A multi-band monopole antenna based on FDTD analysis

0 Introduction

With the development of voice services, narrowband and broadband data services in wireless communications, mobile phones with 3G functions will gradually become the mainstream of the market. At the same time, the design of mobile phones is also changing with each passing day, and the requirements for the wideband characteristics, multi-frequency operation and miniaturization of antennas are more stringent. At present, multi-mode mobile phones generally only use one multi-band antenna that can support multiple wireless standards. Multi-band mobile phone antennas mainly use PIFA antennas and monopole antennas. Compared with other forms of antennas, these two types of antennas have the characteristics of low profile, small size, and convenient design. Therefore, they are widely used in mobile communication terminals such as mobile phones. The basic PIFA antenna is to change the horizontal oscillator of the inverted F antenna into a planar form, thus introducing the planar inverted F antenna. With the in-depth study of PIFA antennas, many new PIFA antennas with good performance have emerged. Faced with the requirements of multi-mode mobile phones for multi-band antennas, the large bandwidth and high gain of monopole antennas are more suitable for the requirements of multi-mode mobile phones for hundreds of megabits of bandwidth. In addition, the structure of the built-in planar monopole antenna is flexible and easy to match with the changing structure of today's mobile phones. Currently, the popular ultra-thin and ultra-small mobile phones on the market generally use this type of antenna.

The commonly used multi-band technologies in the design of multi-band monopole antennas mainly include multi-radiation branch structure, optimized feeder structure or increased input matching lumped components, optimized capacitive load between the radiator and the ground, slotted radiator patch, adjusted radiator shape, and increased current density to obtain high-order modes. The literature adopts a structure with multiple radiating branches and proposes two typical multi-branch monopole antennas. The literature embeds a curved slot inside the rectangular planar monopole antenna to realize a GSM/DCS/PCS three-band antenna.

The multi-band monopole antenna proposed in this paper realizes the GSM/DCS/PCS frequency band by slotting the planar rectangular antenna, and at the same time, through a ground-coupled radiator, it effectively expands the working frequency band of the antenna and can meet the TD-SCDMA standard at the same time. Since the designed antenna can be made on the same printed circuit board as other circuits of the mobile phone, the antenna production price is very low, and the antenna height is very suitable for ultra-thin mobile phones.

1 Antenna Design and Simulation

For a traditional monopole antenna, the length of the radiation branch is about 1/4 wavelength. The radiation resistance and radiation field of a monopole antenna can be calculated using the mirror principle. A simple monopole antenna has a low radiation resistance and a large capacitive reactance. For an infinite ground, its radiation pattern is equivalent to a dipole. If the ground is gradually reduced, an ideal mirror image cannot be formed and the current distribution on the ground will change. In modern antenna design, using electromagnetic field simulation software to simulate antennas has become the main method of antenna design. The electromagnetic field simulation software used in this article uses the finite difference time domain method to perform calculations in the time domain. Since the excitation signal can be a narrow pulse with a very wide spectral component, combined with Fourier transform, the characteristics within the required bandwidth of the calculation object can be obtained through one calculation, so it is particularly suitable for the study of broadband problems.

When using the finite-difference time-domain method to analyze electromagnetic fields, the calculation space is first divided into finite grids, where each electric field component is surrounded by four magnetic field components, and each magnetic field component is also surrounded by four electric field components. This division method satisfies the structural form of the Maxwell curl equation, is suitable for differential operations of the curl equation in space, and can properly describe the propagation process of electromagnetic waves in space. The Maxwell curl equation is discretized in the above-mentioned spatial grid and time, and the following symbols are used to represent the value of any field component F at a point (x, y, z, t):

F(x，y，z，t)→F(i△x，j△y，k△z，n△t)→Fn(i，j，k)

Where: △x, △y, △z are the spatial grid steps in the x, y, and z directions respectively; △t is the time step; i, j, k are integers, so differential operations with second-order accuracy can be used to replace differential operations.

In order to facilitate the calculation and programming, the numbers of space and time are integer values, and the iterative formulas of the components of the passive region Maxwell curl equation (1) and (2) can be obtained as shown in equations (3) to (8):

The space used for numerical calculation is always limited. In order to calculate the electromagnetic field in a limited space, special treatment needs to be done on the surrounding boundaries of the limited space. Using PML (Perfect Match Layer) technology in FDTD, the calculation area can be set to a vacuum, with scatterers and outward waves in the calculation area, and the calculation area is surrounded by a PML absorbing medium, and outside the PML medium is an ideal conductor.

The basic multi-band monopole antenna structure is shown in Figure 1. The PCB main board is made of general FR4 material, with a thickness of 1 mm and a size of 40 mm × 105 mm. The main radiating element of the antenna is slotted to form two radiating branches, and the lengths of the two radiating branches are different. By adjusting the lengths of the radiating branches, the antenna can obtain two resonant frequencies, corresponding to 900 MHz and 1800 MHz respectively. The grounded radiating plate close to the feeder is used to adjust the high-end resonant frequency so that the working range of the high-frequency band can meet the requirements of TD-SCDMA. The final size of the antenna radiating element is determined by optimization of the simulation software.

Figure 2 shows the effect of the grounded parasitic radiation branch on the antenna operating frequency band. As can be seen from Figure 2(a), without the grounded radiation branch, the antenna can only cover GSM900 and DCS1800. By adding the grounded radiation branch, the antenna's operating frequency band is effectively expanded. Figure 2(b) shows the effect of the length L3 of the grounded radiation branch on the antenna's high frequency band. By adjusting its length so that the resonant frequency of the short radiation branch of the main radiation plate partially overlaps, the maximum operating frequency band can be obtained.

The effect of the structural dimensions of the main radiating unit in the antenna on the antenna return loss is shown in Figure 3. Figure 3 (a) shows that the longer the length L2 of the long radiating branch is, the lower the center frequency of the 900 MHz operating frequency band is, the narrower the operating bandwidth is, and the lower the operating frequency of 1 800 MHz is, resulting in a wider bandwidth; Figure 3 (b) shows that adjusting the length L1 of the short radiating branch has little effect on the 900 MHz operating frequency band, while the lower limit of the 1 800 MHz operating frequency band is lowered, widening the operating bandwidth. Through software optimization, the optimal structural dimensions are obtained to meet the design requirements of the antenna.

2 Data simulation and experimental results

Through the above simulation analysis, the antenna structure size was finally determined, with L1=21.3 mm, L2=18.75 mm, and L3=22 mm, while ensuring that the S11 parameter at the antenna input is less than -10 dB. According to the optimized antenna structure, a test antenna was made, as shown in Figure 4. The antenna characteristics were measured using the Agilent vector network analyzer E5701B (frequency range 300 kHz to 6 GHz), and the actual measurement frequency range of antenna S11 is 0.5 to 2.5 GHz. Figure 5 shows the measurement and simulation results of antenna S11. From the simulation data, we can get that the working frequency of the antenna in the low frequency band is 863～973 MHz, and the working frequency of the high frequency band is 1 690～2 100 MHz; while the working frequency of the actual test antenna in the low frequency band is 888～990 MHz, which is slightly wider than the working frequency band of GSM900 (890～960 MHz), and the working frequency of the high frequency band is 167～2 200 MHz, covering all the working frequency bands of DCS／PCS／TD-SCDMA (1 710～2 025 MHz). At the same time, it can be seen from the figure that the actual test results are more consistent with the simulated data in GSM900; in the DCS／PCS／TD-SCDMA high frequency band, the measured data and the simulated data have a slight deviation, and the test antenna has a resonance point at 1 860 MHz, and the resonance frequency of the parasitic radiation branch becomes relatively high.

Figure 6 compares the antenna simulation pattern and the test pattern of the test antenna. It can be seen from the simulated antenna pattern that the antenna pattern is very close to the typical monopole antenna pattern, and the radiation pattern in the vertical plane has relatively good omnidirectional performance. At the same time, the increase in operating frequency increases the lobe in the horizontal pattern, but the vertical plane still maintains good omnidirectional characteristics. Comparing the simulated pattern and the actual test pattern, due to the fluctuation of the test data, the shapes of the two are very different, but the measured pattern is basically consistent with the simulated data in amplitude.

3 Conclusion

The proposed multi-band monopole antenna uses a slotted radiating plate and coupled-fed parasitic radiating branches, which can obtain multiple resonant frequencies in the frequency band of 888 to 2 200 MHz. Therefore, the designed antenna can meet the antenna requirements of GSM900/DCS1800/PCS1900 and TD-SCDMA dual-mode mobile communication terminals. The test antenna made with the antenna structure optimized by simulation has a return loss that meets the design requirements and is consistent with the software simulation results.

The multi-band monopole antenna proposed in this paper has a very low height, but there is still room for reducing the distance from the ground plane. Further research can be conducted on improvement measures for the narrowing of bandwidth caused by shortened distance. At the same time, considering that future communications may use the 2.3-2.4 GHz frequency band, research on monopole antennas with wider bandwidths can continue.