Tuning sampled nonlinearly chirped fiber Bragg gratings with adjustable chirp and fixed center wavelength

and 5.48 GHz, respectively. Obviously, both resonant frequencies significantly decrease. Taking another point of view, the area of the proposed semi-microstrip is much less than that of the conventional circular antenna, which operates at the fixed frequencies. The measured far-field radiation patterns at the two resonant frequencies for the case of S/R 0.5 are plotted in Figures 4 and in 5. Both the 1 st and 2 nd resonant frequencies have orthogonal polarization planes and broadside radiation patterns. However, the cross-polarization levels are different. Good cross-polarization levels for two principle planes at 4.26 GHz are obtained. But for 2.75 GHz, the H-plane cross-polarization is quite high. The different radiation patterns come from the difference of the surface current distributions of the two operating frequencies. From the simulation by IE3D, the current distribution of the 2 nd resonant frequency on the shorting plane has a direction similar to that of the semicircular patch. But for the first resonant frequency, the current distribution of the antenna is distorted. 3. CONCLUSION A semicircular microstrip antenna has been successfully implemented. The remarkable feature of the proposed antenna is dual frequency with a good impedance matching. As the width of the shorting plane decreases, the two operating frequencies are significantly reduced. Comparing with a conventional circular patch, the proposed antenna is smaller in size.