What are the key parameters of antenna performance and how to choose antenna parameters in radio frequency circuits

Introduction] Return loss: The return loss of an antenna indicates how the antenna matches a transmission line (TL) with an impedance of 50Ω, which is shown as the signal feed in Figure 7. Usually, the impedance value of this TL is 50Ω, but other values are also possible. For the industry standard, the resistance of the commercial antenna and its test equipment is 50Ω, so it is recommended that you use this value
Antenna parameters

The following section provides some key parameters of antenna performance.

▪ Return loss: The return loss of an antenna indicates how the antenna is matched to a transmission line (TL) with an impedance of 50Ω, which is shown as the signal feed in Figure 7. Usually, the impedance value of this TL is 50Ω, but other values are also possible. For industry standards, the resistance of a commercial antenna and its test equipment is 50Ω, so it is recommended that you use this value.

Return loss shows: the amount of incident power reflected by the antenna due to mismatch (Equation 1). An ideal antenna will emit all power without any reflections.

If the return loss is infinite, it is considered that the antenna matches the TL perfectly, as shown in Figure 7. S11 is the reciprocal of return loss, and its unit is dB.

In most cases, if the return loss is ≥ 10 dB (ie S11 ≤ –10 dB), it is large enough. Table 1 shows the return loss (dB) and reflected power (%) of the antenna. When the return loss is 10 dB, it means that 90% of the incident power is transmitted to the antenna for transmission.

Table 1 Return loss and reflected power of antenna

▪ Bandwidth: refers to the frequency response of the antenna. It shows how the antenna and 50 Ω transmission line match each other on the entire frequency band used, that is, in the range of 2.40 GHz to 2.48 GHz for BLE applications

Figure 8. Bandwidth

As shown in Figure 8, in the 2.33 GHz to 2.55 GHz bandwidth, the return loss is greater than 10 dB. Therefore, the bandwidth used is around 200 MHz.

In most cases, a wider bandwidth is the first choice, because it can minimize the detuning effect caused by changes in the antenna surrounding environment in actual use of the product (such as a mouse placed on a wood/metal/plastic table, around the mouse Hands etc.). )

▪ Radiation efficiency: refers to the part of non-reflective power consumption (see Figure 7) that is consumed as heat in the antenna. The heat generated is due to the dielectric loss in the FR4 substrate and the conductor loss in the copper wire. This information serves as radiation efficiency. When the radiation efficiency is 100%, all non-reflective power consumption is emitted

Shot into space. For small PCB form factors, the heat loss is minimal.

▪ Radiation pattern: This pattern represents the directivity of radiation, that is, in which direction the radiation is greater and which direction the radiation is smaller. This helps to accurately determine the direction of the antenna in the application. The non-directional antenna can transmit equivalently in all directions on a plane perpendicular to the axis. But most antennas cannot achieve this ideal performance. For a detailed description, please refer to the radiation pattern of the PCB antenna shown in Figure 9. Each data point represents the RF field strength, which can be measured by the Receiver Signal Strength Indicator (RSSI) in the receiver. As expected, the contour image obtained is not circular because the antenna is not isotropic.

Figure 9. Radiation diagram

▪ Gain: Gain provides information for comparing the radiation of the used direction with the isotropic antenna (that is, transmitting from all directions). The gain unit is "dBi", which means the radiated field strength when compared with an ideal non-directional antenna