Mindtribe’s Guide to Chip Antenna Design
As I start the new year and with it new projects both professional and personal, I’m reminded of how we here at Mindtribe don’t stop engineering when we leave the office. We are engineers not only by education and profession, but also by nature. We innovate, we create, we tinker, we build, and most importantly we learn.
In my new year’s landscape I see an enticing list of projects revolving around wireless devices and in particular Bluetooth. My personal project as I leave the holidays behind is to make a Bluetooth controlled light for the wheels of my bike.
Sure, I could have bought a bike light or even decided to make a simple wired one, but what fun would that be?
I’ve decided to use a chip antenna and I’ve spent some time refreshing my knowledge on them. So, in case anyone else out there wants to take a stab at a compact wireless design, here are some thoughts on designing with chip antennas.
The chip antenna is a passive surface mount component often made from ceramic. A chip antenna and an appropriately designed ground plane on the PCB are used in conjunction to create a half-wave dipole. This means that a PCB designer must take into consideration that the chip itself is not the entire antenna.
Ground plane area, hand effects, and dielectric thickness can severely detune these antennas due to their extremely high Q. When doing a design it is important to keep in mind how sensitive the antenna is. Care should be taken in closely following data sheet recommendations including: ground plane, keepouts, stackup, feed line, matching components, and layout.
Antenna datasheets often provide a detailed layout with a specific ground plane size, PCB stackup, keepout area, and impedance-matched trace with matching components. Keep in mind that every piece of performance data on the datasheet was based off of the board they describe.
Therefore, any deviations you make from that design will adversely affect antenna performance. Odds are your design won’t match up perfectly, but the closer you can stay to the datasheet, the more likely you are to have a satisfactory result.
The ground plane may seem optional, but it is critical for creating a half-wave dipole. The chip antenna by itself is only a quarter-wave element. The ground plane serves as the counterpoise that creates the other quarter-wave element to result in a half-wave dipole. Be sure to use vias spaced at about 1/20 of your wavelength around the edge of your ground plane to connect it to other ground layers. This is sometimes called stitching and helps reduce coupling and slot radiation.
In addition to ground plane, these antennas often depend upon strict keepouts to prevent detuning by surrounding objects. These keepouts are a critical part of creating the correct radiation pattern. Therefore, these keepouts should be adhered to in order to achieve the desired level of performance.
The impedance-matched feed line and matching components are a critical part of the PCB design process for chip antennas. They are the system that allows maximum power transfer between your antenna and your RF front end.
As with the antenna itself, any deviation from the datasheet recommendations will degrade performance of this section. Changes in the PCB stackup in particular will severely impact the feed line. If changes in the stackup are made, the appropriate impedance matched feed line for that stackup should be calculated and used.
It is also important to remember that if you are changing the length of the feed line, you should keep the length in the keepout as minimal as possible. If this is not done the feed line may act as a parasitic radiating antenna.
As for the matching components, the ones provided in the datasheet are only useful if the datasheet is followed exactly. Matching component values will need to be different anytime factors like PCB material, stackup, thickness, adjacent components, etc. are changed. So, if you make any changes from the datasheet in these areas, you’ll need to do some impedance matching. This process requires a Network Analyzer, some practice, and probably deserves it’s own blog post.
Another major factor in chip antenna performance is layout. Chip antennas are designed to be oriented on a board in a particular way to optimize their performance. It is critical to carefully incorporate all required layout requirements into your design to ensure the best possible outcome.
If you are making alterations to the matching components and feed line, be sure to avoid creating stubs when placing parallel components. The matching components should be placed close together either near the feed or near the antenna, not in the middle of the feed line. Also, other components near by can negatively affect performance.
Not only are keepout areas critical on the PCB, but they are also critical in the surrounding space. Do not cover the keepout areas with metallic components like batteries. Similarly, carefully consider any housing or enclosure materials and be sure to assess their effect on antenna performance.
With all of this in mind, take some time to carefully select an antenna that is designed to meet your needs. Yes, all the typical parameters: frequency, bandwidth, radiation pattern, peak gain, average gain, polarization, and efficiency. But, take a very close look at size in particular. The size of the chip itself might be very appealing but is your PCB going to large enough to supply the ground plane needed for optimal performance? Can you afford to lose that much component and routing space to keepout?
If not, keep looking, or possibly rethink some of your design. Your thoughtfulness and thorough investigation of appropriate antenna options early on in your project has already saved you both time and money, and quite possibly some heartache.
However, if you can say yes to those two questions, you’re off to a fantastic start.