When designing antennas for mobile devices or other electrically small devices, the antenna operation is tuned with a matching circuit to make the antenna operate at the desired frequency bands. A common misconception is that the goal of impedance matching should be to minimize the reflected power from the antenna due to impedance mismatch. As practically all matching circuits contain losses, these losses may indeed help to obtain a better impedance match, but at the expense of antenna efficiency. In this article we examine the various loss mechanisms in antenna systems and show how the radiation performance can be optimized. The plots and optimization results were obtained from the Optenni Lab RF Design Automation software.
Consider the following example. Figure 1 shows the power balance of an unmatched antenna. Most of the power is reflected back to the amplifier due to impedance mismatch. The antenna has a radiation efficiency of 64.7%, but the radiated power is only 16.2% due to the reflection of power.
Let us add a three-component matching circuit. In Figure 2 we show the result when the impedance mismatch is minimized over a frequency range (at one frequency we could match the impedance exactly, but most antennas need to operate over some frequency band). We are using realistic vendor models of chip inductors and capacitors in the matching circuit. In this case, the reflected power is rather small, but over one third of the available power is absorbed by lossy resonances of the matching circuit.
Finally, consider the case when the matching circuit is optimized to maximize the minimum efficiency over the frequency band. We now obtain a solution which balances the reflected power and component losses to maximize the radiated power.
In a second example the antenna has a so-called aperture tuning port, where a tuning component or a switch can be placed. For the matching circuit design, we need the following data from an electromagnetic simulator (such as Altair Feko):
- The frequency dependent S parameter matrix of size 2 by 2, describing the impedances and coupling between the simulation ports.
- Radiation patterns for each of the ports, such that one port is excited at a time and the others are terminated with 50 Ohm loads.
With this information, Optenni Lab calculate the antenna and circuit response when varying the matching circuit and the component at the aperture port.
Consider first the case when the aperture port is left open. Figure 4 shows the power balance, where most power is absorbed by the poor radiation efficiency of the antenna.
Next we place a coil at the aperture port and optimize the value of the coil together with the matching circuit in order to maximize the radiated power (i.e. maximizing the minimum efficiency over the desired band). Note that now the presence of the coil has improved the radiation efficiency and therefore the total radiated power, although we have some component losses also in the aperture component (indicated as coupling losses).
Figures 6 and 7 show the setup of the optimization of the aperture tunable antenna system in Optenni Lab and the optimized circuit.
From the simulation point of view, the radiation efficiency was calculated in Optenni Lab without the need for running another electromagnetic simulation. This is somewhat surprising, but with the knowledge of the original radiation patterns and S parameters we can recombine the radiation patterns for any port termination, matching and excitation and thus obtain the combined radiation pattern and the radiation efficiency.
To learn more about Optenni Lab, click here.
This guest contribution is written by Jussi Rahola, CEO of Optenni Ltd. Optenni’s main product, Optenni Lab™, is the leading RF design automation software platform that features automatic matching circuit synthesis and optimization, antenna array beamforming with active impedance control, and many assessment tools to evaluate design candidate’s performance. Optenni Lab links with major EM tools, and is typically applied in the design of tunable or other multiband antenna systems and phased arrays. The Optenni Lab RF design automation software is available through the Altair Partner Alliance.
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