Optimizing an LTE Antenna's Matching Network

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A simple antenna for LTE band operation is added to the PC board of a smartphone in XFdtd and the matching circuit is tuned for operation in multiple frequency bands. The component values in the matching network are chosen so that system efficiency is maximized.

Figure 1 shows the antenna to be used, which is a simple strip fed off center. It can be thought of as two back-to-back inverted L antennas of different “top” lengths, though the operating modes will be more complicated than that. Figure 2 shows system efficiency for this antenna when fed directly. Clearly, some matching is required to improve performance.

Figure 1: LTE antenna without matching network.

Figure 2: System efficiency of unmatched antenna.

 

Mobile phones are intended to work over multiple frequency bands defined by an operator. In this example, Table 1 outlines the required LTE bands.

Table 1: Frequency bands defined by the operator.

 

 

In order to provide maximum power transfer and efficiency, a matching network is employed between the feed and the antenna. The design criteria are for the antenna and matching circuit to provide an average system efficiency of at least 65% over all of the bands of operation. In order to satisfy the design goals, the matching circuit of Figure 3 was chosen. Figure 4 shows the antenna and matching circuit in XF and Figure 5 shows the circuit layout in more detail.

Figure 3: Schematic of matching network.

Figure 4: Layout of matching network.

 

Figure 5: Details of matching network layout.

 

The surface-mount components to be used will be available in different families with different incremental values. For this circuit, the capacitors will be restricted to a range of 0.1 pF to 20 pf. These are available in steps of 0.1 pF up to 10 pF and steps of 1 pf above that. For inductors, the allowed range is 0.1 nH to 30 nH in steps of 0.1 nH from 0.1 nH to 10 nH and steps of 1 nH above that.

XF’s Circuit Element Optimizer is used to characterize the system using XF’s full wave FDTD solver. The optimal component values are then determined based on that characterization. As a result, the chosen component values are provided in Table 2 and the corresponding system efficiency for the matched antenna is compared to the unmatched case as seen in Figure 6.

Table 2: Optimal circuit component values.

 

Figure 6: System efficiency of matched and unmatched antenna.

 

 

Since the FDTD solver is used in the system characterization step, all electromagnetic phenomena affecting the performance of the matching network is considered. As validation, the optimal components can be plugged into a prototype and the antenna performance can be measured in a lab.