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Antenna design and matching

17. October 2018 21:25 by Jens Willy Johannsen
Categories: BLE

At the moment, I'm working with Bluetooth Low-Energy devices and it very quickly became apparent that designing a PCB with a BLE antenna (no matter which type) is nowhere near as simple as designing the kind of boards I usually work with (MCU, digital circuitry and senors and a couple of actuators).

Starting to read up on the topic of antenna design only served to confuse me even further. So while this blog post is not a comprehensive guide to antenna design, but more of a brief recap of what I have learned so far, I hope it can help others about to learn antenna design.

Introduction

The basic premise is that we want to build a device consisting of a transmitter and receiver (since BLE is two-way communication) that can communicate as efficiently as possible with another transmitter/receiver device. A higher degree of efficiency means a longer range for the same power usage (sure, you could just pump several megawatts of power into a poorly designed antenna and get a good range out of it but we don't want to do that for many reasons).

Within very few minutes of starting to read about antenna design you will encounter the term "impedance matching". And that is basically what this entire post is about. In a system where impedances are perfectly matched, all of the power leaving the transmitter IC (in our case a Nordic Semiconductor nRF51822 BLE SoC) will be delivered to the antenna. In a system where impedances a not matched at all, none of the power leaving the IC will be converted to RF energy and the resulting range of the device will be zero. The more power that reaches the antenna, the better efficiency we achieve and the better range we get. A perfect match is simply not possible so we're just trying for "as good as possible given the constraints of power, space, time, money and level of ambition".

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