Literature Survey

According to there is a growing demand on antenna design for a brief structure with wideband operation. There are many researches in the open literature related to achieving the wideband performance. In [5]-[8], using parasitic elements as shown in Figure 1.3 (a), for providing additional current paths, can create more resonant frequencies adjacent to the center one and thus get the wideband operation. Modifying the shape of the feed line is another method for enhancing the bandwidth. It was demonstrated that, applying double or triple feeds to the main antenna structure (as Figure 1.3 (b)) can generate a pure and intense current distribution, leading to the improvements in the polarization properties and impedance bandwidth [9]-[12].

(a) (b)

Figure 1.3 Wideband antenna structure. (a) Use the parasitic element, (b) use the triple feed.

Besides, the UWB antennas proposed in the open literature mainly focus on the monopole and slot antenna. The wideband planar monopole antenna is one of UWB antennas that have attracted a lot of attention because it is simple in geometry, easy for manufacture and integration, and low-cost, and exhibits not only a good impedance matching but also stable radiation patterns over the bandwidths. There are many related studies to the type of antenna having been available in the open literature and most researches focus on the planar design antenna such as half-disc [13]-[14], circular [15]-[17], elliptical [18], triangle [19]-[21], rectangular [22]-[25], bow-tie [26]-[28],

hexagonal [29]-[30], and others with smooth edges [31]-[35], which provide the possible shapes of antennas suitable for UWB application. Most of them were designed with printed “fat” monopole structure to achieve UWB performance. There is usually a small gap between the fat monopole and the ground plane edge as shown in Figure 1.4, which is an important factor for impedance matching, especially in the high frequency range. For these antennas, in the lower operation frequency band, the current is mainly distributed over the monopole and the nearby ground plane edge, which is similar to the current of a resonant monopole. In the higher frequency band, the current is distributed around the gap, therefore, acting like a slot antenna. Due to these appropriate current paths across the full band provided by the antenna structure, the wideband operation is thus achieved.

gap gap

gap gap

gap gap

Figure 1.4 Geometry of the typical UWB antenna.

The other one, printed wide slot antennas has an attractive property of providing a wide operating bandwidth, especially for those having a modified tuning stub, such as the fork-like stub [36]-[39], the rectangular stub [40]-[41], and the circular stub [42]

inside the wide slot as shown in Figure 1.5. Moreover, there is some research results related to using the ceramic material or Low Temperature Co-fired Ceramic (LTCC) technology, according to that its high dielectric constant material reduces the antenna size and achieves the UWB applications [43]-[45].

(a) (b) (c)

Figure 1.5 Geometry of the typical wide slot UWB antennas with (a) rectangular feed, (b) circular feed, and (c) fork feed.

The frequency range for UWB systems approved by the FCC is between 3.1 GHz and 10.6 GHz. It might cause interference with the existing wireless communication systems, for example, the WLAN operating in 5.15-5.85 GHz. Therefore, the UWB antenna with a band-notched characteristic is required. To obtain the band-notched function, there are various methods to achieve it. The conventional methods are cutting a slot (i.e., U-shaped, V-shaped, arc-shaped, and a pie-shaped slot) on the patch [46]-[54], inserting a slit on the patch [55]-[57], embedding a quarter-wavelength tuning stub within a large slot on the patch [58], or using the split ring resonator (SRR) structure on the patch [59]-[60]. Another way is putting parasitic elements near the printed monopole as filters to reject the limited band [61]-[63] or introducing a parasitic open-circuit element, rather than modifying the structure of the antenna’s tuning stub [64]. Changing the feed structure is also a method to achieve the band-notched response such as using the lumped and distributed inductors and capacitors integrated on the top side of the substrate in front of the feed port [65] and inserting two quarter-wavelength tuning stubs or a resonance cell into the proposed feeding [66]-[67]. As mentioned above, at the notch frequency, the current mainly concentrates over the area of the cutting slot, the adding stub and parasitic element, thus, achieving the band notch function.

(a) (b) (c)

(d) (e) (f)

Figure 1.6 Geometry of the band notch UWB antenna design. (a) Cut a slot, (b) insert a slit, (c) embed a quarter-wavelength tuning stub, (d) use the SRR, (e) put a parasitic element, and (f) modify the feed line structure.

As regards the miniaturized antenna design for WLAN application, recently many conventional antennas have been successfully designed for WLAN applications, such as monopole antennas, PIFAs, patch antennas, and dielectric resonator antennas (DRA) [71]-[77]. For the kinds of the printed monopole antenna, the antenna is usually directly excited by a 50Ω microstrip line printed on the same (upper) side of the dielectric substrate [71]-[73]. This kind of internal antenna mainly uses the system ground plane as the antenna’s ground. The system ground-plane dimensions, to some extent, can strongly affect the antenna impedance bandwidth. And the other kind of DRA is attracted the attention of antenna researchers because of its wide impedance bandwidth and flexible coupling schemes [74]. Another kind of antenna design is PIFA that provides several merits of compact size, light weight and low cost [75]-[77]. For establishing future wireless transceiver modules provide flexibility by integrating all functional blocks using the multi-layer process and novel interconnection methods, SOP

technology is one solution. Many single-chip wireless transceivers have been designed, such as those fabricated in 0.25 μm at 1.8 GHz for DCS-1800 applications and at 2.45 GHz for Bluetooth applications [78]. To enhance the function of SOP for wireless communications, it has become very attractive to develop the SOP which integrates an antenna with a module efficiently. In the case that the embedded antennas are integrated with the low temperature co-fired ceramic (LTCC) package, semiconductor substrates, or the ceramic ball grid array package (CBGA), interferences between the antenna and the RF blocks may happen in highly integrated modules [79]-[85]. Usually the patch antenna is employed and stacked on the top above the circuit within the package. The configuration leads to an increase in the overall height of the package and results in narrow-band operation as well. In addition, due to the use of high permittivity dielectric material, the radiation efficiency of the antenna is also limited.

Besides, the antenna performance is affected by the operating environment, especially when metal bodies are nearby the antenna. The EM wave radiated from the antenna will induce currents on the metal bodies, which in turn will radiate back to the antenna and thus deteriorate the antenna function. This possible coupling will then cause degradation effects on the antenna performances. But, there are only few references concerned to the operating environment affecting the antenna characteristics, especially when metal bodies are nearby or under the antenna [68]-[70].