**1. Introduction**

294 Ultra Wideband – Current Status and Future Trends

Conference (LAPC) 2011, Loughborough, UK.

[20] Liu L, Weng YF, Cheung SW, Yuk TI Foged LJ (2011) Modeling of cable for measurements of small monopole antennas. Loughborough Antennas & Propagation

> In wireless communication systems, microwave planar bandpass filters are employed in most applications. The broadband and multiband applications are renewing the interest in the design of planar broadband filters with low loss, compact size, high suppression of spurious responses, and improved stopband performances. However, since the ultrawideband (UWB) system covers very wide frequency range of 3.1 to 10.6 GHz and then may be interfered with the existing undesired narrow band from the 5.2 GHz or 5.8 GHz wireless local area network (WLAN) radio signals. Moreover, the WiMAX (3.5 GHz) and RFID (6.8 GHz) communications may interfere with the UWB system within the range defined by the FCC. Therefore, UWB bandpass filters with single- and multi-narrow notched bands are needed to avoid being interfered by the exiting RF signals. In order to obtain operations, several techniques have been reported in literatures based on UWB-bandpass filters with slotted resonators (Meeloon et al., 2007, 2008, 2009), UWB-bandpass filters with slotted resonators and embedded slotted feed (Meeloon et al., 2009, 2011), and UWB-bandpass filters with slotted resonators and embedded fold-slot feed (Meeloon et al., 2010, 2011).

> In this chapter, many advanced UWB-bandpass filters are presented based on slotted linear tapered-line resonator (SLTR) and slotted step-impedance resonator (SSIR) structures for size reduction and improved stopband performances. A comprehensive treatment of slotted resonators and both ends of the resonator with interdigital coupled lines is described. The design concept is demonstrated using two filter examples including one with an SLTR and another one with an SSIR. These filters have not only compact size but also a wider upper stopband resulting from resonator bandstop characteristics. Single-SLTR and single-SSIR filters are designed and constructed and their performances are extensively investigated in simulation and measurement. The proposed filters demonstrate their capabilities in suppression of spurious responses. Also, two-SLTR and two-SSIR filters are designed and

© 2012 Meeloon et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Meeloon et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

fabricated to prove that they improve the passband and upper stopband performances with sharpened rejection skirts outside the passband and widened upper stopband.

UWB-Bandpass Filters with Improved Stopband Performance 297

*g1 w2 w2 w2*

*g1*

*w2 w2 w2*

line has superior performances with better *S11* and *S21* in the passband. This means that it is

(a)

**Figure 1.** Interdigital coupled lines: (a) conventional one with *w*1 = 4*.*0mm, *l1* = 6*.*0mm, l*2* = 6*.*45mm, *g1* = 0*.*2mm, and *w2*=1*.*2mm and (b) optimized one with *w1*=4*.*0mm, *l1*=6*.*0mm, *l2* = 6*.*45mm, *g1* = 0*.*2mm,

(b)

*w2 w2 w2*

**Figure 2.** Compared responses of interdigital coupled lines with *w2* = 1*.*2mm for the conventional one



Fig. 3(a) shows a conventional λ*/*2 microstrip linear tapered-line resonator (LTLR). This resonator has inherently spurious resonant frequencies at 2*fo* and 3*fo*, where *fo* is the fundamental frequency, which may be too close to the desired wide passband. A microstrip SIR has been proposed as shown in Fig. 3(b) for higher stopband performances. In this chapter, microstrip SLTRs as shown in Fig. 3(c) and (d), composed of a microstrip taperedline with slots are proposed. A microstrip SSIR consisting of a microstrip stepped impedance

and *w2* = 0*.*5mm.

and *w2* = 0*.*5mm for the optimized one.

**2.2. SLTR and SSIR characteristics** 

more suited for use as a bandpass filter element than the conventional one.

*w1*

*w1*

*l1 l2*

*l1 l2*

Then, UWB-bandpass filters based on SLTR and SSIR with embedded slot feed structure for notched band are presented. The embedded slot feed at the end of resonators will be comprehensively described. The proposed filters have narrow notches in the passband, resulting from the embedded slot feed. The center frequencies and bandwidths of the notched band can be easily adjusted by tuning the length and width of the embedded slot parameters. The wider upper stopbands caused by resonator characteristics have been also obatained.

After that, UWB-bandpass filters with single-notched and dual-notched bands and improved stopband performance are proposed using SLTR and SSIR as multi-mode resonator (MMR) and embedded slotted feed. To avoid the existing interferences in the UWB passband, two different embedded slotted feed are employed to obtain two narrownotched bands. The center frequency and bandwidth of the notched bands can be controlled by adjusting the dimensions of the embedded slotted feed. To further suppress the upper stopband, the defected slot in λ/2 stepped impedance resonator fed by interdigital coupled line is introduced. Very good agreements between the measured and simulated filter characteristics have been obtained validating the proposed filter prototypes.

Finally, UWB-bandpass filters based on SLTR and SSIR with embedded fold-slot are presented. The proposed filters have narrow notches in the passband and size reduction, resulting from the embedded fold-slot. The length and width of the embedded fold-slot parameters resulting in their performances have been also studied.
