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24 DESIGN007 MAGAZINE I APRIL 2023 RF Antenna Design Basics ere are a few basic points to follow when designing a custom antenna or choosing a commercial off-the-shelf (COTS) antenna for use in your RF PCB. All RF antennas share a few particular characteristics that should be considered during the design phase. Every antenna needs the following elements: • Floating conductive radiator: is is the antenna element from which radiation will be emitted. • Reference: e reference plane or element for an antenna helps determine the struc- ture's directionality in each antenna mode. • Feedline: e feedline routes the input signal from an RF component into the radiating antenna element. • Impedance matching network: An antenna normally has ~10 ohm imped- ance, so it needs to be matched to the feedline impedance to prevent reflection and ensure maximum power transfer at the desired carrier frequency and bandwidth. ere are many standard antenna designs that have been thoroughly studied. You can find many reference designs online, which can then be copied into your PCB layout. You can also find many design formulas for standard antenna structures in microwave engineering textbooks. Finally, if you want to use a COTS RF antenna, there are many inexpensive designs you can find on the market for low cost. No matter which RF antenna you choose to use, you'll need to care- fully place it in your layout to prevent interference between board sections. RF Antenna Layout Tips Once you've designed your antenna, it's time to figure out where it should be placed on the PCB. RF designers should take some tips from mixed-sig- nal designers (most RF boards are really mixed-signal boards) in order to prevent interference between multiple sections in the RF front end, back end, and digital sections. • Efficient radiation: e goal here is to ensure radiation from antenna elements travels away from the board without being picked up by other structures in the PCB layout. • Isolation: Similarly, we don't want multiple sections in the PCB layout interfering with each other. • Electromagnetic compatibility (EMC): Finally, we need to ensure that the layout is resistant to reception of signals from other devices that may emit over a broad range of frequencies. In a real PCB, most design goals are in com- petition, but there are two important points to follow that will help you balance these design goals. 1. Separate Circuit Blocks in Your PCB Layout is is a fundamental mixed-signal PCB design topic, and it applies just as much to RF antenna layouts. You'll need to place the antenna section in a location on the board that is separate from other circuit blocks. Generally, it is best to place the antenna section near the edge of the board and away from other analog components. is confines strong emission to one location on the board and ensures interfer- ence between board sections is minimal. Figure 2: Gridded system layout on an RF PCB.