3.1.2 Measured results

3.1 UWB CPW-fed LPDA for wireless applications

UWB Technology - Circuits and Systems

<sup>50</sup> <sup>50</sup> 0.8 mm3

3.1.1 Geometry antenna and design

applications.

Figure 9.

108

Layout of the proposed log periodic dipole antenna (semi-LPDA) [39].

This section presents a LPDA for UWB applications [39]. The antenna consists of cascading four U-shaped elements of different line lengths with balun circuit to improve the antenna impedance matching. The proposed antenna area dimensions are 50 50 mm<sup>2</sup> built on FR4 substrate thickness 0.8 mm. Simulator HFSS is used for modeling the designed antenna. The pulse distortion is verified by measuring the proposed antenna performance with virtually steady group delay. The simulation and measurement results show that the designed antenna exhibits good impedance matching, stable radiation patterns throughout the whole operating frequency bands, acceptable gain and stable group delay over the entire operating band. LPDA is UWB with the multiple resonance property; its bandwidth can be enhanced by increasing the number of the dipole elements [40–42]. Balanced structure, CPW fed antennas are very good candidates since the feed lines and the slots are on the same side of the substrate. There are many researches done to design LPDA but most of the published papers are not compact and their length is near from wavelength. The compact antenna dimensions, as shown in Figure 9, are

. The proposed USLPDA antenna introduces USUWB with the

multiple resonant property and compact size compared to earlier designs where UWB was realized using a rectangular slot [43], U-shaped dipole elements or stubs [38]. The bandwidth of this antenna at 10 dB reflection coefficient extends from 1.85 to 11 GHz which is wide enough to cover the FCC approved UWB in addition to wireless communications. The antenna exhibits good performance and can operate at wireless

The designed geometry antenna is shown in Figure 9; the antenna is composed

of four different lengths with U-shaped stubs. The lengths and spacing of the elements of LPDA increase logarithmically from one end to the other. The design of the LPDA is used where a wide range of frequencies is needed while still having moderate gain and directionality. The simulator HFSS is used to validate and optimize by simulating the designed antenna. The designed antenna is built on a commercial FR4 substrate with dielectric constant ε<sup>r</sup> = 4.6, and loss tangent tan δ = 0.02. The antenna is fed by a 50 Ω transmission line, which can be easily integrated with other microwave circuits printed on the same substrate. For designing procedure,

The designed antenna is fabricated by using photolithographic technique, as shown in Figure 10, and parameter performance is measured. The simulated and measured input reflection coefficient of the antennas is in good agreement, as shown in Figure 10(b). The designed dipole impedance bandwidth at �10 dB of antenna extended from 1.85 to 11 GHz to cover most of the wireless applications and FCC UWB regulation. The antenna gain data are compared between measured and simulated results as shown in Figure 11. The designed LPDA antenna achieves simulated average gain 5.5 dBi, and the peak realized gain around 6.5 dBi at 2.7 GHz as shown in Figure 11(a). The measured results show very good agreement with simulated results and about �3 dBi difference on average over the operating band. Wheeler cap method [44] can be used to calculate the antenna radiation efficiency. The average radiation efficiency is around 70% over the operating band as shown in Figure 11(b).

Then the radiation efficiency measured result is done by using horn antenna to complete the designed antenna radiation efficiency measurement as shown in Figure 11(b). In the designed antenna, the radiator and the ground plane are contributing to radiation. For UWB applications, omnidirectional radiation pattern is an important requirement. At lower frequency band of operation, the pattern resembles a conventional dipole antenna, but at higher end of the UWB spectrum,


Table 2. Dimensions of the proposed antenna (dimensions in mm) [39].

Figure 10.

(a) GD simulated structures and (b) comparison between measured and simulated GD of LPDA [40].

is to derive Q/ω for |S21| phase. However, it is desirable that the same antenna is used for transition and receiving. High GD variations occur due to the steep phase shift over frequency, which may cause unsatisfactory distortions in the signal. Figure 12 illustrates the simulated GD, and it can be noticed that the average group

Comparison between simulated and measured results (a) gain and (b) radiation efficiency of LPDA [40].

This section presents designed steps to model an UWB monopole antenna. The designed antenna is composed of three different lengths of semi-circular shapes connected with circular disk and half circular modified ground plane. The designed antenna has an area equal 50 50 mm<sup>2</sup> on a low cost FR4 substrate [45]. The antenna demonstrates impedance bandwidth of 10 dB extended from 1.5 to 11 GHz with discontinuous bandwidth at different interior operating bands. Two pairs of SRR as metamaterial structure cells are inserted closely located from feeding transmission line of the antenna to achieve good impedance matching over the entire band of operation and improve the antenna performance. The fundamental parameters of the antenna including reflection coefficient, gain, radiation pattern, and group delay are obtained, and they meet the acceptable UWB antenna standard. Simulator HFSS ver. 14 is used as full wave electromagnetic solver then the prototypes are fabricated and measured. Results show that the antenna is very suitable

3.2 UWB monopole antenna using split ring resonator structure

Passive Components for Ultra-Wide Band (UWB) Applications

DOI: http://dx.doi.org/10.5772/intechopen.88444

for the applications in UWB as well as wireless communication systems.

attractive candidate because of its wide impedance bandwidth, omnidirectional azimuthal radiation pattern, low profile, and ease of integration with active devices and fabrication [46]. The design equation for lower band edge frequency has been reported in the literature. Moreover, these antennas are analyzed by using the frequency domain characteristics like return loss, gain, radiation pattern, surface current distribution, and group delay. Different narrowband services like WLAN, WiMax, GSM, UMTS, Wi-Fi, WMTS, ISM, UNII, DECT, European Hiper LAN I, II, and UWB (3.1–10.6 GHz) applications [1] could be obtained by using single UWB antenna.

For use in UWB systems, printed monopole patch antenna (PMPA) is an extremely

A SRR is one of the electrically smallest resonant elements. It has many applica-

tions ranging from compact filters to advanced metamaterials. SRR has also a significant importance in electrically small antennas [46]. Metamaterials are good candidate for enhancement of the performances of different antennas. There are

delay is about 1.5 <sup>10</sup><sup>9</sup> second.

Figure 12.

111

Figure 11. (a) Fabricated LPDA antenna and (b) |S11|comparison between simulated and measured results [40].

some ripples are observed which are attributed to higher order modes. There are discrepancies observed at higher frequency band spectrum, which arises due to measurement setup.

For UWB applications, group delay is an important factor in communication systems, for example, medical applications systems, security systems, and satellite communication systems, which are used for transmitting. To avoid occurring of distortion, it is recommended that the spectrum is treated in the same manner, over the proposed bandwidth of frequencies. When GD ripples are large, they may cause unsatisfactory distortion in the signal of a transmitting radio system. So, in radio system designs, there are specifications for how much a GD may be accepted. In nonlinear systems, nonlinear distortion happens since the magnitude of frequency response is not constant, and the phase response is nonlinear. The phase distortion could be used to measure GD, the phase characteristics must have a linear slope so that the ratio is constant for all frequencies and this represents a constant GD [44]. To measure the GD between two antennas with spacing d = 1 m, the usual practice

Passive Components for Ultra-Wide Band (UWB) Applications DOI: http://dx.doi.org/10.5772/intechopen.88444

Figure 12.

Comparison between simulated and measured results (a) gain and (b) radiation efficiency of LPDA [40].

is to derive Q/ω for |S21| phase. However, it is desirable that the same antenna is used for transition and receiving. High GD variations occur due to the steep phase shift over frequency, which may cause unsatisfactory distortions in the signal. Figure 12 illustrates the simulated GD, and it can be noticed that the average group delay is about 1.5 <sup>10</sup><sup>9</sup> second.

### 3.2 UWB monopole antenna using split ring resonator structure

This section presents designed steps to model an UWB monopole antenna. The designed antenna is composed of three different lengths of semi-circular shapes connected with circular disk and half circular modified ground plane. The designed antenna has an area equal 50 50 mm<sup>2</sup> on a low cost FR4 substrate [45]. The antenna demonstrates impedance bandwidth of 10 dB extended from 1.5 to 11 GHz with discontinuous bandwidth at different interior operating bands. Two pairs of SRR as metamaterial structure cells are inserted closely located from feeding transmission line of the antenna to achieve good impedance matching over the entire band of operation and improve the antenna performance. The fundamental parameters of the antenna including reflection coefficient, gain, radiation pattern, and group delay are obtained, and they meet the acceptable UWB antenna standard. Simulator HFSS ver. 14 is used as full wave electromagnetic solver then the prototypes are fabricated and measured. Results show that the antenna is very suitable for the applications in UWB as well as wireless communication systems.

For use in UWB systems, printed monopole patch antenna (PMPA) is an extremely attractive candidate because of its wide impedance bandwidth, omnidirectional azimuthal radiation pattern, low profile, and ease of integration with active devices and fabrication [46]. The design equation for lower band edge frequency has been reported in the literature. Moreover, these antennas are analyzed by using the frequency domain characteristics like return loss, gain, radiation pattern, surface current distribution, and group delay. Different narrowband services like WLAN, WiMax, GSM, UMTS, Wi-Fi, WMTS, ISM, UNII, DECT, European Hiper LAN I, II, and UWB (3.1–10.6 GHz) applications [1] could be obtained by using single UWB antenna.

A SRR is one of the electrically smallest resonant elements. It has many applications ranging from compact filters to advanced metamaterials. SRR has also a significant importance in electrically small antennas [46]. Metamaterials are good candidate for enhancement of the performances of different antennas. There are

some ripples are observed which are attributed to higher order modes. There are discrepancies observed at higher frequency band spectrum, which arises due to

(a) Fabricated LPDA antenna and (b) |S11|comparison between simulated and measured results [40].

(a) GD simulated structures and (b) comparison between measured and simulated GD of LPDA [40].

For UWB applications, group delay is an important factor in communication systems, for example, medical applications systems, security systems, and satellite communication systems, which are used for transmitting. To avoid occurring of distortion, it is recommended that the spectrum is treated in the same manner, over the proposed bandwidth of frequencies. When GD ripples are large, they may cause unsatisfactory distortion in the signal of a transmitting radio system. So, in radio system designs, there are specifications for how much a GD may be accepted. In nonlinear systems, nonlinear distortion happens since the magnitude of frequency response is not constant, and the phase response is nonlinear. The phase distortion could be used to measure GD, the phase characteristics must have a linear slope so that the ratio is constant for all frequencies and this represents a constant GD [44]. To measure the GD between two antennas with spacing d = 1 m, the usual practice

measurement setup.

Figure 11.

110

Figure 10.

UWB Technology - Circuits and Systems

varieties of SRR structures that have been reported in the literature like square, circular, triangular, omega, and labyrinth resonator [47].
