August 24, 2015
The Four Critical Factors For Choosing a Bluetooth Antenna
Think like an RF designer and quickly narrow down your Bluetooth antenna options – even if you have no prior RF experience.
This is first in a series of posts to help guide your Bluetooth RF design. It explains the four critical RF factors that impact your antenna choice.
- Is your product wearable?
- Is your product mounted on or enclosed by metal?
- How much PCB real estate do you have?
- What is your desired range?
Knowing just those 4 factors, you can quickly narrow down your search for the right Bluetooth antenna.
If you’re not sure about those factors or why they are important, keep reading, and you’ll learn how to avoid common mistakes that would otherwise waste your time and budget.
When you’re designing a Bluetooth product, the sheer volume of off-the-shelf 2.4 GHz antenna options and reference designs may seem overwhelming.
The official Bluetooth Specification tells you the frequency and bandwidth, but it gives you no guidance to navigate the vast sea of antenna options.
So which antenna should you choose?
If you’re in a hurry, you could always try the “Google, Grab and Go” approach:
- Google “bluetooth chip antenna” or “2.4GHz patch antenna” or “pcb trace antenna design”
- Grab a part or reference design from the web.
- Go design your pcb and cross your fingers.
Who knows? You might get lucky… Or you might not.
Many product designers who otherwise know a lot about electronics have come to us for antenna design help after they chose the wrong antenna – or used a “bluetooth chip antenna” the wrong way.
Expert RF Engineers like Tim Chen, at Doppler Labs, know better. When Tim received a new industrial design that needed an antenna, he knew right away he could not use a chip antenna. Tim asked BluFlux to help simulate the impact so he could adjust his RF design. Within days, we gave him the simulation results he needed to confirm his design path.
You don’t need a Masters in RF Engineering to choose the right antenna for your product. But you do need to understand a few basic principles to think the way Tim did. This post – Step 1 of our Bluetooth antenna design guide – will help you do that in a few minutes.
Our latest mechanical design was even smaller than earlier ones, and the battery was in front of the PCB, so we could no longer use a Bluetooth chip antenna. Because our product is worn inside the ears, I also knew the human body would affect the performance of any new antenna design.
In Step 3 (Select Parts, Design & Debug), you not only choose an antenna but you also choose other RF components and integrate them with your antenna.
Any bluetooth RF design will need to have the same basic types of components. Your exact configuration will depend on your design constraints.
To help you get your bearings so you can set aside all the PCB real estate you’ll need for your RF components, check out this photo of the Nordic nRF52 Preview DK Development kit board as an example.
Note the magnified section of the board that includes the main RF components:
- The nRF52832 SoC Bluetooth transceiver, to generate and receive the bluetooth RF signals;
- The LC matching components for the antenna, to compensate for the effects that the PCB materials and geometry have on the antenna frequency characteristics;
- A breakout test port for directly testing the antenna’s behavior independently from the transceiver;
- A PCB trace antenna which radiates and receives the electromagnetic waves that wirelessly carry information to and from other bluetooth devices;
- The ground plane on your PCB, which impacts antenna performance to such an extent that it is effectively part of the antenna itself;
- A keepout zone adjacent to the antenna.
This reference design happens to have enough space for a PCB trace antenna – here, it’s a bent monopole, which requires ground plane clearance (note the lack of ground plane around the antenna). If your design doesn’t have as much space, you might need to choose a smaller chip antenna or even a custom design.
The bent monopole antenna on the Nordic board has an omnidirectional antenna pattern, whereas a design that requires a directional antenna might place the antenna directly over a ground plane.
By the end of this post, you’ll know the factors in your design that determine whether you need a directional antenna.
Bluetooth only uses one frequency band – the 2.4GHz ISM Band (2.4-2.4835 GHz).
Bluetooth devices hop between frequency channels in order to coexist with the entire menagerie of devices that share the 2.4 GHz ISM band, including other WLAN technologies and microwave ovens.
Bluetooth Low Energy (BLE) has fewer channels (40) than standard Bluetooth (79), but from your antenna’s perspective there’s no difference – it’s all the same frequency band.
So if you’re designing a Bluetooth device, the 2.4 GHz ISM band defines your fundamental antenna parameters for any Bluetooth RF design.
The freespace (in air) wavelength of an electromagnetic wave is calculated by taking the speed of light in freespace divided by the frequency:
In freespace,the speed of light ‘c’ is 299,792,458 meters / second.
Therefore, the range of freespace wavelengths of Bluetooth signals goes from =c=2.4835GHz to c=2.4GHz
Put simply, a bluetooth signal’s wavelength in free space is between 120mm and 125mm.
With no special RF voodoo, we can make a very efficient antenna, with length Lo, (the “unloaded” length an antenna) from a quarter-wavelength conductor, as long as we have a reasonable ground plane to drive the antenna against so the combination of antenna and its ground plane will effectively be a ½ wavelength long.
Lo = Lambda/4= 31mm for the “long” wavelength 2.4GHz end of the band
So 31mm is the desired length of a Bluetooth antenna without “dielectric loading” (see next section).
Here’s a rule of thumb we suggest: if you have a 40mm x40mm square available on your PCB for your ground plane and antenna, you can consider a relatively large off-the-shelf chip antenna or PCB trace antenna for your bluetooth application.
If you don’t have a 40mm x 40mm space for your antenna and ground plane, you can try shrinking your required PCB area by using one of the countless “electrically small” off-the-shelf 2.4 GHz chip antennas that are much less than 31mm in length. For example, the Fractus FR05-S1-N-0-102 and Johanson 2450AT43A100 are both 7mm 2.4 GHz WLAN antennas.
If you hunt around, you can even find “ultra miniature” 0402 SMT antennas that are only 1mm long! So why not just grab a 1mm chip antenna and design your product around it?
To answer that question, you need to consider how the manufacturers can shrink those tiny chips below 31 mm and sell them as 2.4GHz antennas.
Those chips are loaded with special dielectric and permeability materials that slow down the electromagnetic wave inside the antenna package. The slower speed makes the wavelength shorter. Smaller wavelength = smaller antenna.
The resulting ‘loaded length’ of the antenna, is therefore smaller than the freespace length.
But nothing comes for free: a loaded antenna means reduced bandwidth and reduced antenna efficiency. In some cases, that’s ok because:
- The 2.4 GHz ISM band has a narrow bandwidth (only 3.5% of the center frequency). Bluetooth RF designs are therefore more tolerant of dielectric loading than, say, cellphone designs, but be careful: your tuning requirements to dial in that bandwidth will still become more sensitive for an “electrically small” antenna.
- Bluetooth is designed for low-range operation, and many applications only require a few meters of range. Depending on your product’s required range, you might be able to handle the energy lost to the inefficiency that comes as a result of high-dielectric loading.
In other words, you can use small chip antennas for some Bluetooth applications, but if you are considering any loaded, electrically small antenna, be aware:
- Your range could be limited, maybe severely.
- Your sensitivity to surrounding components and enclosure materials could be acute, especially if your product is wearable
- Your overall design may require expert antenna design guidance to integrate the chip into your design successfully.