Select antenna details for low-power wireless applications

The three most common antenna introduction

Describe several parameters that must be considered when selecting an antenna

For applications where space is limited, chip antennas are a good choice, especially for frequencies below 1 GHz.

Whip antennas perform best, quarter-wave antennas provide an efficient solution

Antennas are an important component of an RF system and have a significant impact on performance. High performance, small size, and low cost are the most common requirements for many RF applications. In order to meet these requirements, it is very important to implement a proper antenna and describe its performance characteristics in an overview. This article describes typical antenna types and describes the important parameters that should be tested when selecting an antenna.

When selecting an antenna, the size, cost, and performance of the antenna are the most important considerations. For short-range wireless devices, the three most common antennas are PCB antennas, chip antennas, and whip antennas. The advantages and disadvantages of these three antennas are shown in Table 1.

a.PCB antenna

Designing a PCB antenna is not a simple matter because it requires a simulation tool to get a satisfactory solution. In addition to providing an optimum design, configuring such a tool for accurate simulation is a difficult and time-consuming task.

b. Chip antenna

For the antenna, if the board space is very limited, then the chip antenna can be a good solution. This type of antenna can support a small solution size (even below 1 GHz frequency). The disadvantage of this type of antenna compared to the PCB antenna is that this solution increases material and mounting costs. The typical cost of a chip antenna is between 0.10 USD and 1.00 USD. Even though some chip antenna manufacturers claim that the antenna can be matched to a certain 50 Ω impedance to suit the frequency band, other matching components are usually needed.

c. Whip antenna

If we focus most on performance, not on form factor and cost, an external antenna with a connector would be a good solution. These antennas are usually monopole antennas and have an omnidirectional radiation pattern, which means that the performance of the antenna in all directions on a plane is almost the same. The whip antenna should be mounted on the ground plane for optimal performance. In order to achieve maximum savings, quarter-wavelength antennas provide an efficient solution.

When selecting an antenna, some of the most important factors to consider include: radiation mode, antenna efficiency, and antenna bandwidth.

a. Radiation Mode and Gain

Figure 2 shows how the radiation pattern of the PCB antenna will change with direction in the PCB plane. When interpreting such a radiation pattern change map, it is very important to understand several antenna parameters. In addition to the radiation pattern change map, it is also important to relate the radiation pattern to the configuration of the antenna.

The radiation pattern is usually tested on three planes XY, XZ, and YZ that are at right angles to each other. Although full 3D graphics measurements can be made, this is generally not done because it is a time-consuming task and requires expensive equipment. Another way to define these three planes is to use a spherical coordinate system. The three planes will be defined by θ=90°, Φ =0°, and Φ=90°. Figure 3 shows how to associate a spherical symbol with these three planes.

If no information is given on how to associate the radiation direction on the radiation pattern map with the configuration of the antenna, then 0° is the X direction, and the angle in the XY plane increases toward the Y direction. For the XZ plane, 0° is in the Z direction while the angle is increasing in the X direction. For the YZ plane, 0° is in the Z direction, and the angle tends to increase toward the Y direction. The gain or reference level usually refers to an omni-directional radiating antenna, which is an ideal antenna with the same radiating power in all directions. When the omnidirectional antenna is used as a reference, the gain is in dBi units, or it is specified as an equivalent isotropic radiated power (EIRP). The outer circle in Figure 2 is equivalent to 5.6 dBi, and the lower left 4 dB/div symbol indicates that the transmission level is reduced by 4 dB for all increasing small circles. Compared to an omnidirectional antenna, the PCB antenna will have a high level of 5.6dB radiation in the 0° direction.

As shown in Equation 1, the antenna gain G is defined as the ratio of the maximum radiation intensity to the average radiation intensity times the antenna efficiency.

Among them, Umax is the maximum radiation intensity and Uavg is the average intensity. The ratio of these two values ​​is called the directionality D. The resistance loss of the antenna element and the reflection at the antenna feed point together determine the efficiency e, which is the radiated power obtained by dividing Prad by the input power Pin. High gain does not necessarily mean that the antenna has high performance. In general, mobile systems require an omnidirectional radiation pattern so that their performance is approximately uniform in all antenna directions. For applications with fixed positions, such as receivers and transmitters, higher performance can be obtained when the antenna points to other high gain radiation directions.

In order to accurately measure the antenna radiation direction, it is important to measure only the direct waves of the device under test and avoid reflected waves that affect the measurement results. In order to minimize the pick-up of reflected energy, these measurements are usually performed in echo-free rooms or antenna test fields. Another requirement is that the signal measured in the far field of the antenna must be a plane wave. As shown in Equation 2, the far-field distance Rf is determined by the wavelength λ and the maximum antenna size DIM. Due to the limited space in the non-echo chamber, it is common to test some large, low-frequency antennas in an outdoor antenna test field.

b. Polarization

Polarization is the description of the direction of the electric field. All electromagnetic waves propagating in free space have electric and magnetic fields perpendicular to the propagation direction. In the case of considering polarization, the electric field vector is usually described and the magnetic field is neglected because it is orthogonal to and proportional to the electric field. For best performance, both receive and transmit antennas should have the same polarization. In fact, most antennas in short-range applications produce polarization in multiple directions. Since indoor equipment is subject to many reflections, polarization is not as important as some outdoor work equipment.

c. Bandwidth and impedance matching

Two common methods for determining the antenna bandwidth are: 1) measuring the radiated power while adjusting the carrier on the relevant frequency band; 2) measuring the reflection of the antenna feed point using a network analyzer. Figure 4 shows the first method, which measures the radiated power of a 2.4 GHz antenna, which has an output power variation of nearly 2 dB in the 2.4 GHz band while also having the maximum radiation close to the center of this band. This measurement method is accomplished by adjusting the 2.3 to 2.8 GHz continuous wave signal. This type of method should be performed in an anechoic chamber to obtain the correct absolute bandwidth level. However, this method is very useful even in the absence of an echo-free room.

Measurements in a normal laboratory environment can give a relative result, which indicates whether the antenna has the best performance in the middle of the ideal frequency band. The performance characteristics of the receiving antenna used to make this measurement will have an effect on the measurement results. Therefore, it is very important that this antenna has approximately the same performance in the measurement frequency band. This precautionary measure helps to ensure that the relative performance changes observed in the measurement band are valid. A second method of characterizing antenna bandwidth is to measure the reflected power at the feed point of the antenna. Disconnecting the antenna and using a coaxial cable to connect a network analyzer to the antenna can make such measurements. The bandwidth of the antenna is usually defined as the frequency range, the reflection at this frequency is lower than -10 dB, or the VSWR is less than two. This is equivalent to less than 10% of the effective power reflected by the antenna in this frequency range.

d. Size, cost and performance

The ideal antenna must not only be very small, zero cost, but also have excellent performance. However, in real life, it is necessary to seek a compromise between these indicators. Chip antennas are a better alternative when seeking small antenna solutions. This is particularly true at frequencies below 1 GHz because chip antennas allow for smaller solutions than traditional PCB antennas. However, the main disadvantages of chip antennas are higher costs and typical narrowband performance.