Design and simulate Wireless LAN device antennas

Design and simulate Wireless LAN device antennas

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Source: /21IC Author: How Siang Yap and Bart Van Hecke of Agilent EEsof EDA Department

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By adopting polarization diversity technology, low-cost PCB substrates can be used to create Wireless LAN equipment (WLAN) antennas with good receiver performance. This article will describe how to use the latest 3D electromagnetic field (EM) simulation tool to design and simulate a 2.4 GHz orthogonal polarized printed dipolar antenna, While predicting the surface current and the associated far field radiation.

Different from many articles on the same topic, this article describes how to use EM circuit collaborative simulation to comprehensively consider the effect of baseband circuit components used for antenna polarization switching. Using the method described in this article, the designer can directly motivate the antenna from linear or nonlinear circuit simulation without manual data transfer.

Overview

Consumer wireless applications require antennas to be concealed and installed in wireless products at a low cost and high performance. The following example describes how to meet the above requirements for 2.4 GHz WLAN applications by printing a dual-forward dipolar antenna on the Fr4 PCB. When a PCB is placed vertically, the vertical or horizontal dipolar antenna preferentially emits and receives verticals or horizontally polarized signals, as shown in 1.
This polarization diversity technology can be used to reduce the effect of multi-path reflection and interference on WLAN networks by establishing a circuit that can detect and switch strong signals in the antenna unit. The Design and Analysis of this antenna has been described in some documents in detail. Therefore, this paper uses the electromagnetic field (EM) simulation to quickly analyze the characteristics of the antenna, then, the electromagnetic field and circuit collaborative simulation technology are used to analyze the impact of the switching circuit on the antenna performance.

Figure 1 polarization diversity composed of two printed dipoles and Switching circuits on a PCB

 

Fast Analysis of Antenna Design Using Electromagnetic Field Simulation

 

Figure 2 three-dimensional structure and geometric size of a GHz printed polar Antenna

Figure 2 shows the structure and geometric size of the dipolar antenna.

Use the Momentum Three-Dimensional Electromagnetic Field simulator Antenna Structure of the Agilent EEsof EDA for exact simulation. The results are consistent with the accepted data. The Momentum 3D flat electromagnetic field simulator runs on a typical laptop (HP xw4400 Intel Dual Core 6600 2.4 GHz Win XP 64bit 2 gb ram) and takes one minute. Thanks to this rapid simulation, we can quickly analyze the characteristics of the antenna when variables such as geometric shapes or material parameters change.

Figure 3 shows that the FR4 dielectric constant ranges from 4.2 ~ The effect of changes between 5.0 on the resonance frequency of the dipolar antenna. The higher the dielectric constant, the lower the resonance frequency. As expected, because of the increase in dielectric constants, the wavelength of the substrate material decreases, and the dipolar antenna itself has a large electrical size. These changes often occur when designing products with low manufacturing costs. These factors are particularly important.

Figure 3 Effect of FR4PCB dielectric constant variation on Antenna Resonance Frequency

By viewing the effect of the geometric structure changes shown in Figure 4 on the surface current of the antenna, we can further understand the antenna design. The surface current diagram is very helpful for diagnosing mismatched or unwanted coupling sources. The current density in the diagram is represented in multiple colors, the animation effect is obtained by dynamically refreshing the current phase by performing a 360 ° scan. Now, we can see how the current is introduced into the adjacent structure or where it causes unwanted resonance and further correction. This is much more accurate and efficient than the traditional test method to process and adjust the circuit board multiple times or continuously cut and paste the circuit board.

 

Figure 4 animated display of Surface Current on a printed dipolar antenna helps identify and correct unwanted coupling with adjacent structures, or reflect the position of a harmonious Vibration

The MOM Simulation Technology Used in Momentum assumes that the medium plane is infinitely large. Most applications almost meet such conditions. The Finite Element Method (FEM) can be used to analyze the finite media effect (for example, when the printed dipoles are very close to the PCB edge. Figure 5 shows the simulation of an electromagnetic design system (EMDS) developed by Agilent EEs of EDA. The dipolar antenna is placed at intervals of 5mm and 2mm with the PCB edge, the result shows that the resonance frequency is offset by about MHz.

 

Figure 5 full 3D electromagnetic field simulation shows the effect of a printed dipolar antenna near the PCB edge. When a dipolar antenna is moved from 5mm to 2mm from the PCB edge, the resonance frequency is shifted to 100 MHz.

 

Figure 6 due to the infinite large medium plane assumption inherent in the MOM calculation, the far field map of the dipolar antenna calculated by the MOM method is displayed in the direction of the PCB plane without any radiation. The far-field figure calculated by the more precise Finite Element Method (FEM) On the right is shown in the color gradient, and the spiral distribution of the radiation power is smoother.

Figure 6 compares the far-field radiation map of the dipolar antenna predicted by Momentum and EMDS. Because EMDS does not need to assume the infinite PCB media plane conditions during the calculation process, therefore, the far-field map predicted by the algorithm is more accurate than the far-field map predicted by the method of moment (the simulation result of the method of moment shows that there is no radiation in the hypothetical infinite PCB plane direction ).

Collaborative simulation and Collaborative Optimization of circuit components and Antennas

In order to fully utilize the polarization diversity technology, a switching circuit consisting of a pin Diode can be used to connect with a dipolar antenna to conduct the conduction and shutdown of the dipolar antenna.

In this case, we must consider:

● The Impact of the switching circuit on the overall antenna performance.
● The influence of one dipolar antenna on the other dipolar antenna.
● Match the circuit of the switch circuit between the antenna and the receiver.

Figure 7 electromagnetic field and circuit collaborative simulation can be used to analyze and optimize dual-antenna and switching circuit, and can also be applied to Adaptive Antenna matching and beam forming under DSP control.

The above factors can be analyzed by using Momentum integrated in the Advanced Design System (ADS) platform to perform electromagnetic field and circuit collaborative simulation. Figure 7 shows the collaborative simulation settings of the dual-dipolar antenna and the switching circuit. Here, the PIN Diode following each dipolar antenna is used for bias to achieve polarization selection. Figure 8 shows the S11 reflection coefficient obtained from the shared feed of two dual-polar antennas.

 

Fig 8 reflection coefficient of the polarization diversity dipolar antenna, including the effect of the polarization switch circuit

From now on, if you need to adjust the geometric size of the dipolar antenna and change the parameters of the switching circuit to optimize the resonance frequency or S11 matching of the dipolar antenna, electromagnetic Field and circuit collaborative simulation can be performed in ADS. In Software Defined Radio environments, for example, a single antenna must be able to work at different frequencies and bandwidths, similar technologies can also be used to design adaptive antenna matching or beam networks under DSP control. It also facilitates the Adaptive Switching Circuit for switching the capacitor matrix. When the mobile phone and the user are in different distances, the adaptive circuit tracks and matches the changing antenna characteristics by switching different capacitors.