A Compact Broadband Planar Inverted-F Antenna with Dual-Resonant Modes

Abstract

In this paper, a compact broadband planar inverted-F antenna (PIFA) with dual-resonant modes (TM_1/2,2 and TM_3/2,0) is proposed for 5G applications. By loading a pair of horizontal slots on the patch, the TM_3/2,0 mode can be shifted downward and combined with the TM_1/2,2 mode, leading to a dual-mode operation. Meanwhile, another pair of slots, which are orthogonal to the previous pair, is introduced to improve the impedance matching of the antenna. Moreover, the substrate with a high dielectric constant is used in this design to achieve a compact antenna size. In order to verify the principle and the design method, an antenna prototype was fabricated and measured. Experimental results show that the PIFA has a good performance with a −10 dB impedance bandwidth of 14.5% (3.17–3.67 GHz), a peak gain of 6.23 dBi, a low cross-polarization level of −17.3 dB, and a compact size of 0.45 × 0.26 × 0.03 λ₀₁³ (λ₀₁ is the free-space wavelength at 3.5 GHz). The antenna is predicted to be suitable for 5G terminal applications.

1. Introduction

In this day and age, microstrip patch antennas (MPAs) have been widely used in modern mobile and wireless communication systems due to their features of being low cost, light weight, and so on [1,2]. Compared to traditional MPA, the planar inverted-F antenna (PIFA) usually has a more compact size [3,4], and therefore has become very attractive. Unfortunately, PIFAs are also plagued by narrow impedance bandwidths similar to that of MPAs due to their small sizes and low antenna profiles [5].

Various techniques have been proposed in the past to expand the bandwidths of MPAs or PIFAs. In [6,7,8], the parasitic elements were stacked above the main radiating patch, and additional modes could be introduced to broaden the bandwidth. However, this method increases the antenna profile, thus destroying the intrinsic low-profile property of a planar MPA. In [9,10,11,12], the metasurface antennas were proposed to achieve wideband MPA designs, in which the higher-order modes of the metasurface can be manipulated for bandwidth enhancement. However, the planar sizes of metasurface antennas are usually too large, limiting its applications. In [13,14,15,16,17,18], the feeding schemes were modified to widen the antenna bandwidths. For example, in [16] and [17], by employing a substrate integrated waveguide (SIW) resonator to feed the patch through aperture coupling, the resonant order of the antenna can be increased, and the bandwidth thus can be widened. Nevertheless, these feeding structures will undoubtedly increase the complexity of antenna design and lead to the enlargement of antenna size. In [19,20], the technology of the multi-resonator, such as the slot-patch [19] and conjoint patch-pair [20], is proposed to combine multiple modes for broad bandwidth. However, the bandwidth improvement is still at the cost of enlarging antenna planar sizes.

In addition to the above techniques, the methods of loading shorting pins [21] and etching slots on the radiating patches [22,23,24,25,26,27,28,29,30,31] are also proposed. By using these methods, resonant modes of the radiation patch can be redistributed, and multiple resonant modes can be merged together to achieve a wide bandwidth. In [21], by inserting several shorting pins, three resonant modes of TM_1/2,0, zeroth-order, and TM_1,0 are merged together to obtain a wide bandwidth. In [22], a PIFA is proposed by etching a series of slots, resulting in two resonances of TM_0,1 and TM_1,1 modes. In [23], by simultaneously loading slots and adding shorting pins, the TM_3,1 and TM_1,2 modes are combined to generate a wide bandwidth of 13.4%. In [24], a triple-mode (TM_0,1/2, TM_2,1, and TM_0,1) patch antenna is achieved by optimizing the position of the shorting pins and etching two pairs of slots. In [25,26], with the use of a loading slot and a pair of shorting pins, the resonant frequencies of TM_0,1/2 and TM_0,3/2 modes are reallocated and combined together to achieve an enhanced bandwidth. In [27], by loading a pair of shorting pins, the antenna can achieve a widened bandwidth of 15.3% under dual-mode resonances of TM_0,1/2 and TM_2,1/2 modes. In [28], a novel wideband U-slotted MPA is presented. By loading several shorting vias, the TM_1,2 mode and TM_2,1 mode can be combined together. These methods are simple and clever as they usually keep compact antenna structures. However, most of the designs suffer from deteriorated radiation patterns, such as a tilted mainlobe, a high cross polarization level (>−10dB), the appearance of sidelobe, and so on, partially due to the radiation characteristics of the higher-order modes, and partially because of the loading effect of the shorting pins and slots.

Therefore, the patch/PIFA antenna for terminal usage still needs to be improved in terms of both size and radiation performance. In this paper, a compact and wideband PIFA with good broadside radiation patterns is proposed to solve this issue. Here, it is noted that “good” means that the principal radiation direction is broadside, and the radiation pattern is symmetrical with a low cross-polarization level of less than −15dB and no sidelobe. The antenna operates under the resonances of the TM_1/2,2 and TM_3/2,0 modes, and can cover the 5G n78 band (3.3−3.6 GHz). Specifically, in this design, a pair of horizontal slots are etched at the edge of the patch, which not only reduces the resonant frequency of the TM_3/2,0 mode, but also improves the radiation patterns of both the target modes. With the slots loaded, the current path of the TM_3/2,0 mode is increased, thereby its resonant frequency can be shifted downward to combine with the TM_1/2,2 mode, leading to a widened bandwidth. Meanwhile, the slots change the radiation current distribution of both the two modes, improving their radiation patterns. On this basis, another pair of vertical slots is loaded onto the radiation patch, which is beneficial for achieving good impedance matching. In addition, a single-layer substrate with high permittivity is used to obtain a simple and compact antenna structure. The proposed antenna features a wide bandwidth, a compact size, and stable broadside radiation patterns, making it suitable for 5G terminals.

2. Geometry

Figure 1 shows the configuration of the proposed PIFA, which is implemented on an RO6006 substrate with a permittivity of ɛ_r = 6.15 and a height of Hz = 2.54 mm. It consists of a rectangular patch (W × L), a row of shorting pins, and a ground plane (G × G). The ground plane is printed on the bottom surface of the substrate, while the patch is printed on its top surface. There is a row of shorting pins near the top edge of the patch, which is used to short the patch to the ground plane. A pair of horizontal slots (Slots 1) is etched on the rectangular patch to adjust the resonant frequencies and improve the radiation patterns of the two target modes of TM_3/2,0 and TM_1/2,2. Slots 2, which are orthogonal to Slots 1, are introduced onto the patch to achieve good impedance matching. A 50 Ω coaxial probe is used to feed the antenna. The outer conductor of the probe is connected to the ground, and the inner conductor is connected to the patch.

All the antenna parameters are listed in

Table 1. The detailed dimensions of the proposed antenna.

Parameters	Values (mm)	Parameters	Values (mm)
G	42	Ws1	0.4
Hz	2.54	D1	6.5
L	39	D2	11.5
Ls	4.5	D3	7.25
W	22.2	Cp	19.25
W1	0.5	R	0.5
Ws	11.5

, and the following simulation results are obtained with HFSS 18.9 [29].

3. Principle and Analysis

In this design, our main target is to obtain a wide impedance bandwidth with good radiation patterns within the whole band. Figure 2 shows the design process of the proposed antenna. It can be seen that there are two main design steps, which are the loading of Slots 1 (from Antenna I to Antenna II) and the loading of Slots 2 (from Antenna II to Antenna III), respectively.

3.1. Loading of Slots 1

3.1.1. Closing the Resonant Frequencies of the Two Modes

Figure 3 shows the simulated reflection coefficients (|S₁₁|) of the three reference antennas, where we can learn that with Slots 1 loaded, the resonant frequency of the TM_3/2,0 mode can be greatly shifted down to be very close to that of the TM_1/2,2 mode.

Table 2. Differences between the traditional PIFA (Antenna I) and the slots-loaded PIFA (Antenna II).

Antenna I	Antenna II
Structure
Current distribution of the TM1/2,2 mode
Edge for main radiation	Slots for main radiation
Radiation patterns of the TM1/2,2 mode
Peak gain: 3.03dBi @ 270°	7.2dBi @ 0°
Current distribution of the TM3/2,0 mode
Radiation patterns of the TM3/2,0 mode
Peak gain: 7.46dBi @ 330°	6.36dBi @ 0°

The subheadings in the table have been highlighted to clearly distinguish the figures.

shows the current distributions, and the corresponding radiation patterns of the two target modes. Here, it should be noted that the current intensity increases from 2.2 × 10⁻³(A/m) to the maximum, as can be seen from the color maps. In addition, each radiation pattern is normalized by subtracting its peak gain from the gain in each direction. For quantitative comparison, the peak gains are marked separately in the table. From the current distributions, it can be found that the currents of the TM_1/2,2 mode are mostly parallel to the y-axis, while that of the TM_3/2,0 mode are basically parallel to the x-axis. With the introduction of Slots 1, the currents of the TM_3/2,0 mode are cut off, resulting in an increment in the current path, thereby leading to the resonant frequency reduction from 4.16 GHz to 3.6 GHz. On the other hand, since Slots 1 are parallel to most currents of the TM_1/2,2 mode, there is no significant effect on the frequency of the TM_1/2,2 mode, and the resonance of the TM_1/2,2 mode remains stable at 3.2 GHz.

3.1.2. Improving the Radiation Pattern

Next, the effects of Slots 1 on the radiation patterns of the two modes are also studied. As can be seen from Table 2, when Slots 1 are not loaded, the TM_1/2,2 mode of a conventional PIFA produces a non-broadside radiation [30], while the E-plane pattern of the TM_3/2,0 mode has a −30-degree tilt. Both of the radiation patterns are undesirable. When Slots 1 are introduced, there is a change in the radiation current distributions of both the two modes, thereby improving their radiation patterns. Specifically, the radiation direction of the TM_1/2,2 mode is transformed from non-broadside to broadside. The radiation pattern is symmetrical with a low cross-polarization level of less than −20 dB (within the 3-dB beamwidth) and a peak gain of 7.2 dBi at 0° achieved. On the other hand, the main beam direction of the TM_3/2,0 mode is adjusted from −30° to 0°. The radiation pattern also becomes symmetrical, and the cross-polarization levels are better than −18 dB while the peak gain is 6.36 dBi at 0°. By observing the surface currents of the two modes, the working mechanism is analyzed as follows:

For the TM_1/2,2 mode, the out-of-phase distribution of surface currents along the x-direction leads to the high cross polarization in the H-plane and the radiation null in the broadside direction. Slots 1, which are etched near the shorting wall, are employed to cut the y-direction current in this area. According to the antenna theory, the slots which cut the current will become the radiation source [31]. Thus, with this cutting, Slots 1, instead of the original radiation edge, become the main radiation source of the TM_1/2,2 mode, producing y-direction electric fields. Meanwhile, near the patch’s edge, the effective components of the currents for radiation have also been transformed to be in the y-direction. Thus, a normal broadside radiation pattern with a low H-plane cross-polarization is obtained.

For the TM_3/2,0 mode, the partial out-of-phase currents along the y-direction cause the incline of the E-plane radiation pattern. Slots 1 just cut near the area where the y+ and y- currents meet together. Due to the adverse effect of the slots [32], the out-of-phase currents in the top region realize a 180° flip, and turn to be in phase with the currents underneath. Therefore, the incline of the E-plane pattern can be improved, and a good symmetrical broadside radiation pattern can be achieved.

3.2. Loading of Slots 2

Although these two resonant modes can be combined by introducing Slots 1, the simulated |S₁₁| is always larger than −10 dB, mainly because of the relatively large resistance and reactance values within the frequency range, as shown in Figure 4a. As is known, |S₁₁| is an important index to measure antenna performance. Generally, the port reflection of an antenna is considered acceptable when |S₁₁| is less than −10 dB. To solve this issue, Slots 2 are then loaded onto the radiating patch. This can improve the input impedance of the antenna [27,33], as shown in Figure 4b. Comparing the two figures, it is learned that, with Slots 2 loaded, the resistance within the frequency range between the two modes is smoothed from [22, 116] Ω to [31, 80] Ω, while the reactance is reduced from [0, 62] Ω to [0, 28] Ω. Therefore, the impedance matching of the antenna becomes much better.

3.3. Parametric Studies

To verify the above design principles, the effect of Slots 1 needs to be analyzed in Antenna III. Figure 5a shows the simulated frequencies of TM_3/2,0 and TM_1/2,2 modes with different slot lengths (Cp). It can be observed that as Cp increases, the higher resonant mode TM_3/2,0 significantly shifts down, while the lower resonant mode TM_1/2,2 stays almost fixed, which is consistent with the previous analysis that the frequency of the TM_3/2,0 mode (f2) can be heavily affected by the loading of Slots 1, while that of the TM_1/2,2 mode (f1) is hardly affected. Once an appropriate slot length (Cp = 19.25 mm) is selected, the TM_3/2,2 mode can be combined with the TM_1/2,0 mode to realize a dual-mode operation. Figure 5b clearly depicts the merging process of the two modes from the perspective of the reflection coefficient.

The effect of Slots 2, including the position (Ls) and the size (Ws), is also studied in Antenna III. As shown in Figure 6a,b, it can be found that both the position and the size of Slots 2 have significant effects on the impedance matching. By properly choosing Ls and Ws, for example, Ls from 0 to 4.5 mm and Ws from 6.5 to 11.5 mm, the simulated |S₁₁| gradually reduces to smaller than −10 dB, so that the impedance matching can be progressively improved. With the final parameters (Ls = 4.5 mm, Ws = 11.5 mm) chosen, the reflection coefficient is below −10 dB over a desired wide frequency under dual-mode resonance.

3.4. Design Guideline

Based on the above discussion, a design guideline for extending the proposed antenna for other frequency bands is summarized as follows:

1.

Determining the initial values of the antenna structure. The antenna is supposed to operate under the resonances of TM_1/2,2 and TM_3/2,0 modes. For a conventional PIFA (L/W ≈ 2), the initial size can be estimated by referring to [27].

2.

Loading Slots 1 for wide bandwidth. According to the current distributions in Table 2, the position to etch Slots 1 can be chosen as D₁ ≈ W/3. As shown in Figure 5a,b, tuning the length (cp) of Slots 1 to make TM_3/2,0 and TM_1/2,2 modes merge together can obtain a wide operating band.

3.

Loading Slots 2 to improve impedance matching. Adjusting the length (Ws) and position (Ls) of Slots 2 can lead to better impedance matching, as shown in Figure 6a,b.

4.

Optimizingthe final structure. The parameters of slots (cp, Ws, Ls, D₁, and D3) and feed structure (D₂) can be further adjusted to obtain an optimized performance.

4. Experiment Result

To confirm the predicted antenna performance, an antenna prototype was fabricated and measured. Figure 7 shows the photograph of the fabricated antenna.

The measured and simulated S-paraments and gains of the PIFA are shown in Figure 8. The measured impedance bandwidth (|S₁₁| < −10 dB) is 14.5%, which is from 3.17 GHz to 3.67 GHz, showing good agreement with the simulated result. The measured gain varies from 5 to 6.23 dBi over the operating band, which is slightly lower than the simulated result varying from 5.2 to 6.36 dBi. The discrepancies between the measured and simulated results are mainly caused by the fabrication tolerance, the loss from the SMA connector and its imperfect manual assembly, as well as the possible effects of the measurement setup close to the antenna under test conditions. The measured total efficiency of the PIFA is higher than 83% within the operating frequency band.

The measured and simulated radiation patterns in the E-plane and H-plane of the proposed antenna are illustrated in Figure 9. The measured results show that the antenna has stable radiation patterns with 3-dB beamwidths of 90 ± 2° in the E-plane and 88 ± 2° in the H-plane. Meanwhile, the H-plane radiation patterns have low cross-polarizations of less than −17.3 dB within the 3-dB beamwidths. There is good agreement between the measured and simulated results. The above results all prove that the antenna can have a good radiation performance.

Table 3. Performance comparison with the previous antennas.

Refs.	f0 (GHz)	BW (%)	Peak Gain (dBi)	Efficiency (%)	Radiation Pattern	Resonant Modes	Size (λ03)
[17]	3.50	9.14	7.30	80	Good	TM1,1, TM2,1	0.74 × 0.74 × 0.04
[18]	1.90	10.0	11	85	Good	TM1,0, TM1,2	1.71 × 0.51 × 0.04
[25]	5.5	18.0	5.9	74.1	Asymmetry 35°-tilt	TM0,1/2, TM0,3/2	0.55 × 0.37 × 0.04
[26]	4.10	11.8	3.5	85	Asymmetry 30°-tilt	TM0,1/2, TM0,3/2	0.41 × 0.34 × 0.04
[27]	2.49	15.3	5	-	High cross-pol (over −5dB)	TM0,1/2, TM2,1/2	1.00 × 0.27 × 0.04
[34]	5.5	6.1	10.7	-	Good	TM3,0, TM5,0	1.05 × 0.86 × 0.015
This work	3.45	14.5	6.23	83	Good	TM1/2,2, TM3/2,0	0.45 × 0.26 × 0.03

λ₀ means the free-space wavelength at the center frequency f₀.

shows the performance comparison between the proposed antenna and the previous reported PIFA/patch antennas. It can be seen that the proposed antenna operates with different resonant modes. In this context, the antenna combines the merits of wide bandwidth and compact size. It also demonstrates good radiation characteristics. It is worth noting that although the antennas in [25,27] have slightly wider impedance bandwidths than the proposed values, their overall volumes are much larger, and the radiation patterns have also been deteriorated to some extent. In all, the proposed antenna shows a better overall performance when a number of factors of bandwidth, size, and radiation patterns are comprehensively considered.

5. Conclusions

This paper proposes a compact and wideband PIFA with dual-resonant modes of TM_1/2,2 and TM_3/2,0. Two types of slots are introduced to improve the antenna performance. First, by etching a pair of horizontal slots for perturbating the current distribution of TM_1/2,2 and TM_3/2,0, the dual-mode operation is achieved, and the radiation pattern can be improved as well. Then, another pair of vertical slots is introduced to improve the impedance matching. The antenna was analyzed with parametric studies conducted to verify the operating principle. It was also fabricated and measured for demonstration. The antenna prototype achieves an impedance bandwidth of 14.5% (3.17–3.67 GHz) and a peak gain of 6.23 dBi, with good broadside radiation performance over the whole band. The antenna has a very compact size of 0.45 × 0.26 × 0.03 λ₀₁³. It is therefore considered as a suitable candidate for future mobile terminal applications. It is also worth pointing out that stimulating and utilizing more modes, such as three to four modes in a PIFA to cover both n78 and n79 bands in 5G, could be a meaningful future study.