myself getting one for my brother. However, although you can't see it here, it has an eye-poker antenna and is powered by D-cells. Neither of which I am keen on.

Technologies

At the moment, my initial list of technologies to experiment with includes:

• A PCB antenna (as used in 'shark-fin' car antenna) -- this is the big risk, and subject of my next blog post.
• The  RDA5807M radio receiver IC (I might also try TEA5767 to see which is more sensitive)
• An ATTiny1614 microcontroller 
• A power amp probably D-class. Ideally 10W or more

I've ordered some RDA5807M modules from Aliexpress and started designing a PCB FM broadcast antenna using the research paper I found. I will need both the radio module and the PCB antenna for my first experiments.