F.U.N. with B.L.E.

September 08, 2017 by Ryan Jones

With all these invisible radio frequencies flying through the air, we should know a little bit about what exactly they are!

With all these invisible radio frequencies flying through the air, we should know a little bit about what exactly they are!



Give this project a try for yourself! Get the BOM.



Lately, it is hard to go a day without interacting with some form of technology. As time goes on, more and more devices utilize wireless technology with the help of radio frequencies. At any time of the day, RF signals are buzzing around our heads, connecting our lives across the world, and it's time we give them the recognition they deserve!


RF communication helps us transfer data and information wirelessly by utilizing electromagnetic radiation. Time-varying electrical signals generate electromagnetic energy which propagates in the form of waves. We utilize this technology every day when we connect our phone to the car or a wireless headset to your gaming console.

With the use of ST's STEVAL board, we are able to easily measure the strength of their Bluetooth signal in various environments to see how these signals interact with the outside world. It's like being able to visualize RF signals!



Bluetooth is a form of RF communication most commonly used for, but not limited to, streaming music. Bluetooth Low Energy (BLE) is a form of Bluetooth but, you guessed it, with reduced power consumption. It works well with devices that periodically transfer small amounts of data. While many of our devices are wireless, it doesn't mean they're perfect.

RF signals can be greatly attenuated by different materials and objects in the world. It's why you lose cell service when you go in a tunnel - the satellite's signal isn't strong enough to make it through the layers of cement and hard ground. Even without obstacles, RF signals will not just travel forever and ever.

All electromagnetic radiation follows the inverse-square law. It states that signal intensity decreases with the square of the distance from the source. Basically, they lose strength the further they travel. This applies even in the smallest increment.


Using the STEVAL's associated app, we can connect to the board on our phone via Bluetooth and use its RSSI (received signal strength indicator) to test our signal strength. With the board directly next to my phone I received a reading of -42dBm. Using this number as my reference point, I started small and moved the board a few inches down the table. Immediately the signal dropped to -57dBm. The inverse-square law.

To test its durability through materials, the best option seemed to be to put it in my shoe and stuff it with a shirt and socks. This gave me a reading of -51dBm, about 10dBm below my initial reading, not too bad. The signal seemed to travel well through porous materials but what about something like a Faraday cage? The ol' microwave seemed like the best option. Nothing can get through it, right?

With the microwave unplugged and not powered I placed the board in and closed the door behind it - not forgetting about my hot coffee, of course. Expecting to lose connection completely, I was surprised to see a reading of -78dBm. A significant attenuation in signal with a laggy connection but not a complete loss. I'm not sure if this says more about the Bluetooth signal or the lack of protection from the microwave. 


Inside the STEVAL app


You see, we were able to visualize these signals with the help of the STEVAL app. Though not all signals interact exactly like Bluetooth, the RF interferes with the surrounding world, attenuating our signal strength, just like a phone in a tunnel.

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  • nsaspook September 08, 2017

    Yes, a modern microwave oven is NOT a good Faraday cage. It’s not a sealed space with continuous conductors on all sides. The door gap is designed as a RF choke at the microwave RF frequency with an intentional gap between it and the oven cavity. It is designed to only contain only the frequency used by the magnetron (2.45 GHz).

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  • tonyr1084 September 13, 2017

    In concordance with nsaspook’s comment about microwave frequency, it would be nice to know what frequency(s) bluetooth is running at.  I’m sure it’s not a difficult thing to find out, but why didn’t the author include that information in the article?

    Well, off to google to see what frequency - or frequency range - bluetooth operates on.

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    • tonyr1084 September 13, 2017
      After a quick search, the answer to my question is 2485 MHz. Well, I've never been good with numbers, but isn't that 2.48 GHz? Slightly higher than the microwave frequency, which means shorter wavelengths and more easily escaping through the holes in the glass door of the microwave? As I believe I understand it, the holes are sized as small as they are so that microwave energy can not escape. However, shorter wavelengths, aka higher frequencies, may be able to. So the next questions that begs an answer; is bluetooth dangerous? Or is it the energy behind the wave that makes microwaves dangerous?
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