Introduction

　　The antenna is a key component in the wireless system, and it is responsible for sending and receiving electromagnetic radiation from the air. Designing antennas for low-cost, consumer-wide applications and integrating them into portable products is a challenge that most original equipment manufacturers (OEMs) are facing. The wireless range that the end customer obtains from a certain RF product (such as a coin-type battery with limited power) mainly depends on the design of the antenna, the plastic casing, and a good PCB layout.

　　For systems with the same chip and power supply but different layout and antenna design practices, it is normal for their RF (radio frequency) range to vary by more than 50%. This application note introduces best practices, layout guidelines, antenna tuning procedures, and gives the widest band that can be obtained with a given amount of power.

    A well-designed antenna can expand the working range of wireless products. The greater the energy sent from the wireless module, the greater the transmission distance under the conditions of the given packet error rate (PER) and the fixed receiver sensitivity. In addition, antennas have other less obvious advantages. For example, a well-designed antenna can emit more energy within a given range, thereby improving error tolerance (caused by interference or noise). Similarly, a good debugging antenna and Balun (balancer) at the receiving end can work under extremely small radiation conditions.

　　The best antenna can reduce PER and improve communication quality. The lower the PER, the fewer retransmissions will occur, which can save battery power.

Antenna principle

　　An antenna generally refers to a conductor that is exposed in space. When the length of the conductor is in a specific ratio or integer multiple of the signal wavelength, it can be used as an antenna. Because the power supplied to the antenna is emitted into the space, this condition is called "resonance."

Figure 2. Dipole antenna basics

　　As shown in Figure 2, the wavelength of the conductor is λ/2, where λ is the wavelength of the electrical signal. The signal generator supplies power at the center of the antenna through a transmission line (also called antenna feed). According to this length, voltage and current standing waves will be formed on the entire wire, as shown in Figure 2.

　　The electrical energy input to the antenna is converted into electromagnetic radiation and radiated into the air at the corresponding frequency. The antenna is powered by the antenna feed, the characteristic impedance of the feed is 50Ω, and it radiates into a space with a characteristic impedance of 377Ω.

　　Therefore, regarding the geometry of the antenna, there are two very important things to note:

　　1. Antenna length

　　2. Antenna feed

　　An antenna with a length of λ/2 (as shown in Figure 2) is called a dipole antenna. However, in printed circuit boards, most of the conductors used as antennas are only λ/4 in length, but they still have the same performance. See Figure 3.

　　By placing a ground plane at a certain distance below the conductor, a mirror image (λ/4) of the same length as the conductor can be created. When combined, these pins act as dipole antennas. This type of antenna is called a quarter-wavelength (λ/4) antenna. Almost all antennas on the PCB are implemented in the size of a quarter wavelength on the copper ground plane. Note that the signal is now single-ended feed, and the ground plane is used as the return path.

                                       Figure 3. Quarter-wavelength antenna

　　For the quarter-wavelength antennas used in most PCBs, special attention is required:

　　1. Antenna length

　　2. Antenna feed

　　3. The shape and size of the ground plane and return path

Antenna selection

　　The choice of antenna depends on factors such as its application, available circuit board size, cost, radiation range, and directivity.

　　Bluetooth Low Energy (BLE) applications (such as wireless mice) only require a 10-inch radiation range and a few kbps data rate. However, for remote control applications using voice recognition, an indoor antenna is required. The antenna's radiation range is about 10-15 inches and its data rate is 64kbps.

　　For wireless audio applications, a diversity antenna is required. Diversity antenna refers to: placing two antennas on the same PCB, so that at least one antenna can always receive some radiation, while the other antenna may be blocked due to reflection and multipath attenuation. In the case of transmitting real-time audio data and requiring higher throughput without losing data packets, a diversity antenna is required. It can also be used in beacon applications for indoor positioning.

Antenna parameters

　　The following section provides some key parameters of antenna performance.

　　§ Return loss: The return loss of the antenna indicates how the antenna matches the 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.

　　The return loss indicates the amount of incident power reflected by the antenna due to the 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 completely, as shown in Figure 7. S11 is the reciprocal of return loss, and its unit is dB. According to empirical estimation, if the return loss is greater than or equal to 10dB (that is, S11≤-10dB), it is large enough. Table 1 shows the return loss (dB) and reflected power (%) of the antenna. When the return loss is 10dB, it means that 90% of the incident power is transmitted to the antenna for transmission.

Figure 7. Return loss

Table 1. Return loss and reflected power of antenna

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

                                                                 Figure 8. Bandwidth

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

　　§ 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 into the space. For small PCB form factors, heat dissipation 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 pattern

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

2.4G antenna