1、 A Novel Method for Designing Dual-Frequency Slot Patch Antennas with Two Polarizations WU Di1, OH ISH I BA SH I H idekazu2, SEO Kazuyuk i2, I NA GA K IN aok i3 (1. Department of Communication Engineering , Shang hai University , Shang hai 200072, China； 2. Kojima R &D Center, Kojima Press Industria
2、l Co Ltd , Aichi 470 0207, Japan； 3. Nagoya Institute of Technology, Nagoya 466 8555, Japan) Abstract: This paper presents a new method for designing a dual-frequency patch antenna with circular and linearpolarizations. Dual-frequency operations are behaved by etching two narrow slots close to the r
3、adiating edges of arectangular patch. The circular polarization at the lower resonant frequency of the dual-frequency antenna can beachieved by setting a perturbation segment at an appropriate location in the patch element, and placing the feedpoint on the diagonal axis. Several experimental results
4、 of a designed antenna show the good characteristics for circular and linear polarizations at both resonant frequencies. CLC number： TN821.1 Document code： A Key words: dual-frequency； slot patch antennas； linear polarization； circular polarization In radar and communication systems, dual-frequency
5、operations are oftenrequired. Specially,in modern mobile global position system (GPS), itwill be desirable that one of two frequencies is thecircular polarization. Planar antenna has been investigated for multi-frequency very well since it isadvantageous in low cost, low weight and conformability. E
6、arlydual-frequency planar antenna ismultilayered stacked-patch structure, and the radiating element is used to be circular1, annular2, rectangular3and triangular patches4. Recently, a dual-frequency antenna is introduced in which the structure is dingle layer patch with two slots close to the radiat
7、ing edges, and good performances of simultaneous impedance matching and gain are demonstrated for both resonant frequencies. However, all the conventional investigations are only for the case in which the antenna polarization is linear, while the dual-frequency antenna with circular polarization has
8、 not been touched yet. In this paper, we present a design method of a dual-frequency antenna with circular and linear polarizations based on the conventional investigation for the dual-frequency linear polarization antenna5 7, Two resonant frequencies are determined by adjusting the size of the patc
9、h element and the slot, as well as the location of the slot in the element. The key to obtain a circular polarization for an antenna is to satisfy the condition for exciting circular polarization, ie， the circular polarization can be excited by setting the perturbation segment ,at an appropriate loc
10、ation in the patch element, and by placing the feed point on the diagonal axis. We have experimentally obtained the circular and the linear polarizations at both lower and upper frequencies, and some experimental results will be demonstrated on the reflection loss at input port and the radiation pat
11、tern of the antenna. 1 Configuration of conventional dual-frequency patch antenna Figure 1 shows a configuration of a conventional dual-frequency antenna with linear polarization. Two resonant frequencies are behaved by etching two slots close and parallel to the radiating edges, and the lower reson
12、ant frequency is determined by the outline size of the patch element, while the upper resonant frequency is determined by the size and the position of the two slots in the patch element. Because the slots are closely located to the radiating edges, a minor perturbation of the TM10 mode can be expect
13、ed. As for unperturbed TM30 mode, since the slots are located where the current shall be significant, the current will be strongly modified, and a perturbed TM30 radiation pattern similar to the TM10 mode can be obtained. Our object in this investigation is to obtain a dual-frequency antenna with bo
14、th polarization in which the lower frequency is with a circular polarization and the upper frequency is with a linear polarization. For a square patch antenna with area of S fed by one port, a way to excite a circular polarization is to extend the length of two parallel subtends of the patch element
15、 in order to set a perturbation segment s, and place the feeding point on the diagonal axis. When an appropriate perturbation is selected, a circular polarization with a good axial ratio can be obtained. Fig 1 Configuration of conventional dual-frequency antenna 2 Designing for dual-frequency slot p
16、atch antennas with two polarizations In a conventional investigation for dual-frequency with linear polarization5 7 , to ensure a good radiation efficiency at both frequencies, theaspectratio between the two sides of the patch is taken in the range 0.7 0.8LW (1) and the lower frequency is determined
17、 by 2 ( ) ( / , )L rrcf W W W L t (2) where c is the velocity of light in free space， r is the effective permittivity given by 121 1 1 0( , ) 122ryyxyx (3) and 1 . 5 0 . 4wlWWWL (4) , rLW g tt (5) here 1 0 . 3 3 6 1, 0 . 2 8 0 . 2 7 4 l n ( 2 . 5 1 8 )0 . 5 5 6xyg x y x (6) and the upper frequency i
18、s determined by 2 ( 2 ) ( / , )H rrcf L l d w t (7) In above expression, the parameters of W, L, t, w, d, l are referred to Fig 2, and the r is the dielectric constant of the substrate. However, in this study, in order to obtain a circular polarization at the lower resonant frequency, the aspect rat
19、io of the outline size of the patch will be changed so that the mentioned formulas above will not be completely satisfied. Fig 2 Configuration of dual-frequency We present a design method for dual-frequency slot patch antenna with two polarization as follows. Step 1 Design a square patch antenna wit
20、h the desirable lower frequency f10 by using a conventional method. Step 2 Etch two slots close and parallel to the radiating to behave the dual-frequencies operation by using Eq (7). However the lower resonant frequency f10 will change, since the slots modify the current distribution of the TM10. S
21、tep 3 Adjust the size of the antenna element to obtain the desired lower resonant frequency. Step 4 Adjust the length and the width of the slots to obtain the upper resonant frequency f30. Step 5 Set perturbation segment and place the feed point on the diagonal axis of the antenna to obtain the circ
22、ular polarization at the lower resonant frequency f10. 3 Experimental results and discussion We experimentally designed a dual-frequency patch antenna with two polarizations. Figure 2 shows the configuration of the antenna, and the specifications are listed in Table 1. In this configuration of the antenna, we extended the L edge of the patch antenna to set the perturbation segment, and placed the feed point on the diagonal axis to achieve the circular polarization. Table 1 Specifications of a dual-frequency patch antenna with two polarization GPS VICS
