magnetic software. The simulated |S11| for the antenna design steps are depicted in Figure 3. However, the overall impedance bandwidth for the proposed U-shaped log periodic dipole model is much wider.
The introduced design started by conventional dipole with length 45 mm as shown in Figure 2(a) which resonates at 2.4 GHz as shown by dashed black line in Figure 3. The second step of design is adding balun circuit to improve the antenna bandwidth as shown in Figure 2(b) and the corresponding result is shown as solid red line in Figure 3. First US-LPDA is added in the third step of design as shown in Figure 2(c), this adds two extra resonant frequencies as
(a) (b) (c) (d) (e)
Figure 2. From (a) to (e) evolution of the design steps of the proposed US-LPDA.
Figure 3. Design procedures of the USLPDA antenna.
shown as blue dashed line in Figure 3. Continuing the design by adding the second U-shaped element, as shown in Figure 2(d), the response is shown as green line in Figure 3. In addition, a third element is added as shown in Figure 2(e) and its response is show in Figure 3 as brown dashed line. Final design as shown in Figure 1 and the corresponding |S11| results are shown in Figure 4. There are two orientations of the elements arrangement with the same lengths either from small size element length to large size element or vice versa as shown in Figure 4(a). The reflection coefficients |S11| of both orientations are shown in Figure 4(b). The orientation from small to large size elements gives lower antenna resonant frequency at 1.5 GHz with poor impedance matching, while the other orientation from large to small size elements gives resonant frequency at 1.85 GHz and good impedance matching.
The effects of each arms of the proposed antenna are also studied and the simulated reflection coefficient of varied each arm and kept the other arms fixed are shown in Figure 5.
Figure 5 Shows the effect of varies L1, L2, L3 and L4 and the corresponding results are shown in Figure 5(a) to Figure 5(d). Optimized antenna dimensions are shown in Table 2. Simulated current density distributions of the USLPDA with four elements are shown in Figure 6 at different resonant frequencies take place at 1.85 GHz, 2.45 GHz, 3.5 GHz, 5.5 GHz, 7.5 GHz and 10 GHz. The current distribution of the proposed antenna is studied to verify the operation of the USLPDA. The largest element fundamental resonant frequency of the multi arms is 1.75 GHz as shown in Figure 6(a). The highest magnitude of current (red) is related to the corresponding element of radiation.
Group Delay is an important factor in communication systems especially ultra-wideband for example medical applications systems, security systems and satellite communication systems which are used for transmitting wideband data, because the distortion causes retraction of the S/N ratio  -  . Flat and consistent GD with frequency is important. 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
Figure 4. (a) Two different orientations of USLPDA and (b) the corresponding reflection coefficient |S11.|.
(a) (b) (c) (d)
Figure 5. (a) to (d) Simulated S-parameters of proposed LPDA with varies arms L1, L2, L3 and L4, respectively.
Table 2. Dimensions of the proposed antenna (dimensions in mm).
unsatisfactory distortion in the signal of a transmitting radio system. So, in radio system design there is usually a specification for how much a GD that may be accepted. In nonlinear systems nonlinear distortion happens since the magnitude of
(a) (b) (c) (d) (e) (f)
Figure 6. From (a) to (f) surface current densities for the USLPDA at 1.85, 2.45, 3.5, 5.5, 7.5 and 10 GHz, respectively.
frequency response is not constant and the phase of frequency response is nonlinear. By using GD the phase distortion could be measured, the phase characteristics must have a linear slope so that the ratio is constant for all frequencies and this represents a constant GD  . To measure the GD between two antennas with spacing d = 1 m, the usual practice is to derive Q/ω from |S21| phase. However, it is desirable the same antenna be used for transition and receiving antenna. High GD variations, due to the steep phase shift over frequency, may cause unsatisfactory distortion in the signal. Figure 7 illustrates the simulated GD, and it can be noticed that the average group delay is about 1.5 × 10−9 second.
4. Implementation and Measured Results
Prototype of the proposed antenna is fabricated on FR4 substrate by using photolithographic technique, as shown in Figure 8 and performance parameters are measured. The simulated and measured input reflection coefficient of the antennas is in very good agreement, as shown in Figure 8(b). Impedance −10 dB bandwidth of the proposed dipole antenna extended from 1.85 GHz to 11 GHz to cover most of wireless applications and FCC UWB regulation. The measurements were carried out by using a Rohde & Schwarz ZVA67 vector network analyzer operating from 50 MHz to 67 GHz. The comparisons between measured and simulated results of antenna gain and radiation efficiency are also studied as shown in Figure 9. The USLPDA 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 9(a). The measured results show very good agreement with simulated results and about ±3 dBi difference on average over the operating band. Wheeler
Figure 7. (a) GD Simulated structures and (b) comparison between measured and simulated GD of USLPDA.
Figure 8. (a) Fabricated USLPDA antenna and (b) |S11| comparison between simulated and measured results.
Figure 9. Comparison between simulated and measured results (a) gain and (b) radiation efficiency of USLPDA.
cap method   can be used to calculate so that the antenna radiation efficiency was simulated for the proposed antenna by using. The average radiation efficiency is around 70% over the operating bands as shown in Figure 9(b). Then the measured result of the radiation efficiency is done by using horn antenna to complete the proposed antenna radiation efficiency measurement as shown in Figure 9(b). Simulation and measured results for the two dimensional radiation patterns of two main planes (XZ and XY) are depicted in Table 3 at
Table 3. Simulated and measured results of the proposed antenna radiation patterns in both XY and XZ planes at different frequencies. 1.85 GHz, 2.45 GHz, 3.5 GHz, 5.5 GHz and 7.5 GHz.
different resonant frequencies 1.85 GHz, 2.45 GHz, 3.5 GHz, 5.5 GHz, and 7.5 GHz, respectively. In the proposed antenna, the radiator and the ground plane are contributing to radiation. Omnidirectional radiation pattern is an important requirement for UWB applications. At lower frequencies of operation, the pattern resembles a conventional dipole antenna, but at higher end of the UWB spectrum some ripples are observed which are attributed to higher order modes. Some discrepancies are observed at higher frequency band spectrum which arises due to measurement setup. The simulated and measured results suggest that the proposed antenna shows satisfactory omnidirectional radiation characteristics throughout the UWB band.
A new ultra-wideband antenna consists of U shaped log-periodic dipole antenna (USLPDA) has been proposed in this paper. The dipole is cascaded with four-U shaped elements to create an ultra-wideband extended from 1.85 GHz to 11 GHz. The proposed technique not only results in miniaturization of the antenna but also provides very stable radiation patterns throughout the whole frequency band. The proposed antenna can be easily fabricated on any commercially available substrates using the present design guidelines. This antenna has an average gain of 5.5 dBi and 70% average radiation efficiency over the operating resonant frequencies. These features make the proposed antenna suitable for different wireless communication systems as well as UWB applications.
This work is funded by the National Telecom. Regulatory Authority (NTRA), Ministry of Communications and Information Technology (MCIT), Egypt through a contract with Electronics Research Institute.
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