Y. Belhadef and N. Boukli-Hacene
Planar inverted-F antennas (PIFAs) are miniature designs that offer great versatility for both mobile and wireless applications. Such antennas offer multiband, broadband operation, omnidirectional radiation patterns, high efficiency, and small size. By using commercial High Frequency Structure Simulator (HFSS) modeling software from ANSYS, two PIFA designs were created in support of different wireless standards, with their dimensions modified to allow operation across two and three simultaneous wireless bands, respectively.
Rapid growth in wireless communications worldwide has driven the development of multiple wireless standards and portable communications devices capable of numerous operating modes. To handle multiple frequency bands and standards, a versatile antenna such as a PIFA is required, and a recently proposed solution involved the use of a PIFA like an internal antenna for portable devices.1
By integrating this type of mobile-telephone antenna into a handset, several advantages are possible compared to conventional antennas, such as monopole or spiral antennas.2 PIFAs are less easily interrupted in operation, suffer less absorption by the head of the user, and are less sensitive to the geometry of the mobile communications product. One of the most suitable antenna configurations for mobile product integration is the PIFA, which is widely used in mobile telephony. This type of antenna supports the majority of integrated multiband systems in portable terminals, along with a large number of variants, including insertion of slots and the addition of parasitic elements to modify the basic antenna behavior.3
The insertion of a slot in the radiating element is used to create new resonant frequencies and band coverage, as well as aid in achieving miniaturization of antennas for multiband wireless devices.
A PIFA is a rectangular antenna charged by a feed probe. It is called an inverted-F antenna because the side view of this antenna and its air dielectric resembles the letter F with its face down. The higher plate of a PIFA and the location of the feed wire located in a corner of the rectangular plate can be determined approximately.4
The PIFA antenna evolved gradually from two inverted-L antenna (ILA) and inverted-F antenna (IFA) structures, with modifications made to help overcome limitations in their initial radiation patterns of those antenna types (Fig. 1).
An ILA consists of a short vertical monopole with the addition of a long horizontal arm at the top. Its input impedance is nearly equivalent to that of the short monopole with the reactance caused by the horizontal wire above the ground plane.5 The antenna's input impedance consists of a low resistance and high reactance. Since loss due to mismatch decreases radiation efficiency, it is desirable to modify the ILA's structure to achieve nearly resistive input impedance that is easily matched to a standard coaxial line.
The ILA's structure is commonly modified by adding another inverted-L element to the end of the vertical segment to form an IFA. The addition of the extra inverted-L element behind the feed point tunes the antenna input impedance. One disadvantage of an IFA that is constructed with thin wires is the low impedance bandwidth. Typically, a single IFA element presents an impedance bandwidth of less than 2% of the center frequency.5 One way to increase the bandwidth of the IFA is to replace the top horizontal arm with a plate that is oriented parallel to the ground plane to form a PIFA. This antenna, developed by T. Taga and T. Suneskawa,6 is used for basic reception within a large part of the Nippon Telegraph and Telephone (NTT) wireless mobile communications system in Japan (Fig. 2). The PIFA enjoys the advantages of small size and broad bandwidth.
The resonant frequency, Fr, for a PIFA can be calculated from the following:
Fr = c/
where:
c = the speed of light in free space,
H = the height of the radiating element, and
L = the length of the radiating element.
Figure 3 presents a dual-band PIFA antenna excited by a coaxial-type feed. Two radiating elements are partially separated by a U-shaped slot. The external radiating element, having a size of L2 W2, is assembled above a ground plan with a short-circuit plane of height h. The internal radiating element has a size of L3 W3. The two PIFA antenna resonant frequencies are mainly given by (L2, W2) and (L3, W3), respectively.7 The antenna's impedance can be easily fixed at 50 Ω by controlling the feed position, d. For fixed height, h, the widths of the two operating bands are considerably dependent upon the short-circuit plane width S and the two radiating elements ratios W2/L2 and W3/L3. To design a dual-band PIFA for wireless applications, the PIFA antenna slot U dimensions are taken equal to: L2 = 17 mm, L3 = 9 mm, W2 = 17 mm, W3 = 7 mm, h = 5 mm, and d = 9 mm.8
The authors modeled the slotted PIFA antenna by means of the High-Frequency Structure Simulator (HFSS) electromagnetic (EM) simulation software from ANSYS (Fig. 4). The design leads to a miniature dual-band antenna. The operating bands predicted by HFSS cover center frequencies for Bluetooth and Wi-Fi systems.
Figures 5(a), 5(b), and 5(c) show the return loss, input impedance locus, and two-dimensional radiation patterns for the PIFA design, respectively. The return loss for the modified antenna (L3 = 9.5 mm, h = 5 mm, d = 10.5 mm, S = 1 mm, and the addition of a slot with size 3 0.5 mm perpendicular to slot U) yields two resonant peaks. The first peak of -21.5 dB is observed at 2.480 GHz and the second of -25 dB can be seen at 5.378 GHz. On a Smith chart, the input impedance for the two frequency bands is almost at the center of the chart, indicating null reflection.
The radiation characteristics of the PIFA in terms of its polar coordinates in the E- and H-planes show similar geometric patterns, indicating that the radiation patterns for these antennas is almost globally omnidirectional. Along with the initial PIFA designed for Bluetooth and WiMAX, a second PIFA was designed and simulated with HFSS for use in three different frequency bands. In this second design, the radiating element is 40 22 mm, assembled on a ground plane with dimensions of 100 50 mm. Between these two elements, 7-mm-long feed wire and short-circuit wires were used. The two slots inserted on the patch are L W = 23 16 mm and L W = 11 4 mm as shown in Fig. 6.9
Once the antenna was modeled by means of the HFSS simulation software, it was apparent that modifications were needed to meet the requirements of the different communications standards (Fig. 7). Some of the changes, as indicated by HFSS, including a reduction in the width of the large slot, which became 2.9 mm, insertion of a strip of width 0.6 mm between the ground plane and the radiating element, and the addition of another U form slot at the end of the radiating element. Making these changes led to good results with the HFSS simulator in terms of desired multiband performance. In fact, three peaks were observed in the three bands used for various mobile and wireless telecommunications standards, equal to -27, -19, and -29 dB at 2.854, 3.663, and 6.855 GHz, respectively, corresponding to null reflected power Fig. 8(a)>. The three bands correspond to MBWA, UWB, and WiMAX frequency bands, respectively.
To further understand the PIFA designs, input impedance loci were analyzed on the Smith chart. It was noted that the loci are around 50 O for each of the three frequencies Fig. 8(b)>. The polar radiation patterns in the E- and H-planes are also presented for analysis in Fig. 8(c).
Y. BELHADEF, Researcher
N. BOUKLI-HACENE, Researcher
Laboratoire de Tlcommunications
Dpartement de Tlcommunication
Facult de Technologie
Universit Abou-Bekr Belkad-Tlemcen
BP 230, Ple Chetouane, 13000
Tlemcen, Algeria
FAX : 213-4328-5685
Belhadef_y@yahoo.fr
bouklin@yahoo.com
References