IC Substrate - Evaluation and optimization of mobile phone antenna with tunable aperture

The electromagnetic (EM) environment in which smartphones and other portable wireless devices operate often changes, and this is mainly caused by user actions. The user itself will fundamentally affect the radiation characteristics of the device antenna due to the coupling between the EM and the user's body. Users use different ways, so random factors must be considered when designing antennas. Wireless links are usually degraded by this coupling, and designers must be careful to minimize any problems that may occur due to different physical effects.

For the design of the tunable aperture antenna, we have introduced a novel calculation to evaluate the potential bandwidth and radiation efficiency of the antenna under different usage conditions. This will help design engineers evaluate suitable antenna designs early in the PCB design process. We imported the antenna pattern and S parameters into the radio frequency design automation software platform OptenniLab, evaluated the characteristics of the antenna, and synthesized a matching circuit to optimize the total radiation efficiency of the antenna.

For the aperture tunable antenna, we first find the best value of the aperture tuning element to evaluate the theoretical performance limit of the antenna, so as to achieve the maximum radiation efficiency. In the later stages of the design process, we optimize the matching circuit and the tuning circuit, and then compare their efficiency with the best radiation efficiency.

Simulation model for use case configuration
The main phenomena that affect antenna performance due to being close to the body are:

1) Antenna detuning caused by dielectric load. Since the dielectric load lengthens the electrical length of the antenna, the frequency of the antenna will decrease.

2) The loss caused by the absorption of power by the body. This loss directly affects the radiation efficiency of the antenna.

The simulation model in this article consists of the electric small smart phone antenna shown in Figure 1, which is essentially non-resonant at the design frequency. Use ANSYS HFSS EM simulator to simulate antennas in three configurations:
We refer to these configurations as "free space configuration", "hand configuration" and "head configuration".
New performance characteristic diagram for aperture tuning antenna
There are two basic tuning methods for mobile phone antennas, the so-called aperture tuning and impedance tuning (Figure 4). In aperture tuning, the tuner component will change the current distribution in the structure, thereby affecting its impedance and its radiation efficiency. The ability to optimize radiation efficiency is one of the main reasons for the popularity of aperture tuning, and we have also proposed a new method that visually presents antenna performance as a function of aperture components.
In OptenniLab, when the normalized radiation pattern of the electromagnetic simulator is imported together with the S-parameter matrix of the electromagnetic system, the software can calculate the total radiation pattern when the terminal circuit is placed on the port by superimposing the appropriate weighted port radiation pattern. The resulting total radiation pattern can be used to directly calculate the radiation efficiency associated with the tuner, facilitating the study of some representative tuner component values.

The feed port impedance as a function of tuner components is usually not very useful information for small non-resonant antennas, because the antenna is designed to work only with a lumped matching circuit. Instead, we should ask how wide bandwidth can be obtained at a given reference return loss level? OptenniLab has a bandwidth potential calculation tool [1], which can help answer any question about the value of the aperture element.

For Figures 5(a)-(c), in each environmental configuration, we have constructed a "mapping" for radiation efficiency and available bandwidth as a function of several selected aperture component values. The target return loss level calculated by the bandwidth potential is 10dB.

Physical performance limitations of different configurations
When considering the ultimate performance limits of each configuration, we must find the optimal value of the aperture component to maximize radiation efficiency. The optimal value depends on the frequency band and configuration. As we set optimization goals and evaluate the performance of design candidates, it is very useful to understand the final limitations.

In this study, we consider two scenarios: the satellite navigation frequency band Beidou B1-2 (approximately 1587-1592MHz) and 3GPP frequency band 1 (1920-2170MHz). For single-aperture tuners, the best radiation efficiency can be easily found by adjusting the value of the aperture component-the RF design automation software platform can recalculate the radiation efficiency in real time. The results are as follows:

Beidou B1-2

· Free space: hrad, max=41% (-3.9dB), L aperture=1.4nH

· Hand: hrad, max=24% (-6.2dB), L aperture=3.4nH

· Head: hrad, max=6%(-12.2dB), aperture=open circuit

3GPP band 1

· Free space: hrad, max=45% (-3.4dB), L aperture=1nH

· Hand: hrad, max=32%(-5.0dB), L aperture=3nH

· Head: hrad, max=6%(-12.2dB), L aperture=5nH

Theoretical and actual matching circuit performance
The radiation efficiency gives the physical upper limit of the total efficiency of the antenna at a given frequency. It is impossible to reach this physical upper limit in practice, because it requires perfect lossless impedance matching over the entire frequency band. Moreover, for different configurations, the best possible impedance matching circuit is not necessarily the same. Considering the theoretical closed-loop aperture tuning, in which the aperture component adapts to changes in the environment, we can assume the best aperture component value for any configuration. But even so, we still have to accept the compromise of impedance matching on the frequency band and configuration.