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.
Navigating The Sea of Bluetooth Antenna Options
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.
5 Necessary Steps for any RF design (even Bluetooth!)
- 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.
- 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.
- 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.
- 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.
- 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.
If you follow this sequence, you’ll choose the right antenna and get the communication range and throughput your product needs. If you don’t, all bets are off.
Basic RF Design Components
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.
The 2.4 GHz ISM band Bluetooth Standard Defines Your Antenna Length
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.
Why not just use the smallest Bluetooth chip antenna you can find?
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.
Why the PCB Ground Plane and Clearance Area Requirements?
Quarter wavelength antennas must be combined with a ground plane of sufficient length to make an effective ½ wave antenna. You also need your antenna to radiate without unwanted interference from the ground plane, and that requires clearance for an omnidirectional antenna (see below). And you’ll need to set aside some space for matching components that will tune the antenna’s operating band to match the ISM band for your particular PCB layout and surrounding components.
Most chip antenna manufacturers will provide layout guidelines that account for all these PCB design elements, because they are absolute requirements for the antenna to even work at all for Bluetooth.
Any layout guidelines worth using will follow the quarter wave rule as a bare minimum: you need at least a quarter wavelength-long ground plane in the dimension of polarization.
For Bluetooth, this is 31 mm, but longer is always better (for example Johanson’s layout guidelines specify 40mm ground plane length for their 2.4 GHz WLAN antennas).
The antenna’s directivity determines the required position and orientation of the antenna with respect to the ground plane.
The nearby materials and nature of the application determine whether you need a directional or omnidirectional antenna.
Below, you can read about how your product’s use case and materials determine the directivity you need from your antenna.
For an omnidirectional monopole or IFA antenna fed against the thin edge of a PCB, we need at least a 31 mm long ground plane, and preferably about that same dimension in width.
For a directional antenna like a patch or PIFA resting on the broad surface of a PCB (more on those distinctions in a minute), we like to see a symmetric ground plane about 31mm x 31mm (fits within the 40 x 40 mm rule-of-thumb along with the antenna).
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.
Critical Factor #2: Is your product enclosed in or mounted on metal?
Solid metal in the path of transmission will outright block your antenna’s E-M waves. So if your product is enclosed in solid metal, you’ll need to mount your antenna external to the product enclosure, and it will need to radiate away from the enclosure, not into it – that restricts you to directional antenna types like a patch or PIFA (planar inverted f antenna).
If your enclosure’s metal coverage is only partial, careful antenna location will still be critical, and it may still need to be external, depending on the exact geometry.
A smart meter mounted on sheet metal or a metal wall will not be able to send radiation through the sheet metal, so it will also need a directional antenna type to get maximum range in the field of view.
We’re not at the tuning step yet, but it’s also worth noting that all conductors near your antenna will affect your tuning requirements.
And on the subject of tuning, nearby dielectric materials like plastics, glass, or ceramics can throw off the resonant frequency of the antenna if they aren’t taken into account in the antenna matching and tuning process. Account for this in your antenna test and tuning plan.
If you’re not sure about your materials, just select your product’s design constraints in the form below. We’ll help you evaluate your entire application, including the materials you’re considering.
Critical Factor #3 – PCB Real Estate – Do you have a 40mm x 40 mm area on your PCB for your RF components, antenna and ground plane?
This minimum PCB area requirement will impact the efficiency, installed impedance match, and patterns of your antenna. If you choose a Bluetooth chip antenna – pretty much without exception – it will require a PCB as a ‘counterpoise.’ The PCB is as much a part of the antenna as the chip structure itself.
The quarter-wave rule – preferably bigger – should be followed for peak efficiency. If a reliable and efficient signal over a relatively long range is not important to you, then you can ‘commit some sins’ here.
However, if you want to extract as much performance out of your Bluetooth radio as possible, improving data throughput over longer ranges with lower power consumption, then pay attention to the these rules of thumb and the app notes on PCB areas!
If you don’t have a 40mm x 40mm space available, your range/throughput requirements and nearby materials may require a custom antenna designed specifically for your product.
Critical Factor #4 – Bluetooth Range
The next question is the desired range of your product. If a reliable Bluetooth connection is important to you over distances that are on the higher end of what Bluetooth is designed for (50m-100m), then you should stay away from Bluetooth chip antennas that are less than 20mm.
As you learned above, miniature antennas have been loaded with dielectrics (or high permeability materials) so they can operate in the electrically small regime.
Antenna efficiency drops fast when length gets shorter than 2(20mm for Bluetooth).
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.
If you need to reach across long ranges, and you have the space for it, choose the biggest 2.4 Ghz chip antenna you can fit. Is this a generalization? Maybe. But here’s what’s always true: an antenna that is not “electrically small” (loaded) will always outperform one that is.
Your Next Steps…