Antenna Design: Chip Antenna Implementation
Chip antennas offer small, compact solutions for wireless products. They often only need a few millimeters of space, and a little bit of ground clearance. In return, you get performance that is slightly below that of larger antennas like PIFA, patches, and dipoles. Initially, to the unsuspecting engineer, this may seem like a great way to save space and money. However, there are several caveats that a person must be aware of.
Be aware of the ground plane!
A chip is actually only half the antenna design. The other half is the ground plane of the PCB. Similar to a monopole, a chip antenna’s radiation pattern and tuning are directly related to the size and shape of the ground plane. Most datasheets will present the peak gain, bandwidth, return loss, radiation pattern, and other parameters for the antenna. They will also include a drawing like the one shown below in Figure 1. Notice the dimensions of the PCB? All the stated performance values are based on measurements from this exact ground plane. When you integrate a chip antenna into your product, more than likely it will be on a different size PCB. Which means your performance will differ from the datasheet. Additionally, the performance measurements were done in open space, your antenna will most likely be located inside a product.
Figure 1: Chip Antenna Design
Ground Keep out and Matching Components
Similar to the ground plane, the datasheet will also provide ground keep out and matching recommendations. These are very important and must not be ignored. The ground keep out is instrumental in the development of the electromagnetic fields surrounding the chip antenna, which results in formation of the radiation pattern. Additionally, the ground keep out will affect the tuning of the chip antenna as well. Lastly, the matching components provided in the datasheet are only valid for a chip antenna implementation that follows the datasheet exactly: same ground, same keep out, same orientation. Re-matching the antenna is not easy, and requires a network analyzer. However, it is crucial to performance to have the antenna matched appropriately. Proper matching can be the difference between 20 feet and 200 feet!
A chip antenna is designed to have metal near it only on certain sides. The chip shown in Figure 1 is meant to be placed in a corner, with ground on two sides as shown. Taking this chip and orientating it such that it’s perpendicular to the ground (now 3 sides with no ground) would be incorrect. As previously discussed, the ground keep out directly affects the antenna. If the ground is closer than recommended, performance decreases. If the ground is farther away than recommended, performance changes as well.
The key in using a chip antenna in RF design is to be aware of how your implementation will differ from the datasheet. It often takes an experienced antenna designer to understand the tradeoffs and how to maximize the chip antenna performance while properly placing the antenna.