الفهرس 
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Abstract
This thesis addresses the design and analysis of multi-band circularly polarized microstrip antennas used for applications in modern wireless communications.Nowadays, the mobile personal communication systems and wireless networks are commonly used. Experience has revealed that the antennas suitable for these applications should be with small size and operating in different frequency bands. In addition, to prevent interference problems due to microstrip antenna, an efficient electromagnetic band gap (EBG) structure is introduced. For this purpose, multi-band new circularly polarized microstrip antenna design introduced in this work includes two antennas with each antenna has three configurations. The proposed antennas have produced a higher efficiency, an improved operational bandwidth, and a higher gain relative to the conventional microstrip antenna.The first antenna is a circularly polarized (CP) multi-band square microstrip antenna with diagonal U-slots and a pair of truncated corners. This antenna has a simple geometry and impedance tuning capability. The dimensions of this antenna are optimized to improve the impedance matching bandwidth. This antenna has been presented in three configurations. The first form is the truncated corners circular polarized microstrip antenna with two U-slots along the direction of the diagonal at 450 to achieve the multi-band. This form satisfies the circular polarization at a frequency of 2.775 GHz where the axial ratio (AR) bandwidth of (2.76 - 2.78 GHZ) is 0.72% with AR value is less than 3dB. This band is suitable for WIMAX application and at a frequency of 4.375 GHz where the axial ratio bandwidth of (4.338 - 4.39 GHZ), 1.19% with AR less than 3dB. This band is suitable for C-band applications. It is the U-slots, which actually produced multi-bands, but the circular polarization is realized only in two of the bands. The second form is the same antenna but with diagonal holes in the substrate. The addition of the holes structure is intended for increasing the bandwidth and the gain. This satisfies the circular polarization at a frequency of 2.225 GHz where the axial ratio bandwidth is (2.209-2.24 GHZ) i.e. 1.36% and at a frequency of 2.875 GHZ where the axial ratio bandwidth is (2.84-2.888 GHZ) i.e. 1.67% with AR < 3dB. These bands are suitable for WIMAX application and at a frequency of 4.325 GHZ where the axial ratio bandwidth is (4.313 - 4.358 GHZ) i.e. 1.05% with AR < 3dB. This band is suitable for C-band application. The third form is the same previous antenna but with EBG on all the area under the patch to improve the antenna efficiency which satisfies the circular polarization at a frequency of 3.35 GHz where the axial ratio bandwidth of (3.334 - 3.3689 GHZ) is 1.04% with AR < 3dB. This band is suitable for WIMAX application and at a frequency of 5.475 GHz where the axial ratio bandwidth of (5.4466-5.502 GHZ) is 1.01% with AR < 3dB. This band is suitable for WIFI application.However, many researchers are focusing on the development of patch antenna for performance enhancement. This has been relatively achieved using U-slot shapes inserted in the patch. Therefore, a compact circularly polarized N-shaped slots inserted in a microstrip patch antenna has made it significantly smaller than the conventional patch antenna is presented. This antenna includes three versions. The first design is the truncated corners circularly polarized microstrip antenna with three N-slots inserted axially with the feed for achieving the multi-band mode at a frequency of 2.31 GHz where the axial ratio bandwidth of (2.304-2.325GHZ) is 0.918% with AR < 3dB. This band is suitable for WIMAX application. This design realized another matched frequency of 4.3 GHZ where the axial ratio bandwidth of (4.273 - 4.33GHZ) is 1.356% . In addition to a third matching frequency of 4.78 GHZ where the axial ratio bandwidth of (4.767-4.797GHZ) is 0.617% with AR < 3dB. These bands are suitable for C-band application. The second design is the same previous antenna but with diagonal holes which increases the bandwidth and gain as well as satisfies the circular polarization at a frequency of 2.36 GHz. It realizes an axial ratio bandwidth of (2.374-2.386GHZ) i.e. 0.508% with AR < 3dB. This band is suitable for WIMAX application. It produces another matching frequency of 4.42 GHZ where the axial ratio bandwidth is (4.3896 - 4.44GHZ) i.e. 1.208% with AR < 3dB. This band is suitable for C-band application. The third design is the same previous antenna but with EBG structure on all the area under the patch to improve the antenna efficiency which satisfy the circular polarization at a frequency of 3.08 GHz where the axial ratio bandwidth is (3.04 – 3.112 GHZ) i.e. 2.179% with AR < 3dB. This band is suitable for WIMAX application. The results have revealed an increase in the bandwidth and the gain, as well as have shown improved efficiency. That improvement can be attributed to the smaller effective dielectric constant of the substrate because of the inserted air holes. In addition, it has been noticed that the number of resonant frequencies are less than the previous two cases with only some of them have realized the CP operation due to the stop band filter used (the EBG structure). However, the performance has improved because of the efficiency improvement, which has been improved due to the increased number of air holes used that caused a lower effective dielectric constant and lower dielectric losses. Another effect of lowering the effectiver is the shifting up of the fundamental resonance frequency to 3.08 GHz with wider CP bandwidth.