“With the development of modern wireless communications, mobile devices have higher and higher requirements for antennas. Miniaturized antennas that cover multiple frequency bands and are low-cost are the main research directions today.
With the development of modern wireless communications, mobile devices have higher and higher requirements for antennas. Miniaturized antennas that cover multiple frequency bands and are low-cost are the main research directions today. The realization of the artificial metamaterial, especially the composite left-right-handed transmission line (CRLH-TL), provides a new method for the miniaturization of microwave devices. The unique zero resonance (ZOR) phenomenon of CRLH-TL can break through the traditional resonance. The size of the device is limited by the resonant wavelength, and the miniaturization of the antenna is realized. A variety of miniaturized antennas using zero resonance have been proposed, but most of the antennas have narrow bandwidth and complex loading structure, which limits the scope of application.
This paper proposes a planar monopole antenna loaded with a zero-resonance element. On the basis of a planar monopole fed by a coplanar waveguide, a single zero-resonance element is realized by loading a capacitor and an Inductor, which is outside the high-frequency resonance of the original monopole. Introduce a new lower frequency resonance frequency, and then realize the multi-frequency characteristics. The designed antenna working frequency band can cover the communication frequency bands of two mainstream wireless local area network (WLAN) protocols, WiFi (2.40 GHz-2.48 GHz, 5.15 GHz-5.80 GHz) and WiMAX (3.3 GHz-3.8 GHz). The antenna structure is designed on a single-layer board, and the structure is simple and effectively reduces the manufacturing cost.
The main structure of the antenna is shown in Figure 1. The antenna body is composed of a rectangular monopole patch and a zero resonance unit composed of an interdigital capacitor and a grounded fine wire inductance. The antenna is fabricated on a FR-4 substrate with relative permittivity εr=4.4 and thickness h=1.6 mm. The size of the substrate is a×b=40mm×30mm. The antenna is fed by a coplanar waveguide with an impedance of 50 Ω, and all the structure of the antenna and the floor are etched on the same side of the dielectric board.
The main body of the antenna is a monopole, and its total length L2 corresponds to a quarter wavelength of the high resonance frequency. The left end of the monopole forms a zero resonance unit by loading the interdigital capacitor in series and the grounded fine line inductance. The number of interlacing is N and the length lc, the length W2+L2 and width wl of the fine wire inductance control the capacitance and inductance value to determine the zero resonance frequency, so that independent control of the high and low frequency bands can be realized.
In the antenna design process, first design the monopole to work at around 5.2Ghz, determine the length of the monopole, L1, and then load the interdigital capacitor and the fine line inductance on the left side of the monopole to introduce a lower resonance frequency, and the number of interleaving is N The length lc and the total length W2+L2 and width wl of the thin line can be obtained by approximate formulas. Finally, optimize the design in the full-wave simulation software HFSS to determine the specific parameters. It can be seen in Figure 2 that by loading the zero resonance unit, a new low frequency band is effectively introduced without increasing the length of the monopole.
Experimental results and analysis
In order to verify the effectiveness of the stand-alone self-antenna design method loaded with zero resonance units, based on the antenna design dimensions given in Figure 2, we processed a physical sample and measured the return loss, pattern and other parameters. The sample antenna The photo is shown in Figure 3.
The simulated and measured return loss of the antenna is shown in Figure 4. It can be seen from the figure that the test results are basically consistent with the simulation results. The measured return loss of the antenna 10dB bandwidth can cover the communication between WiFi (2.40 GHz-2.48 GHz, 5.15 GHz-5.80 GHz) and WiMAX (3.3 GHz-3.8 GHz) Frequency band to meet the design requirements.
The current distribution of the antenna simulation is shown in Figure 5. From the figure, it can be seen that at about 2.45 GHz, the current is concentrated on the fine line inductance and the interdigital capacitance, and the oscillating current has the same phase, which is in line with the characteristics of zero resonance. The current around 3.5 GHz oscillates from the feeder and the inductor to the interweaving concentration. The current density of the main body of the monopole is relatively large at around 5.5 GHz, and the monopole is the main radiator. At the same time, the load of the interdigital capacitance and the fine line inductance has a certain influence on the current distribution.
Figure 6 shows the simulation and test pattern of the antenna in the three working frequency bands. It can be seen that the antenna has better omnidirectionality at lower frequencies, and the radiation direction is more consistent, in the high frequency band. As the frequency increases, the radiation pattern deteriorates to a certain extent. The reason is that the loading of the interdigital capacitance and the fine-line inductance has an effect on the current distribution of the original monopole, resulting in a certain deviation of the radiation direction. In the test process, the xy plane cross-polarization test result has a big deviation compared with the simulation result due to the limitation of conditions. At the same time, the antenna gain simulation and test curves are also given in Figure 7. It can be seen that the antenna gain is above 2.2dBi in the working frequency band, which can meet its application in small WLAN equipment.
This paper proposes a three-frequency monopole antenna loaded with zero resonance units. The antenna has a simple structure, is easy to manufacture, and the antenna body is small in size. The test results show that this antenna can cover three of the two wireless LAN protocols, WiFi and WiMAX. A working frequency band, suitable for small wireless terminals, has good application value.
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