The design parameters of Bluetooth influence antenna form.
The pros and cons of different antenna hardware implementations.
Reviewing some of the most useful microstrip antennae.
Early Bluetooth for PCB antennae can be found in enabled USB dongles
The burgeoning yet already expansive Internet of Things (IoT) field is engaging consumers and manufacturers in ways that were impossible a decade before. As technology further enmeshes with everyday life, the need for infrastructure that is built for, rather than harnessed by, IoT continues to grow. For the majority of cases, Bluetooth or one of its design forks provides the perfect mix of capabilities and power efficiency to support this growing network.
Communication protocols are only one half of the equation, however. RF engineers are responsible for the hardware design of the antennae that enable wireless communication. Designing antennae within the constraints of a complex device that is already juggling a bevy of constraints is no simple task; incorporating Bluetooth for PCB antenna design is a delicate balancing act between performance, safety, and size.
Requirements of Bluetooth for PCB Antenna Design
Electromagnetic signals tend to act slightly quirky at high speeds. Minor imperfections in design and manufacture can result in excessive EMI that can shelve a board for further revisions. Conversely, many of these peculiarities can be harnessed as transceivers that allow for wireless communications without direct contact.
Antenna theory bestows a framework for RF engineers to build these transceivers in a variety of formats, shapes, and configurations to maximize the efficiency of space and signal integrity. Among the different protocols, Bluetooth fills the niche of a close-range technology for devices operating within low bandwidth conditions. The release of Bluetooth Low Energy (BLE) as part of Bluetooth 4.0 and improvements alongside Bluetooth 5 have been major driving forces in the IoT architecture.
PCB antenna design will have to contend with many conflicting factors during production, the foremost being a center frequency of ~2.45 GHz for Bluetooth-enabled transceivers. Specifically, Bluetooth operates in the range of 2.4 to 2.4835 GHz; to avoid conflicting with other wireless protocols that operate in the popular, unlicensed 2.4-2.4835 GHz band. Bluetooth switches between 80 1 MHz channels 1600 times a second. Begin by calculating the wavelength λ (lambda), where c is the speed of light in a vacuum and f is the center frequency:
The wavelength and frequency of light have a negative correlation, as constrained by the speed of light
Approximately, the targeted wavelength is 122.4 mm, with a half-wavelength and quarter-wavelength of 61.2 mm and 30.6 mm, respectively. These values are often present in different PCB antenna design configurations for a variety of optimization reasons. These measurements aren’t excessive for many boards, but consider how Bluetooth might avail itself to smaller electronic devices that are already densely packed: a full wavelength would be nearly as long as the cellphone itself, and that’s before any additional considerations such as the feed or ground plane concerns arise.
Comparing Antenna Hardware Implementations
There’s a wide variety of antennas available based on the particular needs of the implementation. At a level above the types of antennas, Bluetooth-enabled antennas exist across three categories:
- Microstrip - The most popular style of antenna uses the same manufacturing steps as traces, but additional processing may be required due to changes in high-speed signal behavior. Microstrip antennae allow for a multitude of design options and are highly scalable as an extension of standard subtractive copper etching with the lowest profile of all the listed options. The earliest microstrip antenna is the patch antenna (a planar polygon), which can be assembled in an array to boost certain antenna qualities without altering the shape – and the targeted frequency, among many things – of the individual antenna.
- Metal plate - The metal plate is like a halfway point between a printed antenna and a component. Similar to the microstrip, a multitude of shapes can be formed, but these are provided by manufacturers as opposed to design during the manufacturing process. As an SMT component, there are additional design options available for layout, although a keep-out/clearance area underneath the antenna must be maintained to prevent signal integrity issues. Despite its hybrid nature, metal plate antennas possess the best efficiency for the same antenna type, albeit with a commensurate greater power draw.
- Chip - The chip antenna is the most design-inflexible antenna, but it is packaged in an extremely efficient form factor due to a high permittivity ceramic at the center tha concentrates the electric field. Chip placement, as well as some other antennae, will operate without ground or metal beneath. The best placement for the chip antennae is beyond the extent of the ground plane (given proper clearance). A cutout at an edge/corner of the plane can be viable, but chip antenna placement above ground or a cutout that does not border an edge is unlikely to operate as expected.
The issues of ground plane coverage and surrounding metal also affect the microstrip and metal plate styles. Typically, balanced antennae are placed or routed off-ground – no metal or ground plane is suggested underneath the antenna for optimal performance, so long as the common-mode current is filtered on input. As a rough estimate, on-ground placement can lead to a dB loss close to two. However, some antennae, such as vertical and end-fed varieties, require the ground to perform as expected. It’s up to the engineer to maximize antenna functionality and provide guidance to the layout designer for stackup and routing instructions.
Popular Styles of Microstrip Antennae
Antenna form follows function; the number of designs and subtle variations to tweak the antenna characteristics can be positively overwhelming. Two fundamental models best suited and common to microstrip production follow:
Patch antenna - As briefly mentioned, the patch antenna is the basis of microstrip antennae. Simple shapes combined with microstrip transmission lines are etched directly into the board, providing a low-profile antenna that is at a Goldilocks distance away from the backing ground plane to optimize performance and efficiency. For a rectangle, the edges of the antenna perpendicular to the transmission line account for the impedance: widening these edges decreases the impedance and vice versa.
Planar inverted F antenna (PIFA) - The PIFA trades some performance of the half-wavelength patch antennae for a quarter-wavelength package that is especially desirable for tight designs like cell phones. The design of the antenna also radiates omnidirectionally away from the ground plane, which in a cell phone ensures the energy absorption of human tissue remains within acceptable limits.
Microstrip antenna design also includes variants like the L microstrip and bow that serve as entry points for RF solutions. Modifying any antenna by altering physical dimensions, adding a shorting pin/plate, etc., results in changes to performance, even for seemingly minor alterations.
Confidently Rollout RF Designs With Cadence’s Suite of Tools
Bluetooth for PCB antenna design not only requires a lengthy design phase, but no antenna is complete without necessary testing to assure it operates as expected, as well as any adjustments to meet its expected settings. Not only do RF engineers need to confirm the operation of the antennae, but they also need to ensure that the device itself remains EMI/EMC compliant; verifying that radiation is within acceptable limits is expensive, more so if devices are unable to pass compliance tests on their first try. To prevent costly delays, Cadence’s package of PCB Design and Analysis Software offers tools to build and simulate antenna properties, ensuring designs remain on the inside track to approval. The Cadence ecosystem of products then seamlessly integrates with OrCAD PCB Designer, supporting users with an industry-leading PCB layout design environment.
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