The low-frequency improvement with loading soft magnetic ferrite films for multiband antenna applications

Abstract

This work presents an easy way to improve the low-frequency properties in a small-size LTE/WWAN smartphone antenna by just loading a soft magnetic ferrite film on the strong radiation area. The base non-magnetic antenna, which only takes up an 11 mm × 30 mm no-ground space, can achieve hexa-band operation covering 822–914/1710–3244 MHz bands by co-designing a meandering strip and a T-shaped feeding strip. With loading a small-size ferrite film (2 mm × 15 mm) at the strong radiation strip, the frequencies can be shifted to cover the range of 692–1191/1698–3020 MHz, which shows a fully covering of octa-band operation, especially including the best long-term-evolution band, LTE700, without enlarging the size of base antenna. In addition, the radiation characteristics of the magnetic antenna with ferrite film loading show a limited change at upper bands (1710–2690 MHz) but an acceptable level at lower bands (698–960 MHz). Both experimental and simulated results have been taken out and shown the consistent tendency. This optimization method of low-frequency properties by introducing a commercial ferrite film on the antenna has provided a simple and convenient way to solve the frequency deviation in the practical smartphone applications.

Keywords

Multiband antenna ferrite loading frequency modulation LTE700 band

1. Introduction

In the modern handset communication systems, such as WWAN (wireless wide area network), LTE (long-term evolution) and GPS (global positioning system), the antenna has been playing a critical role for information transmission. The needs for antenna have been continuously developed with small size, low cost, light weight, easy fabrication, and wide-band covering [1]. To fulfill these requirements, different promising antennas for handset devices have been proposed, such as the inverted-L antennas [2], the planar inverted-F antennas (PIFAs) [3–5], the frequency reconfigurable antennas [6,7] and the loop antennas [8–12]. However, in the real applications, the operating frequencies will not only be determined by the antenna structure but also the fabrication technique, which will bring the frequency deviation between the proposed and fabricated antennas. In addition, the antenna size is also limited to the inverse relation between the resonant frequency and radiation length. Therefore, it should be an emergent demand for finding a practicable method to meet the antenna requirements for miniaturization, easy modulation and low cost.

Microwave ferrite is a magnetic dielectric and can allow the penetration of electromagnetic wave, thereby permitting an interaction between the wave and magnetization within the ferrite. This interaction can be used in the microwave devices to extend the knowledge to meet demands for miniaturization, broader relative bandwidths, higher operating frequencies, and reduced costs [13]. The microwave ferrite materials have been introduced on the design of antennas due to their high permeability, high permittivity and low loss at microwave frequencies, showing significant effects on the performances of microwave antennas. In the references [14–17], the antenna substrates are composed of ferrite and polymer materials as to miniaturize the antenna by using their high permeability. However, these materials or composites are limited to be <600 MHz because of their excessive magnetic loss tangents. In the references [18–21], self-biased magnetic thin films and polymer composite are combined as antenna substrates, which can achieve higher operation frequencies with μ_r > 1 and f > 1 GHz. Moreover, the spinel ferrite thin film instead of bulk ferrite using in the embedded antenna can bring a broad bandwidth with an acceptable radiation efficiency and gain by shortening the wave path [22].

In this work, we have introduced a soft magnetic ferrite film to optimize the operating frequencies on the design of LTE/WWAN smartphone antenna. With loading a small piece of microwave ferrite film on the strong radiation area of the base antenna, the operation frequencies (VSWR 3:1, −6 dB impedance) of the antenna can be shifted from 822–914/1710–3244 MHz to 692–1191/1698–3020 MHz. The improved frequencies can cover two more bands at low-frequency operations, LTE700 (698–787 MHz), and GSM900 (880–960 MHz), showing a fully octa-band covering (LTE700, GSM850, GSM900, DCS1800, PCS1900, UMTS2100, LTE2300, LTE2500) without changing the size of base antenna. Meanwhile, the radiation characteristics of the magnetic antenna with ferrite film loading show a limited change at upper bands (1710–2690 MHz) and acceptable degradation at lower bands (698–960 MHz). This easy frequency optimization in the design of multi-band LTE/WWAN smartphone antenna with loading microwave ferrite film may show great potential for applications in mobile wireless communication systems.

2. The mechanism of using ferrite materials in the antenna design

The geometry of the base non-magnetic antenna used for mobile-phone applications is shown in Fig. 1(a). The 0.8 mm-thick FR4 substrate with relative permittivity of 4.4 and dielectric loss tangent of 0.02 is served as the system circuit board, which has a standard dimension of 115 mm × 60 mm. The antenna area occupies a size of 11 mm × 30 mm on the left corner above the system circuit board. The ground plane is printed on the back side of the FR4 substrate, which includes a no-ground plane area (30 mm × 11 mm) corresponding to the opposite side of antenna area. Figure 1(b) gives the detailed dimensions of proposed antenna, which is positioned on the non-ground portion. The base antenna is mainly composed of a T-shaped feeding strip, shorted strip, and parasitic strip. Point A of the T-shaped (with a rectangular slot) strip is the antenna’s feeding point, which is connected to 50 $\Omega$ mini coaxial feed line. Point B of the parasitic meandered branch is directly connected to the ground plane through a via-hole in the system circuit board. The operating bands are determined by the radiation coupling of T-shaped feeding strip and the dual-branch parasitic shorting strip.

Fig. 1.

Proposed antenna configuration (unit: mm). (a) Geometry of the base non-magnetic antenna design, (b) detailed dimensions of the metal pattern in the antenna area, (c) simulated surface current distractions at 800 MHz in the base non-magnetic antenna, (d) compared optimum S-parameters for the antenna without and with ferrite film in the HFSS simulation.

Figure 1(c) shows the simulated surface current distributions at 800 MHz in the base non-magnetic antenna. It can be clearly seen that the strong current is on the parasitic meandered branch, which has been chosen as the ferrite action area to permit a strong interaction between the wave and magnetization within the ferrite film. The covering extent of ferrite film and datum reference are shown in Fig. 1(b). Figure 1(d) has taken out the comparison of simulated S-parameters for the non-magnetic antenna and the optimized magnetic antenna with ferrite film loading. Without loading the ferrite film in the base antenna, the resonant frequency covers the range 857–1127 MHz and 1658–2944 MHz as shown in Fig. 1(d), enabling hexa-band operations by covering GSM900/DCS1800/PCS1900/UMTS2100/LTE2300/LTE2500 bands. However, with loading the ferrite film on the strong radiation area as shown in Fig. 1(b), which is determined by the surface current distributions, the optimum operating frequency can be clearly improved at low band and be extended to cover the range of 677–1142 MHz and 1664–2913 MHz, which can cover LTE700 and GSM850 bands without enlarging the size of antenna. The coupled feeding method and the loading microwave material technology adopted in the antenna design can effectively improve the impedance matching and increase the bandwidth, especially at the LTE700 band of 698–787 MHz, which is a critical band for long term evolution and future 5G communication. Finally, to best understand the effect of ferrite film size on the operating frequencies in the next part, we defined three characterized frequencies in Fig. 1(d), the crossing points of −6 dB return loss for mobile phone application and resonant frequency at low band as f ₁ and f ₂, the center resonant frequency at low band as f _c, and the bandwidth mentioned below as f ₂ − f ₁.

The main mechanisms of using ferrite materials in the microwave devices can be explained in terms of one or more of the magnetic effects, such as Faraday rotation, ferromagnetic resonance (FMR), field displacement, nonlinear effects, and spin waves, et al. In the patch antenna design, the resonant frequency can be expressed by the following equation [23,24]: $\begin{eqnarray}\displaystyle f_{r}={\displaystyle \frac{c}{2L\sqrt{{\mu}_{r}{\varepsilon}_{r}}}} & & \displaystyle\end{eqnarray}$ (1) where, μ_r is the relative permeability, ϵ_r is the relative permittivity of the substrate, c is the speed of light in free space, and L is the effective length of the rectangular patch. Microwave ferrite materials show relatively high permeability and high permittivity (ϵ_r ∼ 10–15), as well as low loss at RF/microwave frequencies, and can miniaturise the antenna by using moderate values of permeability and permittivity. However, the excessive magnetic loss tangents associated with various loss mechanisms will confine the operating frequencies of ferrite materials to be less than 600 MHz, in which FMR is the major loss mechanism. Therefore, the modulation of radiation frequency in the antenna will be limited by the FMR frequency of ferrite films, leading to the main frequency modulation around gigahertz frequency [25].

The bandwidth (VSWR = 2) for the antenna with magneto-dielectric layer can be given by [26]: $\begin{eqnarray}\displaystyle BW={\displaystyle \frac{96\sqrt{{\mu}_{r}/{\varepsilon}_{r}}t/{\lambda}_{0}}{\sqrt{2}\left[4+17\sqrt{{\mu}_{r}{\varepsilon}_{r}}\right]}} & & \displaystyle\end{eqnarray}$ (2) where t is the thickness of the substrate and 𝜆₀ is the wavelength at the resonant frequency. The loading of microwave ferrite film in the antenna will lead to an increased permeability, subsequently to reduce the radiation frequency and enhance the bandwidth [24].

The proposed antenna was also fabricated and tested to validate the design strategy, as shown in Fig. 2. The S-parameters, radiation patterns, efficiency and gain of the antenna were compared when the commercial NiZn-ferrite films (PHF15010, Shenzhen PH Functional Materials Co. Ltd.) were out of or on the critical strip, and the corresponding effects were investigated combined with the simulation results.

Fig. 2.

Photos of the fabricated antenna. (a) Base non-magnetic antenna, (b) magnetic antenna with ferrite film loading.

3. Results and discussion

To fully comprehend how the ferrite’s sizes affect the antenna’s performance and show how to optimize antenna’s parameters to fit in the strict design specification, detailed ferrite size dependent S-parameters were investigated both in simulation and experimental process.

Figure 3 shows the simulated S-parameters with varying the size of ferrite films, and the loading position of ferrite films is shown in Fig. 1(c). As can be observed in Fig. 3(a), with fixing the width of ferrite films as 2 mm and increasing the length of ferrite films from L = 0 mm to L = 16 mm, the reflection coefficient can reach about as high as −48 dB, and the bandwidth at low operating frequencies can be clearly broadened while there is no obvious changing in the bandwidth at high frequencies. The dependence of low-frequency resonant point f _c, low-band crossing point f ₁ and low-frequency bandwidth with the length L of ferrite films is shown in Fig. 3(b). As the length L of ferrite film increases, the f _c would shift to the lower frequency point and the bandwidth shows an obvious increase, leading to large improvement in the low-frequency operating bands. As the L increases to a critical point of 11 mm, the low-band crossing point f ₁ decreases to below 700 MHz, meaning that the LTE700 band would be covered with loading the ferrite film. To best cover the LTE700 band, we have chosen the L = 13 mm as the optimal point of the length of ferrite films, which shows the f ₁ = 675 MHz, f _c = 860 MHz, and low-frequency bandwidth of 460 MHz. Then, the length of ferrite film was fixed as 13 mm, and the width dependent S-parameters has been investigated, as shown in Fig. 3(c). The most direct improvement is also observed at the low frequencies, and the detailed varying of frequency parameters can be observed in Fig. 3(d). With slightly enlarge the width W of ferrite films, the low-frequency resonant point f _c and low-frequency bandwidth will be enhanced, and low-band crossing point f ₁ will be lower than the LTE700 point at a critical W value. Therefore, with the optimum loading of ferrite films in the HFSS simulation, a hexa-band base antenna can be enhanced to cover octa-band operations, especially to cover the critical band of LTE700 for long term evolution without enlarging the whole size of antenna.

Fig. 3.

Simulated S-parameters of the proposed antenna as a function of the size of loaded ferrite film. (a) Dependence of S-parameters with the length of loaded ferrite film, (b) dependence of f _c, f ₁ and bandwidth with the length of loaded ferrite film, (c) dependence of S-parameters with the width of loaded ferrite film, (d) dependence of f _c, f ₁ and bandwidth with the width of loaded ferrite film.

To confirm the effect of loading ferrite film on the proposed antenna, the commercial NiZn-ferrite films with μ_r of 150 have been chosen in the experiments. Figure 4 displays the measured S-parameters with varying the size of ferrite films, which is the verification of simulated design. In the 500–3000 MHz frequency range, the non-magnetic antenna can generate multiple resonant modes at lower and upper bands for hexa-band LTE/WWAN operation covering 822–914/1710–3244 MHz bands. As shown in Figs 4(a) and 4(b), when the ferrite film is loaded on the antenna, the lowest resonance frequency f _c will shift from 862 MHz down to 757 MHz and low-band crossing point f ₁ will shift from 822 MHz down to 684 MHz by changing the L from 0 mm to 17 mm. The −6 dB reflection coefficient bandwidth is also enhanced from 356 MHz to 519 MHz. These obvious improvements all appeared in the low-frequency band (<1500 MHz), and the covering of LTE700 band without enlarging the antenna size compared to the non-magnetic antenna was of special importance in the real antenna practice. Comparably, at the high-frequency band (1710–2690 MHz), the loading of ferrite film leads to no obvious change on the operating frequencies. The expansion of low-frequency bandwidth and the low-frequency shift of S-parameters can also be observed in the antenna with varying the width W of ferrite film from 0 mm to 3 mm as shown in Figs 4(c) and 4(d). The detailed length and width dependent return loss curves at low operating bands were inserted in Figs 4(a) and 4(c), and visible changes of crossing points that affect the operating frequencies can be observed. In Figs 4(b) and 4(d), by carefully adjusting the sizes of ferrite films, the optimum operating frequency covering can reach to 692–1191/1698–3020 MHz at L = 15 mm and W = 2 mm. Therefore, a fully covering of octa-band operation, including LTE700, GSM850, GSM900, DCS1800, PCS1900, UMTS2100, LTE2300, and LTE2500 bands, can be reached by just loading the microwave ferrite film on the antenna without changing its size. In addition, the simulated results slightly differ from the measured results, which may be due to the imperfect bonding between antenna and ferrite film, the measurement tolerances and the feeding cable effects in the experiments.

Fig. 4.

Measured S-parameters of the proposed antenna as a function of the size of loaded ferrite film. (a) Dependence of S-parameters with the length of loaded ferrite film, and low-band frequency varying is enlarged in the inserted figure, (b) dependence of f _c, f ₁ and bandwidth with the length of loaded ferrite film, (c) dependence of S-parameters with the width of loaded ferrite film, and low-band frequency varying is enlarged in the inserted figure, (d) dependence of f _c, f ₁ and bandwidth with the width of loaded ferrite film.

The radiation characteristics of the proposed antenna are measured in the SATIMO anechoic chamber. The measured radiation patterns in the xy-, xz-, and yz-planes for the non-magnetic antenna and the antenna with optimal microwave ferrite film at 900, 1920, and 2560 MHz are shown in Fig. 5. The radiation patterns keep approximately the same shape by loading the ferrite film on the antenna, showing weak effects on the radiation characteristics. Moreover, a good omnidirectional radiation can be obtained in all planes, which indicates that the radiation characteristic can keep stable at all antenna’s communication bands. This good radiation characteristic is advantageous for practical smartphone applications.

Fig. 5.

Measured 2-D radiation patterns for the proposed antenna without ferrite and with ferrite film at (a) 900 MHz, (b) 1920 MHz, (c) 2560 MHz.

The measured total efficiency and gain of the fabricated antenna by unloading and loading ferrite film are presented in Fig. 6. As shown in Fig. 6(a), at the upper bands of DCS1800/PCS1900/UMTS2100/LTE2300/2500 (1710–2690 MHz), the total radiation efficiency for the magnetic antenna was not much lower than that of the base antenna, which can reach up to be about 61%–82%. However, at the lower bands of LTE700/GSM850/900 (698–960 MHz), the corresponding total radiation efficiencies for the antenna show an obvious decrease from 80% to 65% at the center frequency by loading a ferrite film, but a visible improvement from 3.7% to 18% at LTE700 band (<791 MHz). This result is consistent with previous report for that the spinel ferrite film will bring a deterioration on radiation efficiency but can be controlled at an acceptable level with thickness decreasing [22]. As can be observed in Fig. 6(b), the antenna gains show the same tendency with the total efficiencies as loading the ferrite film. The minimum and maximum gains for the magnetic antenna are about −5 and 5.9 dB at 698 and 2350 MHz, respectively. The degradation on the total efficiency and gain by loading the ferrite film on the antenna mainly appear at low-frequency bands (698–960 MHz), and a limited change can be observed at the high-frequency bands (1710–2690 MHz), which coincides with the interaction frequency range by FMR limitation of the ferrite film [26,27]. As a result, this simple design by loading the commercial ferrite film on the fabricated antenna may enhance the application of practical frequency adjustment in the multiband antenna.

Fig. 6.

Measured (a) radiation efficiency and (b) gain of the proposed antenna without ferrite and with ferrite film.

4. Conclusion

A multi-resonant LTE/WWAN smartphone antenna with commercial NiZn-ferrite film loading has been successfully designed, fabricated and tested in this work. The base non-magnetic antenna with hexa-band operation, including GSM850, DCS1800, PCS1900, UMTS2100, LTE2300, and LTE2500 bands, can be achieved by the coupling of meandering strip and a T-shaped feeding strip in an 11 × 30 mm² no-ground space. When loading the ferrite film on the base antenna, the operation frequencies can be shifted from 822–914 MHz/1710–3244 MHz to 692–1191 MHz/1698–3020 MHz, which provides a fully coverage of octa-band operation, especially leading to the covering of the LTE700 band, the best band for long term evolution, without enlarging the size of antenna. The simulation process and experimental results have both been taken out to verify the optimized behaviours with loading the ferrite film on the strong radiation area of the antenna. Acceptable radiation characteristics including the total efficiency and gain for the antenna at the low-frequency band (698–960 MHz) as well as a limited change at high-frequency band (1710–2690 MHz) have been attained. This paper offers a convenient way for the frequency optimization in real multiband antenna application, which have both practical and theoretical importance for developing future mobile wireless communication systems.

Footnotes

Acknowledgements

This work was supported by the Funds of the National Natural Science Foundation of China under grant nos. 11902316, 11972333, and 51902300, the Natural Science Foundation of Zhejiang Province under grant nos. LZ19A020001 and LQ19F010005, the Open Foundation of Institute of Flexible Electronics Technology of THU under grant no. 2019KF1001, and the Open Project Funding of the Key Laboratory of Electromagnetic Wave Information Technology and Metrology of Zhejiang Province under grant no. 2020KF0002. The Antenna’s efficiency, gain, and pattern were measured in Hangzhou RFID Center-CAS. The authors would like to express their sincere appreciation to these supports.

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