Design Techniques for Planar UWB Antennas for Communications

JasonYoo

Design Techniques for Planar UWB Antennas for Communications [Copy link]

Since the UWB (Ultra Wide Band) system uses a broadband of more than 500MHz to transmit data at high speed, the UWB antenna must have good frequency characteristics. In recent years, OFDM (Orthogonal Frequency Division Multiplex) and Mono Pulse communications have gradually become popular, and the application of UWB has attracted more attention from all walks of life. The Federal Communications Commission (FCC) of the United States defines the bandwidth of UWB as 3.1~10.6GHz. Considering that the frequency used by WLAN and wireless TV is mostly 2.4GHz, the design of UWB antennas must cover 2.4GHz and the frequency domain specified by the FCC. In addition, in order to smoothly exchange information with the third/3.5th generation portable terminal devices with a bandwidth of 31.92~2.17GHz in the future, and to support IEEE802.16a terminal devices using 2~11GHz, the applicable frequency range of UWB antennas is more reasonable at 1.9~11GHz. In addition, the UWB antenna must be able to be powered by a 50Ω coaxial cable, and the antenna's directivity must be uniformly radiated. In view of this, this article will further explore the design techniques of next-generation planar ultra-thin UWB antennas that can cover the 1.9~11GHz frequency range based on the above conditions.

Photo 1 of the characteristics of the UWB antenna shows the actual appearance of a UWB antenna composed of radiating metal plates such as a large elliptical element, a small elliptical element, and an inverted U-shaped element. The large elliptical element is provided with an elliptical hole of almost the same size as the small elliptical element. Both elements are powered by a coaxial cable. The two elliptical elements are connected to the central conductor of the coaxial cable, and the inverted U-shaped element is connected to the conductor outside the coaxial cable. The size of the antenna is 58mm (height) x 28mm (width). 　 　 Next, we will review the structural characteristics of the antenna. Figure 1(a) is a Volcano Smoke antenna, which requires a large three-dimensional ground plane. Figure 1(b) shows that the radiating element of the Volcano Smoke antenna is changed to a plane of arbitrary shape, but a large ground plane is set up to obtain broadband characteristics. Figure 1(c) shows that an Image Element with upper and lower symmetry is used. Although researchers have reviewed the integration method of the conical line, the balanced type is a high impedance structure, which easily leads to the lengthening of the conical line. Figure 1(d) is a countermeasure for omitting the ground plane, which uses an inverted U-shaped element that can obtain broadband characteristics to replace the Image Element, thereby improving the impedance integration at low frequencies. Figure 1(e) shows that in order to improve the impedance integration at high frequencies, a small high-frequency element is used, but the characteristics of the combination of the two elements are further degraded. Figure 1(f) shows that in order to reduce the mutual combination, a hole is set in the low-frequency element to integrate the impedance of the entire frequency domain. 　 Antenna structure design Figure 2 is an example of the structure design of the UWB antenna. As shown in the figure, assuming that the wavelength of the lowest frequency is W z , the height of the large elliptical element and the inverted U-shaped element is about 1/4 W z . This design uses 0.23 and 0.22 wavelengths respectively. The height of the small elliptical element is 0.16 W z . The parameters that have the greatest impact on the impedance integration characteristics in Figure 2 are the interval "d" between the two elliptical elements and the gap "g" between the elliptical element and the inverted U-shaped element. The actual size of this antenna is found by experimental review and then optimized through simulation analysis. 　 　 Simulation Analysis As described above, the optimization design is performed using simulation analysis based on the Moment method. The simulation analysis uses NEC (Numerical Electromagnetic Code). FIG3 is an example of an actual simulation analysis mode. The maximum number of segments is 3422. 　 　 FIG4 shows the calculated value of the VSWR characteristic of the frequency when the distance d between the two elliptical components is changed. At this time, the gap g between the large elliptical component and the inverted U-shaped component is set to 1 mm. It can be seen from FIG4 that when d=6 mm, the VSWR becomes minimum. 　 　 FIG5 shows the calculated value of the VSWR characteristic of the frequency when the gap g between the large elliptical component and the inverted U-shaped component is changed. At this time, the distance d between the two elliptical components is set to the optimal parameter of FIG4, that is, d=6mm. It can be seen from FIG5 that the VSWR becomes the minimum when g=1.2mm, and the VSWR is lower than 2.4 when the frequency range is 2.4~10.6GHz. 　 　 FIG6 is the calculation result of the three-dimensional radiation pattern of the antenna. As shown in the figure, the radiation patterns at 2.5 GHz and 5 GHz are donut-shaped. Its radiation characteristics are similar to those of a dipole antenna. It can be seen from the figure that a Null occurs in the back direction at 8 GHz and a Null occurs in the front and back directions at 10.6 GHz. The main reason for this phenomenon is the asymmetry of the radiation element of the antenna. 　 　 The UWB antenna made by the parameters obtained by simulation analysis was fine-tuned experimentally and d was set to 5mm. Figure 7 shows the test results of the Return Loss characteristics of the UWB antenna. As mentioned above, the Federal Communications Commission (FCC) of the United States defines the bandwidth of UWB as 3.1~10.6GHz. According to the test results, this antenna has a good radiation pattern in the frequency range of 2.4~12GHz, at which time the Return Loss is less than 9.5dB and the VSWR is less than 2.0dB. 　 　 Figures 8 and 9 are the actual test results of the radiation patterns of the Azimuth plane and the Elevation plane, respectively. The radiation pattern of the Azimuth plane in Figure 8 shows that the UWB antenna can obtain a radiation pattern with almost no directivity, which is very suitable for applications in wireless PAN (Personal Area Network) and other fields. According to the radiation pattern of the Elevation plane in Figure 9, the UWB antenna can obtain a directivity close to a figure 8, and the test results of Figures 8 and 9 are very consistent with the calculation results of Figure 6; Table 1 is a summary of the characteristics of the UWB antenna. 　 　 　 project performance bandwidth 2.4~10.6GHz VSWR 2 Peak Equalization 2.4GHz 2.6dBi 3.1GHz 2.8dBi 5.0GHz 3.0dBi 8.0GHz 5.2dBi 10.6GHz 2.9dBi Polarization 　 Antenna size high 58mm Width 28mm thick 5mm Table 1 Summary of UWB antenna characteristics

The UWB antenna in the above photo 1 is made of metal plate. Considering the future mass production and characteristic stabilization, the antenna printed circuit board is not only more advantageous, but also can be thinned to 5mm. This makes it easier to package it into wireless PAN and other communication products that have very strict thickness requirements. If the data transmission application of the 3rd/3.5th generation terminal with a bandwidth of 2GHz is considered, the UWB antenna must be able to support the frequency domain of 1.92GHz. If IEEE802.16a is used, it must support the upper limit of 11GHz. In other words, even if the antenna circuit is substrate-based, it must be able to cover the frequency domain of 1.92GHz to 11GHz. When the antenna circuit is substrate-based, the printed circuit board method is selected. Since the upper frequency limit of this UWB antenna is 11GHz, a Teflon substrate is used, which has very low loss even at the upper frequency limit. In fact, the substrate thickness is tentatively set at 0.8mm, and the specific dielectric constant εr=2.6. Then, the Moment method is used to optimize the size design with IE3D simulation software. Figure 10 is the VSWR characteristics obtained by simulating and analyzing the height hs of the small elliptical component. As shown in Figure 10, the height of the small elliptical component is 28mm, and the VSWR is the smallest at the frequency of 2GHz. Then, the optimal size of each part is determined accordingly. 　 　 FIG11 is the final surface and back structure and related dimensions of the printed circuit board. FIG11 shows that the basic structure of the antenna is very similar to that of photos 1 and 2. The only difference is that the connection between the large and small elliptical components is changed to a through hole method. The related dimensions are converted to W L based on the minimum operating frequency of 1.92 GHz . If compared with the structure of FIG2 and converted to wavelength, the height of the small elliptical component is increased by about 12%, and the outer dimensions of the large elliptical component are almost unchanged, and its low frequency can still cover up to 1.92 GHz. The dimensions of FIG11 have been experimentally miniaturized through actual trials, so that the VSWR value can become the smallest final dimension. 　 　 FIG12 is the test result of the electrical characteristics of the UWB antenna after printed circuit substrate. It can be seen from the figure that the Return Loss of the antenna from 1.92 to 11 GHz is lower than -9.5 dB, that is, the VSWR of the VUWB antenna is lower than 2.0, which shows that the electrical characteristics of this antenna are very good. 　 　 Figures 13 and 14 are the radiation pattern characteristics of the antenna's Azimuth and Elevation surfaces, respectively. As shown in Figure 13, the UWB antenna has a radiation pattern with almost no directivity after being printed on the circuit board. The fluctuation in the Azimuth plane is 2.9dB at 1.92GHz, and the peak equalization is 3.1dBi at 1.92GHz. The radiation pattern characteristics in the Elevation plane of Figure 14 are in the shape of an 8. Table 2 is an overview of the characteristics of the UWB antenna after being printed on the circuit board. According to the above information, this antenna is very suitable for the 3rd/3.5th generation terminals, ISM Band 2.4GHz WLAN, and the UWB Band specified by the FCC, and the 1.92~11GHz frequency domain covered by IEEE802.16a. Photo 2 is the actual appearance of the external UWB antenna, and the dimensions of the antenna are 75mm (height) x45mm (width) x20mm (thickness). 　 　 project performance bandwidth 1.92 ~ 11 GHz VSWR 2 Peak Equalization 1.92GHz 3.1dBi 2.4GHz 1.8 dBi 3.1GHz 2.3 dBi 5.0GHz 2.8 dBi 8.0GHz 4.7 dBi 10.6GHz 4.3 dBi Polarization Vertical Antenna size high 67 mm Width 29 mm thick 0.8 mm Table 2 Overview of UWB antenna characteristics after printed circuit substrate

Conclusion The above introduces the design techniques of the next-generation planar ultra-thin UWB antenna. Basically, the UWB antenna is composed of radiating metal plates such as large elliptical elements, small elliptical elements, and inverted U-shaped elements. It can cover the frequency range of 1.92~11GHz. As for the printed circuit substrate, the size optimization design is carried out using simulation analysis, and then experimental micro-strips are carried out through actual trials to minimize the VSWR value and determine the final size. According to the electrical characteristics test results, it is obvious that this antenna is very suitable for the 3rd/3.5th generation terminals, ISM Band 2.4GHz WLAN, and the UWB Band specified by the FCC, and the 1.92~11GHz frequency domain covered by IEEE802.16a.

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