A new circuit structure is proposed for design of dualband bandpass filters with a wide upper stopband. The unit cell of the circuit consists of two two-port networks in shunt connection; one is a coupled-line section of λ/4 long followed by a transmission line segment of identical length, and the other has the same elements but cascaded in reverse order, where λ/4 is the wavelength at arithmetic mean frequency of the two passbands. Higher-order circuits can be obtained by directly cascading two or more such cells and show improved frequency selectivity. In addition to on both sides of the passbands, transmission zeros are also created in the rejection bands. Analysis of the unit cell circuit is formulated by the transmission line theory, and design curves for one-and two-cell circuits are provided. In realization, interdigital capacitors are incorporated with the λ/4 coupled sections for compensating the effect of unequal modal phase velocities on the filter performance in the rejection band. Two circuits are fabricated and measured to validate the analysis.
IntroductionThe fourth-generation communications system (4G) makes the multimedia products attract more attentions of people when the multimedia commodity can be used in any time and in any place. In this paper, we propose a kind of planar monopole antenna for DVB-H system. For the reception of terrestrial television, the traditionally Yagi or whip antennas are often used. Due to their large size, they cannot be used in the reception of digital television on handheld devices (DVB-H), which is operated at the frequency band from 470 to 702 MHz [1]. DVB-H technology is widely applied to cellular phone, portable computer and PDA but a small internal DVB-H antenna solution suitable for handheld devices is still under development [2].While the antenna structure is kept as small as possible, it is not easy to match the antenna over the whole DVB-H band, from 470 to 702 MHz (relative bandwidth 30-40 %), having a typical matching criterion, e.g. -6 or -10 dB return loss. There are two familiar options to shrink DVB-H antenna size. First, we can use high relative dielectric constant material to reduce antenna size [3]. Second, we can use fractal or meander-line structure to reduce antenna area and volume. Our design approach is similar to the later method. Antenna Design MethodWe use a metal sheet to design a wideband digital TV small antenna with a size comparable with a common business card. As shown in Figure 1(a), the antenna body contains a square metal sheet and two metal strip lines of different lengths which are used as the feeding network. These two strip lines are folded vertically along the edge of the square metal sheet and separated from the square metal sheet with a slot gap s = 2 mm to avoid touching the square sheet antenna except the end points which are respectively connected with the square metal sheet. Such arrangement of this feeding network is to get a longer electrical length due to a larger phase constant. Figure 2 shows that these two strip lines attract the largest part of electric currents and so behave like two main current paths.When coupled with the square sheet, the strip line will have a resonant frequency according to its strip length. A short strip line will have a good high frequency response and a long strip line will lead to a good response at low frequency band. In order to generate multiple resonant frequencies for broadband antenna impedance bandwidth, we use two strip lines with different lengths to have two resonant frequencies of about 580 MHz and 800 MHz respectively. These two resonant frequencies are used to achieve the wide bandwidth performance as shown in Figure 3. The size of antenna body is 90×50 mm 2 and that of reference ground plane is 115×50 mm 2 . The physical lengths of the two slots gaps are 88 mm and 40 mm respectively. The strip lines are connected with the right corner of the front-side of the square metal sheet. To attain a broad bandwidth, we can extend the antenna tail length and fold the antenna tail downward 10 mm to attain a good matching performance at lo...
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