Because the errors exist during manufacturing and testing, although the frequency bandwidth and AR bandwidth are diminished in tested results compared with the calculated results, the 20.6% (VSWR [lt] 2) bandwidth and the 27.4% 3-dB AR bandwidth are achieved, respectively, which are more than five times that of the conventional circularly polarized antennas. Figures 8 and 9 show the simulated and tested results of the radiation patterns in E plane and H plane of the antenna with PBG structure.
CONCLUSIONThe PBG structure used in the circularly polarized microstrip antennas can suppress the propagation of surface waves, enhance the efficiency of radiation, improve the circular polarization, and broaden the bandwidth, which have been proved in the calculated and tested results. However, two problems should be taken into account about the PBG structure: first, the PBG structure is used mainly in the substrates with high dielectric constant. This diminishes the dimensions of the patch and suppresses the propagation of the surface wave, but the dimensions of the substrate will be enlarged because more air columns are used to improve the band-gap. This means that the effective area for radiation is enlarged and the direction coefficient is increased effectively. So the effect of PBG structure used in the antenna is not more evident than the conventional antenna with large substrate. Therefore, how to select the number of columns and the lattice constant is very important. Second, the analysis of the PBG structure is mainly focused on the experiments, because the academic study is not mature at present. The performance should be improved when the PBG structures are used in high frequencies.antenna fed by a three-stub hybrid coupler, Microwave Opt Technol Lett 40 (2004), 38-41. 5. J.-W. Wu and J.-H. Lu, 2ϫ2 Circularly polarized patch antenna arrays with broadband operation, Microwave Opt Technol Lett 39 (2003), 360-363. 6. K.L. Chung and A.S. Mohan, A systematic design method to obtain broadband characteristics for singly-fed electromagnetically coupled patch antennas for circular polarization, ABSTRACT: In this article, we achieve the output observation in time and frequency domain of a rational harmonic mode-locked fiber laser (RHMLFL). The pulse amplitude of pulse train is equalized by operating at nonlinear modulation transfer function region of active modulator in RHM-LFL. Without adding any other additional predistortion components, amplitude equalized short pulses up to the fourth order rational harmonic modelocking are successfully demonstrated for 7.5, 10, and 15 GHz. One can extend such a method for various mode-locked lasers even with much higher repetition rates.Figure 5 RF spectrum of RHMLFL with repetition rate of (a) 7.5 GHz, (b) 10 GHz, (c) 15 GHz by amplitude equalization method 960