■ Frequency response: Return Loss (RL) depends on the radio frequency. Designers should check the datasheet for the antenna’s frequency response to ensure RL is minimized at the intended operating frequency (Figure 2) ■ Directivity: this measures the directional nature of the antenna’s radiation pattern. Maximum directivity is defined as D max ■ Efficiency (η) : the ratio of total radiated power (TRP, or P rad ) to input power (P in ) is calculated from the formula η = (Prad/Pin) * 100% ■ Gain: this describes how much power is transmitted in the
Figure 1: Radiation patterns graphically represent how the antenna radiates or absorbs radio energy in 3D space. Datasheets typically show the maximum extent in the XY and YZ planes when the antenna is mounted as intended. Image source: Amphenol
example antennas from Amphenol to illustrate suitable choices for a Bluetooth Low Energy (LE) or Wi-Fi sensor, an IoT asset tracker with GNSS satellite positioning capability, a Wi-Fi access point (AP), and a LoRa IoT device. Interpreting the datasheet An antenna’s final performance is subject to engineering decisions such as mounting position and the design of impedance matching networks. A good implementation
Poor impedance matching increases return loss (RL). The voltage standing wave ratio (VSWR) indicates the impedance matching between the transmission line and the antenna (Table 1). High VSWR values result in high power losses. A VSWR below 2 is generally acceptable for an IoT product
direction of peak radiation. It is usually referenced to an isotropic antenna with a designation of dBi. It is calculated from the formula Gain max = η * D max
VSWR
Return Loss (dB) % Power/Voltage Loss
requires a careful review of the antenna’s datasheet. Key parameters include: ■ Radiation pattern : this
1
-
-
1.25
-19.1
1.2/11.1 11.1/33.3 18.2/42.9 30.9/55.5 44.7/66.6 67.6/81.8 81.9/90.5
2
-9.5 -7.4 -5.1 -3.5 -1.7
graphically defines how the antenna radiates (or absorbs) radio energy in 3D space (Figure 1) ■ Maximum power transfer: good
2.5 3.5
5
10 20
power transfer between the antenna and receiver occurs when the transmission line impedance (Z 0 ) is matched to that of the antenna (Z a ).
-0.87
Table 1: The VSWR indicates the impedance match between the transmission line and the antenna. A VSWR below 2 is generally acceptable for an IoT product. Table source: Steven Keeping
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