A wireless design primer explains antenna selection and placement, as well what matters in board design for the antenna's RF performance.
The design cycle is a little different when the product is wireless and requires an antenna. Antennas change the design process, because it needs to be located with care, in the best position on the PCB, and it’s important to consider its relationship with some of the other components in the design.
Ideally, the designer should arrange the RF elements of the design before considering the other components. In this article, we look at the additional stages that an antenna adds to the design cycle, starting with the selection of the most suitable antenna.
The most popular embedded antennas are the surface-mounted device (SMD) group. They are popular chiefly because of their efficient use of board space, but also because they can achieve excellent performance within a device. These antennas are tiny; they can be as small as 1-mm across and are reflowed directly onto the host PCB during the board assembly process. Generally, they are manufactured from high-grade dielectric laminate substrates.
The designer might also consider using a ready-made antenna module. These contain the antenna in a small package with other components, ready to drop into the design. The chief advantage of choosing an antenna module is the fact that it can be popped into the design, and the RF circuitry is provided ready-made.
Flexible printed circuit (FPC) antennas can be an interesting option where the component layout of the design is restricted in available board space, or where, for some reason, an SMD antenna will not easily fit. The FPC comprises a thin layer of copper cover tape with an integral cable and a UFL connector to join them to the circuit board. It’s thin enough to be slightly bent to a curved surface, and tucked into a small space within the device, maybe fixed inside the outer casing of the design. FPCs are a good choice where space is tight, and are popular in small handheld devices.
If the design for the device contains materials that might compromise performance, the designer might choose an external antenna. Embedded antennas tend to not achieve strong performance close to metal components, so if large metal features are incorporated into the design, a terminal or case-mounted antenna placed to the outside of the device may provide the best performance.
Figure 1 Antenna options include (left to right) an SMD, an FPC antenna, or terminal antennas for external use. Source: Antenova
These antennas are manufactured with their own insulating material to isolate the RF signal, and if there are metal parts close by, they will still perform with minimal losses. Case-mounted antennas free up space for other components on the PCB, and as they are not as sensitive to the other parts in the design, they are easier to integrate.
Of all the components in a typical wireless design, the antenna is probably the most sensitive to its position. Therefore, it is recommended that the placement of the antenna be decided right at the start.
The SMD antenna is soldered directly to the host PCB, and the position of the antenna has implications for its RF performance. The antenna will radiate in six directions along an axis, usually along the length of the antenna. This means that to perform well, it should have as many directions as possible free from reflective and absorptive obstructions. For this reason, antennas are often placed on the corner of the PCB, or designed to be used on one of the edges of the PCB. Manufacturers design their antennas to operate in different positions, and the datasheet for each antenna will specify exactly how the antenna radiates, and how to place it on the host PCB to optimize performance.
Figure 2 An SMD antenna is placed on the long edge of a PCB. Source: Antenova
There are certain components that need to be placed as far away from the antenna as possible, because they create noise and are likely to cause impedance to the radiated performance of the antenna. The chief culprits for causing interference are motors, batteries, and any components with a high-metal content such as LCDs.
Finally, the outer casing for the device may also cause interruptions for the radiated fields of the antenna. If the device has a plastic cover, be careful, because plastic has a higher dielectric constant than air, and can likely detune the resonant frequency of the antenna.
Ground planes and board design for RF
SMD antennas typically require a ground plane to radiate. In an embedded design, the ground plane is a section of the PCB which provides a flat contiguous surface for the RF signal to reciprocate from. The ground plane must be of a certain length, which is related to the longest wavelength of the antenna. It’s therefore critical to provide the correct amount of space for the ground plane on the PCB, as this will allow the antenna to radiate efficiently.
Again, it will be explained in the antenna manufacturer’s datasheet. Sometimes the ground plane is below the antenna, and sometimes it’s adjacent to it; this will vary from antenna to antenna, and will be a factor in your choice of SMD antenna.
As well as requiring a ground plane, antennas often require a certain space around them to be free of any other components—a keep-out area. The keep-out requirements for each antenna are also unique for each individual antenna, and these areas will need to be kept clear of other components, possibly through several layers, if not all of the board beneath the antenna.
The RF performance of the device will be best if the RF trace lines are kept as short as possible from the radio to the antenna. This is because longer transmission lines are more prone to reflections and signal energy losses in the copper trace, which can degrade the overall radiated performance of the device. Therefore, we recommend that the RF elements of the design be placed as close as possible to the antenna.
Some designs will benefit from a lumped element matching circuit—such as a Pi matching circuit—within the RF circuitry to tune the antenna for improved working bandwidth.
Figure 3 An antenna design with an active tuning circuit can overcome the bandwidth reduction seen with a smaller ground plane. Source: Antenova
Gerber review and RF testing
Before the design is finalized, a Gerber file layout review provides a good check of the RF circuits and transmission lines in the layer stack-up of the PCB design and will flag any areas that are not quite correct. The Gerber review checks that the antenna, the module, the transmission lines, vias, and PCB materials are all optimized for good RF performance. Some antenna design companies charge for Gerber reviews, while others offer it free of charge, or you may use a software design package for this purpose.
The next test will be to measure how well the antenna performs on its PCB. This is done in an anechoic chamber. However, the antenna may work well in the perfect conditions of the chamber, and then behave differently in its final application, where people and objects in the environment could affect how the antenna radiates. So, with a design for a wearable device, or a medical device to be used close to the human body, the antenna should be tuned and tested with a phantom head or phantom hand in the anechoic chamber.
A few more tests can assess how well the design will work in real life: passive testing, over-the-air (OTA) testing, and specific absorption rate (SAR) testing. The results will be measured for efficiency, spurious emissions, total radiated power, and total isotropic sensitivity.
It’s critical to test the design, to be sure that the device will perform correctly in day-to-day use, and will not create emissions or interference. These tests are critical where the design requires carrier network approval, and it’s usual to use a specialist RF testing service.
Finally, every design for the cellular networks must be certified by the mobile network carrier to gain approval to be used on its network.
This article was originally published on EDN.
Geoff Schulteis, an RF antenna application specialist, leads technical support for Antenova’s North American customer designs.