How to use multiband embedded antennas in IoT Designs
products is to use a separate impedance-matched antenna for each frequency, but that adds considerable complexity, size, and expense. An alternative is to use a single embedded antenna and design circuitry to ensure good impedance matching to cover a range of operational frequencies. Antenna selection and placement There are several vendors offering mature embedded antenna designs. With knowledge of the end product’s intended operational frequency band(s), it is relatively simple to narrow down the shortlist of suitable antennas from a supplier catalogue. For example, Ignion (formerly Fractus Antennas) offers a range of components suitable for IoT products, including the ALL MXTEND NN02-220 antenna and the TRIO MXTEND NN03-310 antenna. The NN02-220 is a multiband antenna suitable for cellular 2G, 3G, 4G, and 5G, plus NB-IoT/LTE-M cellular applications and is supplied in a 24 x 12 x 2mm package. With appropriate system design, the antenna can reach an efficiency approaching 70% and a VSWR of less than 3:1. It features an omnidirectional radiation pattern, providing approximately equal transmission and reception in all directions. The NN03-310 covers the same frequency bands as the NN02-220
Figure 1: The Ignion NN03-310 is an embedded antenna for cellular, GNSS, short-range RF, Wi- Fi, and UWB. Image source: Ignion
The position of the antenna on the pc board also has a large influence on the design’s transmit power and receive sensitivity. Manufacturer guidelines recommend placement at the corner of the IoT device. It is also important to place the chip antenna as far as possible from other active components that could generate electromagnetic interference (EMI) during operation. For the transmission power levels typical of cellular IoT devices, a minimum clearance area of 20mm from other components is satisfactory. The ground plane should also be kept clear of this area. The PC board pads and traces connecting the chip antenna to the rest of the circuitry should be the only copper conductors in the clearance area. It’s also good practice to keep the antenna away from housing screws, brackets, and other metallic parts. For example, on the Nordic Semiconductor nRF6943 cellular IoT development board, the antenna is placed at one side of the board with a
but adds GNSS, Bluetooth LE, Wi-Fi 6E, and ultrawideband (UWB). It measures 30 x 3 x 1mm and has performance figures similar to its sister product with efficiency approaching 65%, a VSWR of less than 3:1, and an omnidirectional radiation pattern (Figure 1). Once the embedded antenna has been selected, the next step is to consider the ground plane. The size of the ground plane has a large impact on antenna efficiency. For example, at an operational frequency of 900MHz, in a like-for- like comparison, a 10cm2 ground plane might exhibit 30% efficiency, whereas a 40cm2 ground plane would boost the efficiency to 60%. Therefore, within the constraints of the end-product form factor, it is good design practice to use as large a PC board as possible and then dedicate one complete layer of the pc board to the ground plane. Note that as the frequency increases, the ground plane size has less impact on antenna efficiency. Above a few GHz, the impact is negligible.
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