Compact Multi-band Planar Monopole Antenna for LTE Terminals

Thanh-Nga Mai, Anne-Claire Lepage, Bernard Huyart, Yenny Pinto Institut Mines-Telecom, Telecom ParisTech - LTCI CNRS UMR 5141

46 rue Barrault 75634 Paris cedex 13, France thanh-nga.mai@telecom-paristech.fr, anne-claire.lepage@telecom-paristech.fr, bernard.huyart@telecom-paristech.fr

Abstract— This article introduces a compact multi-band planar monopole antenna which is made of a driven monopole antenna and an additional strip. It is etched in a no-ground part, which is small compared to the Printed Circuit Board (PCB) size. The antenna which can operate in GSM/UMTS bands and most of LTE bands (LTE700, LTE2500, LTE3400 and LTE3600 bands) has the size suitable for mobile phones. A prototype has been realized and there is a good agreement between simulation and measurement.

Index Terms—multi-band, monopole, PCB, LTE.

I. INTRODUCTION 4G telecommunication system is being exploited globally

today. However, not all of present cell phones can support this technology, which demands new antenna designs able to operate in multi bands [1]. There are several techniques which are proposed to achieve multi-band antenna designs with small size [2] – [8]. Planar strip monopole which has been proposed in [2] is low-profile, low-cost and easy to fabricate, so that it is selected to conduct our project.

The original monopole antenna in [2] obtains only the low frequency band around 800 MHz, but our design can achieve more bands, such as GSM1800, UMTS2100, LTE3400 and LTE3600, thanks to the use of a different substrate and a different extended ground element. To obtain LTE2600 band, instead of using a parasitic shorted strip [2] which takes more time to simulate and is difficult to fabricate, our antenna implies a simple additional strip as suggested in [3]. Our design has a larger band around 700MHz (700 – 960 MHz) than in [3] (700 – 800 MHz), but a narrower band around 2500 MHz. The optimization of the antenna design has to be improved to fulfill this frequency range. In general, the proposed antenna can operate for GSM/UMTS and most of the LTE mobile phones.

II. DESIGN OF PROPOSED ANTENNA The topology of the antenna is shown in Fig.1. In the front

side of PCB, there is a folded monopole whose length is around a quarter of wavelength at 700 MHz [2]. In order to cover LTE2600 band, an additional strip having the length of a quarter of the wavelength at 2600 MHz is added with the gap of 0.5 mm, as proposed in [3]. Besides, an extended ground (16 x 13mm) in the back side is designed to achieve LTE3600 band. This extended part is added to the ground plane at the

distance of 22.5 mm from the left edge of PCB (as illustrated in Fig. 1). More details of this design are shown in Table 1.

The planar antenna is printed on a low-loss substrate having the relative permittivity of 2.55, loss tangent of 0.005, and a thickness of 0.762mm. It is fed with a 50 microstrip line. The surface occupied by the monopole is 25.5 × 63 mm2 (0.06 × 0.14 at 700 MHz), and the overall size of PCB is 124.5 × 63 mm2 .

The simulations have been conducted with CST Microwave Studio (Transient solver) and the prototype has been realized. This antenna design is simple, but operates in several bands.

Fig. 1. Layout of the proposed antenna in xy plane

TABLE I. Parameters of antenna

la 22 mm lg 99 mm d 22.5 mm de 13 mm le 16 mm

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I. SIMULATION AND MEASUREMENT

A. Impedance bandwidth The simulated magnitude of the reflection coefficient is

shown in the red line in Fig. 2. In the range below 5000 MHz, the monopole antenna provides 3 bands: 700 – 960 MHz, 1750 – 2300 MHz, and 3100 – 4380 MHz with -6dB matching criterion. Another frequency band from 2560 to 2650 MHz is due to the additional strip. There is an agreement between the measured and simulated results. The frequency shift at high frequencies above 2600 MHz may result from the inaccurate length of the additional strip. While the frequency shift at the lowest resonant frequency can be caused from the variation of the length of the ground.

Fig. 2. Simulated and measured reflection coefficient

B. Radiation pattern Fig.3 presents the radiation pattern in term of realized

gain, where all losses are taken into account, including the mismatch loss. The maximum realized gains at different frequencies are listed in Table 2.

At low frequencies, the antenna has an omni-directional radiation pattern like a dipole (in Fig. 3.a). This pattern is changed at high frequencies (in Fig. 3.b and 3.c) due to the influence of the ground. Compared to the radiation pattern at 800 MHz, the level of cross-polarization at 2600 MHz is higher, as demonstrated in Fig. 3b.

As seen, the dotted curves represent the simulated radiation patterns and the solid curves represent the measured ones. As observed, the measured co-polarizations at E and H-plane are quite the same as the simulated ones at 3 frequencies. The measured cross-polarizations at 3800 MHz are also similar to the simulated ones. However, at low frequencies (800 and 2600MHz), the level of cross polarization of measurement is higher than in simulation (not shown). The cause of the discrepancy has not been found yet and an additional measurement has to be performed.

TABLE 2. Maximum realized gain at E-plane

Frequency Simulation Measurement 800 MHz 2 dB 1.9 dB

2600 MHz 2.7 dB 3.2 dB 3800 MHz 4.1 dB 4.6 dB

3a. At 800 MHz

3b. At 2600 MHz

3c. At 3800 MHz xz plane yz plane

Fig. 3. Radiation pattern

C. Surface current

a. At 800MHz b. At 2600MHz c. At 3800 MHz

Fig.4. Simulated surface currents (absolute values)

Fig.4 shows the surface currents of the antenna at different frequencies. According to the surface current distributions, the driven monopole antenna is excited at the frequency of 800 MHz. At 2600MHz, the surface current is on the additional strip; then extends on the monopole due to the coupling effect.

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The driven monopole antenna is also coupled with the extended ground element at 3800MHz.

II. PARAMETER STUDY

A. Length of the ground plane (lg) In general, this parameter has influence on the value of

|S11|. In order to cover the LTE700 band, we focus on the variation of |S11| at low frequencies. Fig. 5 demonstrates the change of |S11| from 500 to 1000 MHz in respect of altering the length of the ground plane. It is noted that the variation of lg does not change the resonance frequency but the level of |S11| and the bandwidth. When decreasing the length of ground plane, the operation frequency band is shifted up and the bandwidth is reduced. However, the size of PCB should not be too large, then the length of 99 mm is selected.

B. Position of the extended ground element (d) The presence of the extended ground element is necessary

to obtain the LTE3400 and LTE3600 band. Looking at the surface current, it is noted that this extended element works as a radiator, especially at high frequencies. Using it, the highest resonant mode is shifted from 3100 to 3800 MHz and the bandwidth is enlarged significantly (as shown in the Fig. 6). It is noted that the choice of the position of this extended element is tricky. The distance of 22.5 mm from the extended ground element to the right edge of the PCB is chosen.

C. Length of the additional strip in the front side (la) In order to cover the band 38 of LTE operating bands

(2570 – 2620 MHz), an additional strip is added, which has no impact on S11 at frequencies less than 2600 MHz. Its length is set around a quarter of wavelength at 2600 MHz; then it is optimized to get the expected bandwidth. As observed in Fig. 7, the length of 22 mm is a good choice.

Fig. 5. Variation of the length of the ground plane

Fig. 6. Variation of the position of the extended ground element

Fig. 7. Variation of the length of the additional strip

III. CONCLUSION This paper presents a compact planar monopole antenna

covering 4 bands: 700 – 960 MHz, 1750 – 2300 MHz, 2560 – 2650 MHz, and 3100 – 4380 MHz, which can operate in GSM/UMTS and most of LTE bands. The prototype is finalized and there is an agreement between simulation and measurement. Besides, the details of parameter study are written in order to demonstrate the behavior of this antenna. The antenna can be a good choice for portable devices which need to support 4G technology.

ACKNOWLEDGMENT The research leading to these results has been conducted in

the framework of the Celtic project SPECTRA.

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[3] I.Dioum, A.Diallo, C.Luxey, S.M.Farsi, “Dual-band monopole MIMO antennas for LTE mobile phones,” IECOM, Dubrovik, pp. 1-4, Sept.2010.

[4] J.Volakis, C.Chen, K.Fujimoto, Small antenna: Miniaturization Techniques & Applications, McGraw-Hill 2010.

[5] X. Begaud, Ultra Wide Band Antennas, Wiley, Oct. 2011, pp. 202-203. [6] S.C. Del Barrio, M.Pelosi, O.Franek and G.F.Pedersen, “Tuning Range

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[7] J.Pei, A.G.Wang, S.Gao and W.Leng “Miniaturized Triple-Band Antenna With a Defected Ground Plane for WLAN/WiMAX Applications”. IEEE Antennas and Wireless Propagation Letters, 10, 298–301,2010.

[8] A.R.Bhatti, S.Yi and S.Park. “Compact Antenna Array With Port Decoupling for LTE-Standardized Mobile Phones”, Antennas and Propogation letters, p: 1430–1433, 2010.

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