The biggest problems experienced in wireless design is the first two steps. - RF Design - Necessary Steps for any RF design (even Bluetooth!)

Overview  Bluetooth only uses one frequency band – the 2.4GHz ISM Band (2.4-2.4835 GHz) At 2.4GHz, any antenna radiation that enters t...

Overview 

• Bluetooth only uses one frequency band – the 2.4GHz ISM Band (2.4-2.4835 GHz)

  At 2.4GHz, any antenna radiation that enters the human (or any mammal’s) body will be absorbed by the body.

  If a reliable Bluetooth connection is important then stay away from Bluetooth chip antennas that are less than 20mm

  Manufacturer guidelines on ground plane length, width, clearance and matching components are usually defined to extract as much performance and signal out of your antenna as possible, but small antennas simply limit your efficiency and therefore your range.

   

   

 

 

Necessary Steps for any RF design (even Bluetooth!)

The biggest problems experienced in wireless design is the first two steps.

• Assess Your RF Design Concept.
• Define Your Antenna Requirements

 

1. Assess Your RF Design Concept.  The subject of this post – and where every RF design journey should start.  For all but the simplest designs (and luckiest designers), taking a few minutes to do this step now will help you avoid range and throughput problems down the road.
2. Define Your Antenna Requirements.  Based on your four design constraints from step 1, you can quickly define your antenna requirements – the type, size, etc. that you’ll need to order or design.  The next post in this series will explain how to do that.  Or if you fill in the form on this page, we’ll walk you through step 2 – for free – so you can choose the right part for you design.
3. Select, Design, and Debug.  From the available antenna options that can meet the requirements from Step 2, you’ll select an antenna and the associated RF components (see the next section for the list).  If you did the preceding 2 steps, the antenna you choose in Step 3 will be compatible with your PCB layout and will deliver the efficiency you need to cover the distances appropriate for your product.  Yes, we’ll cover that in a separate blog post, too.
4. Test and Tune.  Most modern bluetooth antennas require a tuning step.  If you choose an electrically small chip antenna, you will ALWAYS need to retune after you place your antenna part on your pcb and add your enclosure.  Tuning requires at least a network analyzer (if you don’t have one, you can a cheap one and do your own tuning, if you know how) and preferably an anechoic chamber.
5. Validate and Certify.  The last step before you launch is system and field testing.  You might also need to go through certification – for instance FCC or even cellular carrier testing if your device also supports cellular communications.  This last step can also require a final tuning iteration, depending on your operating conditions and testing requirements.

Critical Factor #1: Is Your Product Wearable?

Wearable devices pose particular challenges for antenna design and selection.
Omnidirectional coverage is a myth for most Bluetooth wearable devices.
At 2.4GHz, any antenna radiation that enters the human (or any mammal’s) body will be absorbed by the body.  It won’t be received by another Bluetooth device on the other side of the body. 
While many product developers will say ‘I need omnidirectional coverage for a wearable,’ the reality is that when the device is near the body, they lose all the energy to the body anyway.
The actual use case is also important to consider.  If your product is worn inside the ear, any radiation sent into the body will be ineffective for signal transmission.
Cell phones can operate when they are not being actively held near the body, so a wide field of view can be useful, but when the cellphone is held near the head, the energy radiated into the body is lost for signal purposes.  That’s why extending an antenna further from the head and/or hand improves its ability to communicate with nearby devices.  That’s the priniciple behind the BluFlux range-extending cellphone case patent. 
A smart watch on the wrist has less human tissue nearby than an earbud in the ear or cellphone held near the head.  Each device and use case will have a different set of design constraints.
The key principle is that a Bluetooth will not be able to radiate through nearby human tissue.
The body will also impact the antenna tuning requirements.  In general, BluFlux recommends tuning the antenna to mitigate destructive interaction with the body.  That will preserve performance in every other direction else, versus trying to go omni, which would corrupt performance everywhere.  However, if the device is used in a variety of orientations with respect to the body, you may have no choice but to attempt omnidirectional coverage and sacrifice performance in certain use cases.