RF antenna design and layout demand careful attention to detail. These guidelines, drawn from mixed-signal practice, outline basic techniques to help ensure good RF isolation and signal integrity for designers new to high-frequency analog work.
Today it is rare to find a consumer product without an antenna. Even a garage door opener can connect to a phone via Bluetooth or Wi-Fi. Each additional RF antenna in a PCB layout introduces new challenges for RF designers, especially as analog design skills regain importance. As RF functions are integrated into modern PCBs, designers must ensure signals are not degraded and that signal integrity is maintained.
Simple layout measures can prevent RF signals from being weakened by nearby digital components. Those same measures also help avoid interference among multiple analog signals. While many RF design factors must be considered for mixed-signal or all-RF systems, antenna design and placement are among the most important elements. The following sections summarize the essential concepts and practical layout approaches for PCB RF antennas.
RF antenna design fundamentals
Floating conductive radiator: The element that radiates energy.
Reference: A reference plane or element that helps determine the antenna pattern orientation.
Feedline: The transmission line that carries the input signal from the RF component to the radiating element.
Impedance-matching network: Antennas often have a low impedance (the article notes around 10 ohms), so matching to the feedline is required to prevent reflections and to maximize transmitted power at the desired carrier frequency and bandwidth.
Many standard antenna designs and reference designs are available online, and microwave engineering textbooks contain design formulas for common antenna structures. Commercial off-the-shelf antennas are also widely available. Regardless of the antenna type, careful placement on the PCB is required to avoid interference between different sections.
RF antenna layout techniques
After designing the antenna, determine its placement on the PCB. RF designers should coordinate with mixed-signal designers, since most RF PCBs are mixed-signal, to prevent interference among RF front-end, RF back-end, and digital sections.
Effective radiation: Ensure the antenna radiates away from the PCB and is not absorbed by nearby PCB structures.
Isolation: Prevent mutual interference between different functional areas on the PCB.
Electromagnetic compatibility (EMC): Ensure the PCB does not unintentionally receive emissions from other devices that may transmit across a wide frequency range.
These design goals often compete. Two practical guidelines help balance them.
1. Separate circuit blocks on the PCB
This basic mixed-signal PCB principle also applies to RF antenna placement. Place the antenna area apart from other circuit blocks. Generally, locating the antenna near the PCB edge and away from other analog components confines strong radiation to a defined area and minimizes interference between sections.
Gridded layouts must ensure that return paths do not interfere across regions, which can cause noise coupling and crosstalk. Field solvers integrated into advanced PCB design tools can reveal return-path deviations during layout. For high-frequency designs, continuous ground planes help maintain coherent return paths.
2. Isolate the antenna area
Modern smartphones and cellular devices are examples of advanced isolation techniques. Isolation involves placing shielding structures around RF-sensitive elements to block wave propagation between transmitters and receivers. The following approaches can be used in the antenna area to isolate components, feedlines, and antennas from each other and from external noise sources.
Isolation structures are placed between RF elements to prevent noise coupling or power exchange. Selecting the appropriate isolation structure to preserve antenna signal integrity is a complex design decision that benefits from electromagnetic field solvers. Use EM solvers to evaluate the effects on feedlines and antenna impedance as well as the achieved isolation. When possible, use finite element method (FEM) solvers rather than applying Fourier transforms to convert FDTD results.
Figure 1: Advantages and disadvantages of various RF isolation structures
Although antenna design and layout require additional attention to detail, this effort improves RF isolation and preserves signal integrity.
Key points
- RF antennas take many forms, from on-chip planar antennas to copper antennas printed directly on the PCB.
- When designing a PCB with one or more antennas, ensure adequate isolation between different circuit modules on the board.
- Use CAD and EM tools to design isolation structures, transitions, and even printed antennas on the PCB.
