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Design and Simulation of a Planar Inverted-F Antenna

Parent Category: 2020 HFE

By Johannes Steigert

Introduction

Today’s mobile devices often serve different frequency bands with multiple antennas optimized for performance and designed for the smallest possible footprint. To address these concerns, engineers at CommScope implemented an inverted-F antenna (IFA) for sub 1GHz band mobile communications using a combination of antenna theory, circuit analysis and electromagnetic (EM) simulation. This type of antenna is a variation of a monopole antenna and is commonly used in hand-held wireless devices.

The inverted-F design has two advantages over a simple monopole: the antenna is shorter and more compact, making it desirable for compact applications, and the impedance matching can be controlled by the designer without the need for extraneous matching components. IFAs have been used in military applications since 1958, and in modern communications systems are used in a planar version (PIFA) on PCBs. The systems typically operate in frequencies below 1GHz and often meander the radiating element (M-PIFA) to reduce the resonator dimensions.

IFA Design

The specifications for the mechanical dimensions of the IFA severely limited the available space for the antenna. Consequently, difficult tradeoffs from the theoretical design had to be made regarding antenna gain, efficiency, and broadband performance. An IFA design places the feed from the ground plane to the upper arm that runs parallel to the ground plane. The upper arm of the IFA has a length that is roughly a quarter of a wavelength (Figure 1).

2020 12 HFE antenna1

Figure 1 • Construction of basic IFA with shorting stub (left of feed point) to compensate for capacitive effect of open stub resonator.

 

The upper arm is shorted to the ground plane to the left of the feed, which is closer to the shorting pin than to the open end of the upper arm. Antenna theory calls for the ground plane to be at least as wide as the IFA length (L), and the ground plane should be at least lambda/4 in height. If the height of the ground plane is smaller, the bandwidth and efficiency will decrease. The height of the IFA (H), should be a small fraction of a wavelength.

Assuming zero current at the middle of the resonator (quarter wavelength) enables the designer to model the IFA performance by using a single half plane. Zero current paired with a voltage maximum is characteristic for an open-ended quarter wave transformer as shown in the right half of Figure 1.

The open resonator can be seen as a capacitive load in parallel to the nominal impedance. The reflection coefficient is therefore shifted away from the ideal matching point. By using a shorted stub as compensation for the capacitive resonator, the designer can ensure proper matching and optimum efficiency performance of the antenna design.

This type of antenna, being a planar structure, is well-suited for EM analysis using the Cadence® AWR Design Environment® platform, specifically the AWR® AXIEM® EM simulator. The arms of the antenna are modeled as copper traces on an infinite FR4 substrate with finite metal grounding structures on both sides of the board strapped together with vias.

The AWR AXIEM EM simulator was used to analyze and optimize the in-band return loss performance of the antenna. The current distribution of the antenna shown in Figure 2 is comparable to those demonstrated in theoretical background material. The maximum current is at the intersection between the short and open stubs. The minimum current is located at the open end of the resonator and the skin effect is at the edges. The far-field radiation pattern can also be annotated over the 3D view of antenna, as shown to the right in figure 2.

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Figure 2 • EM simulation provide current distribution information annotated over the copper traces defining the antenna structure (left) and the resulting donut-shaped radiation pattern (right).

 

The width and length of the resonator were derived from literature and quickly implemented as a meandered line to fit within the 60 mm width of the overall antenna footprint. The targeted operating band, antenna gain, and matching performance is shown in Table 1.

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Table 1. UL/DL bands, antenna gain, bandwidth, return loss and physical footprint

 

In Figure 3 • the measured return loss of a prototype is shown at the bottom, which included test cables, possible manufacturing deviations in the FR-4 substrate material, and non-ideal free space conditions, showed a 15MHz shift in frequency, which could be compensated for by adjusting the length of the resonator either on the prototype or by using the simulation model.

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Figure 3 • The antenna response can be tuned by adjusting the length of the resonator.

 

The top of Figure 3 compares the simulated E- and H-plane antenna radiation patterns versus measurement data. The maximum antenna gain at mid-band in the xz plane is 1.66dB, well within the target performance range.

While the center frequency can be shifted by tuning the resonator length, it is also possible to use a matching network of surface mount passive components to shift the frequency response of the input match. To demonstrate, the network synthesis option in AWR Microwave Office® circuit design software was used to develop an impedance matching network for the antenna operating at LTE Band 13. With the latest version of the network synthesis wizard, the lumped element components used in the matching network can be real component models selected directly from the AWR vendor library by the synthesis tool.

Lastly, yield analysis was used to verify the LTE Band 13 matching for a variation of relative permittivity in the substrate material between 4.0 and 5.0, as well as likely tolerances of the matching network component values as shown in Figure 4 • The antenna return loss under all circumstances was within the desired 10dB over the full bandwidth, demonstrating that the substrate material should not pose a yield problem for the expected tolerances.

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Figure 4 • Yield analysis used to verify the LTE Band 13 matching performance.

 

Conclusion

The CommScope team successfully realized a planar IFA for LTE Band 14. Theoretical calculations were demonstrated with AWR AXIEM EM simulations and simulation performance was verified with real measurements. In addition, the antenna was modified for LTE Band 13 operation using external matching components.

The capabilities of AWR Microwave Office circuit design software combined with the speed of the AWR AXIEM EM simulator helped the team cut the number of various prototype design spins and delivered a first-time-right solution for the given problems. The software detected the influence of the mechanical tradeoffs and helped the team compensate for those problems during the simulation phase.

About the Author

Johannes Steigert serves as an RF Engineer at CommScope.

February 2021

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The February 2021 Online Edition is now available for viewing and download!

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