VSWR of six or more indicates poor efficiency and the design should be revised (Table 1). A further complication is that antenna impedance changes with frequency. This is not a problem when the system is tuned to a single frequency, but IoT products often use radios operating at multiple frequencies. This is necessary to accommodate a mix of multiple interfaces, such as Bluetooth LE (2.4 GHz), Wi-Fi (2.4,
antennas feature compact dimensions and can cope with several different frequencies while offering good performance. However, there is a trade-off: in a like-for-like application, the performance of a multiband embedded antenna will fall short of a single-band strip antenna. This makes it even more important that the designer closely adheres to key design guidelines to maximise the embedded antenna’s efficiency across all operational frequencies. These guidelines extend beyond just antenna selection and positioning; the embedded component forms just one part of the ‘antenna system.’ To construct an efficient system, the antenna must be carefully paired with a suitable printed circuit board (PC board) ground plane and impedance matching circuit to optimise performance. The design of each part of the system significantly affects the overall antenna system efficiency, and the design of the impedance matching circuit can be particularly challenging for multiband embedded antennas.
Antenna basics An antenna converts voltage and current to produce the transmitted RF signal, and in turn, it converts an incoming RF signal to voltage and current at the receiver. Optimising the antenna’s efficiency ensures it converts as much of the transmitter power into radiated radio energy and harvests as much energy as possible from the incoming signal to feed the receiver. The efficacy with which it performs these roles largely determines the range and throughput of an IoT device.
VSWR Loss (dB)
Antenna efficiency (typically measured in decibels (dB)) is
1:1
0
determined by several factors, but a key factor is impedance. Significant mismatch between the antenna’s impedance (which is related to the voltage and current at its input) and the impedance of the voltage source driving the antenna, results in poor antenna efficacy. The key to boosting efficacy is to equalise the two impedances. Any power reflected by an antenna on a transmission line due to impedance mismatch interferes with the forward traveling power and creates a standing voltage wave. A common measure of how well the impedance is equalised is the voltage standing wave ratio (VSWR). A VSWR of 1 indicates no impedance mismatch loss, while higher numbers indicate increasing losses. For example, a VSWR of 3.0 indicates about 75% of the power is delivered to the antenna. A
2:1
0.51
3:1
1.25
6:1
3.1
10:1
4.81
20:1
7.41
Table 1: High VSWR causes greater losses. The designer should consider revising the design if VSWR exceeds 6:1. Image source: Steven Keeping
This article provides a brief introduction to antennas and
the challenges facing designers of wireless IoT devices. It then introduces multiband embedded antennas and explains how to design them in and ensure they are matched with the ground plane and impedance matching circuit to optimise performance.
5, and increasingly 6 GHz), LTE-M/ NB-IoT cellular (operating on several bands in the 700 to 2,200 MHz allocation), and GPS (1,227 and 1,575MHz).
One option for multiband
we get technical
33
Powered by FlippingBook