How to use multiband embedded antennas in IoT Designs
a multiband antenna system with a single matching circuit can fail to perform well. One solution – employed on Nordic’s nRF6943 – is to incorporate more than one matching network, with each accessed as needed through an MCU-controlled switch. The benefit of doing this is improved performance across all frequency ranges, with the downside that cost, and complexity increase when compared to a single matching circuit. These downsides are mitigated to an extent because each matching circuit only needs to transform the impedance for a single frequency band and will consist of just a few components. Figure 3 shows an example of the NN03-310 used in a reference design on a small PC board using three matching circuits (or matching networks (MN) as Ignion calls them). MN sections a, b, and c form the matching circuit for operation in the 824 – 960MHz and 1,710 – 1,990MHz cellular bands;
Figure 4: Matching circuit for sections a, b, and c (cellular operation) shown in Figure 3 with component values calculated using the Ignion design software. Image source: Ignion
MN sections d and e suit the 1,561 – 1,606MHz GNSS frequencies; and MN section f is the matching network for 2.4GHz (Bluetooth LE or Wi-Fi) operation. Figure 4 shows the design and component values for the cellular matching circuit (section a, b, and c), and Figure 5 shows the simulated performance of the complete design.
Testing the antenna system
Even though the matching circuit software will provide a good estimate of antenna system frequency response and efficacy, an actual prototype must be tested to ensure it demonstrates not only the predicted radiative efficiency, but that it is also approximately omnidirectional. The first test can be done by connecting a 50 Ω micro-coaxial cable to the antenna, grounded at three or four points on the pc board, and then connecting that cable to a network analyser. The results will not only indicate efficiency but also frequency response and bandwidth. The test typically reveals if some adjustment to the matching circuit components is needed. Ignion has made initial testing easier by supplying evaluation
Figure 3: Reference design using the NN03-310 antenna in a multiband design showing the matching circuit positions. Image source: Ignion
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