Keywords

1 Introduction

The challenges for modern portable devices are getting higher as most of devices are going for compact and slimmer design. Most of cellular communication system are using microstrip and printed antenna that is flat in nature of appearance. Planar Inverted-F antenna (PIFA) is widely used due to its flat nature and PIFA capable to cover several wireless communications services such as WiMax, WLAN, Long Term Evolutions (LTE), Bluetooth and many more [1–5].

PIFA has been introduced in 1987 by Taga for GSM Band Applications [6, 7]. PIFA was known for its inverted F-shape antenna with a shorting arm to its ground plane [2, 3, 8]. PIFA is an improvement from monopole antenna where the top radiating arm is been folded to reduce its size which forms F-shape [8]. In late 2009, authors in [9] has proposed Coplanar Inverted-F Antenna (CIFA), where the radiator and the ground plane are on the same planar substrate [9, 10, 5]. Figure 1 shows fundamental antenna of PIFA design. In this antenna, the folded radiator creates capacitive loading with the ground edge. Thus, an inductive shorting pin to ground is introduced to compensate the imbalance reactance.

Fig. 1
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PIFA configuration. Basic design (left) and antenna cross-section (right)

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On the other hand, compared to monopole, rod and helix antenna, PIFA has an advantage of easy integration with most of handheld devices [2]. However, compact PIFA antenna that covers lower WWAN band such as LTE700M are difficult to design due to its large radiator and ground size. As the resonant frequency goes lower, the overall antenna dimension becomes larger [8], and it will be difficult to integrate it into portable devices such as smart phones.

The limitation in PIFA is due to its narrow bandwidth achievement at lower band [1–4, 6, 8] and its overall dimensions. In literatures, several techniques have been studied in order to enhance the impedance bandwidth; one of the technique that are used is ground slot near the main radiator in order to improve the operating bandwidth [1–4, 9–13]. Other bandwidth enhancement techniques such as addition of stairs near the main radiator have been explained in [14–16]. Addition of stair creates longer electrical current path, hence creates better impedance bandwidth. Furthermore, fractal and spiral radiator techniques have also been used to achieve compact and multiband features as reported in [9, 17].

In this paper, two antennas of new PIFA designs that mainly design for LTE700M with additional capabilities to support others wireless services such as GSM900, WLAN2.5G and WiMAX3.5G are proposed. The antenna introduces a stair-shaped PIFA radiator design to increase the bandwidth. The antenna provides dual band operations with nearly omnidirectional radiation characteristics and good impedance matching.

2 LTE700M PIFA Antenna Designs

PIFA is created at quarter wavelength of the lowest desired frequency, in this case, 690 MHz. The radiator is folded into three stages in order to reduce its size, which named as Arm1, Arm2 and Arm3. Figure 2 shows the initial design of PIFA. Folding main radiator into several arms creates capacitive loading which shift the resonant frequency. A shorting arm is introduced to compensate the capacitive loading that is created after folding the main radiator parallel to the ground at the bottom layer. The antenna is created on an FR4 with dielectric constant ε = 4.3 and thickness = 1.6 mm. The antenna design was modeled using CST Microwave Studio. Following subsection are the details of proposed PIFA designs.

Fig. 2
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Proposed LTE antenna design descriptions

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2.1 Antenna #1: LTE700M PIFA Antenna

In this antenna, there are two approaches used that are the width arm increment and addition of stairs at radiator in order to enhance the bandwidth for LTE700M. Figure 3 shows the front and back view of PIFA antenna structure. This overall antenna dimension (l x w x h) is 125 × 60 × 1.634 mm, where the copper thickness is 0.017 mm and substrate thickness is 1.6 mm. At bottom layer, the finite ground size is 93 × 60 mm. Meanwhile, at the top layer, there are two stairs added at the radiator. The stair 1 dimension is 16 × 9 mm, and stair 2 dimension is 14 × 10 mm. The PIFA is fed by 50 Ω microstrip feedline that is positioned at (0, 47) and width of 3 mm. The shorting arm was located at (57, 93) and width of 3 mm.

Fig. 3
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Antenna #1 design geometry. a Front view. b Back view

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2.2 Antenna #2: Miniaturized of LTE700M PIFA Antenna

Based from antennas #1, the size of the antenna finite ground plane is miniaturized in order to have compact design. In the miniaturization, the length of the ground is reduced from 125 to 115 mm, a 10 mm length reduction. Due to this reduction, the antenna stair-shaped geometry is re-optimized to achieve the original LTE700M band resonance. After miniaturization, the overall antenna (l × w × h) is 115 × 60 × 1.634 mm and stair 1 and stair 2 dimension is 22 × 6 mm and 15 × 13 mm respectively (Fig. 4).

Fig. 4
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Antenna #2 design geometry. a Front view. b Back view

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3 Simulation and Fabrication Results

3.1 Results of Antenna #1

With the introduction of stair-shaped geometry at the radiator, there is a significant frequency shifting and bandwidth enhancement that can be observed in Fig. 5a compared to the antenna without the stair-shaped geometry. As simulated data shows the bandwidth achieves at −6 dB is from 0.68 to 0.96 GHz, which met the LTE700M requirements.

Fig. 5
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Antenna #1 results. a Simulated results with and without stairs. b Simulated versus measured data

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Figure 5b shows both the simulated and measured results for the input reflection coefficient of antenna #1. Good correlation has been observed between measured and simulated results.

The measured results show that minor shift in the lower band LTE700M which may probably due to fabrication error. The measured results of antenna #1 at −6 dB return loss is covers from 725 up to 980 MHz, and at −10 dB return loss it cover from 2.6 up to 4.2 GHz.

3.2 Results of Antenna #2

This antenna is the improvement of antenna #1, whereby the total length was reduced by 10 mm. Figure 6a shows the geometry difference observed between antenna #1 and antenna #2.

Fig. 6
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Antenna #2 results and antenna design. a Antenna #1 and #2 design geometry. b Simulated versus measured data

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Figure 6b shows both the simulated and measured results for the input reflection coefficient of antenna #2. The measured results show that minor shift in the lower band LTE700M. The measured results of antenna #2 at −6 dB return loss is covers from 720 up to 980 MHz, and at −10 dB return loss it cover from 2.38 up to 4.1 GHz.

4 Analysis and Discussion

The folded mechanism is basically for the purpose of miniaturization process where it help to reduce antenna overall dimension. The shorting pin plays an important role to achieve zero reactance at the desired resonance frequency. Figure 7a shows the Smith Chart characteristic where default impedance point is located at −ve reactance (capacitive). As mentioned by author in [9, 10], by adding shorting pin to the ground, it help to improve reactive excitation thus the overall impedance point near zero. Figure 7b shows the S11 parameter response of shorting pin effect.

Fig. 7
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Shorting pin effect analysis. a Smith chart characteristic. b Shorting pin effect in term of S11 response

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By having the shorting pin, PIFA achieve better impedance matching. The introduction of stairs act as series capacitance which helps broaden the bandwidth from 690 to more than 960 MHz. Figure 8 shows the effect of addition of stairs in term of Smith Charts.

Fig. 8
figure 8

Addition of stair effect based on Smith chart

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The simulated surface current distribution also been studied for PIFA antenna #2 for three frequency band which are f1 = 700 MHz, f2 = 800 MHz and f3 = 2400 MHz. As shown in Fig. 9a–c, the excited surface currents flowing near the stairs have maximum strength and begin to decreases slowly before turn to max peak again at Arm3. The longer path taken by this surface current creates better impedance matching at particular frequencies.

Fig. 9
figure 9

Simulated surface current distribution on PIFA Antenna #2. a f = 700 MHz. b f = 800 MHz. c f = 2400 MHz

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Figure 10 shows the simulated VSWR characteristic of both antennas. Typical antenna design required VSWR characteristic of less than 3:1, whereby lower VSWR indicates better reflection coefficient, hence better impedance matching.

Fig. 10
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Simulated VSWR characteristics for proposed antennas

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In term of excitation position, antenna #1 center resonant frequency occurs at 764 MHz and 3.01 GHz, while antenna #2 center resonant frequencies occurs at 834 MHz and 3.03 GHz. Table 1 shows the design summary of both PIFA antennas.

Table 1 Summary of fabricated results
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4.1 Radiation Pattern: Antenna #1

The simulated radiation pattern of the antenna is shown in Figs. 11 and 12. The patterns are nearly omnidirectional which is suitable for wireless portable devices.

Fig. 11
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Simulated 2D radiation pattern for 700, 850 and 960 MHz in antenna #1

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4.2 Radiation Pattern: Antenna #2

Fig. 12
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Simulated 2D radiation pattern for 700, 850 and 960 MHz in antenna #2

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5 Conclusions

In this paper, two new PIFA that operates at LTE700M have been presented. PIFA Antenna #1 operates at 0.715 up to 0.96 GHz at 6 dB return loss reference and at 2.672 up to 4.23 GHz at 10 dB return loss. Meanwhile, PIFA Antenna #2 operates at 0.72–1.18 GHz at 6 dB return loss and also operates 2.32–3.82 GHz at −10 dB return loss reference. The overall Antenna #2 size has achieved reduction in volume by 46.6 % in length, 33.7 % in width, and 5.45 % in height when compared to its reference antenna [18]. Therefore, with its compact size and easy integration with modern handheld devices, the antenna is a promising candidate for several wireless services such as LTE700, GSM850/900, WLAN2.45G and WiMAX3.5G.