| The far radiation field of the microstrip antenna with rotational symmetric structure has been analyzed using the method of mathematical modeling, and 13 inferences of the phase and amplitude of the radiation field were obtained. Then, based on these inferences,8 rotational symmetric wideband or multiband omnidirectional microstrip antennas for communications have been designed and fabricated.1. Low-profile omnidirectional circularly polarized microstrip antenna with narrow band Based on the center-fed circular microstrip structure, a low-profile narrow band omnidirectional circularly polarized microstrip antenna is composed of 6 vortex slots and 6 shorting vias which are symmetrically employed to control the horizontal and vertical polarizations, respectively. The proposed antenna is small in size, low in profile, simple in structure, good in omni-directionality in the azimuth plane, and broad in beam-width in the elevation plane.2. Wideband omnidirectional circularly polarized antennaBased on the low-profile narrow band omnidirectional circularly polarized microstrip antenna, a wideband omnidirectional circularly polarized antenna works with the broad bandwidth and keeps the circularly polarized properties by decreasing the radius of the ground plane, partly removing the substrate, and increasing the profile. The proposed antenna works at three modes which include the TMoimode, the disc top-loaded monopole mode, and the TM02 mode. They can be coupled together which results the broad operation bandwidth. The measured results show that the proposed antenna radiates the right handed circularly polarized (RHCP) wave with the impedance bandwidth of 57.9% and the axial ratio (AR) bandwidth of 51.7%.3. Wideband ultra low-profile microstrip circularly polarized antennaBased on the center fed circular microstrip structure, a wideband ultra low-profile microstrip circularly polarized antenna is composed of shorting vias and slant branches which are symmetrically introduced to control the θ-and Ψ-polarizations. The proposed antenna works at three modes, simultaneously. The first and the third modes are the TM01 and TM02 modes of the circular patch, respectively, and the second mode which is controlled by the slant branches locates between them. With the conical radiation pattern, the proposed antenna radiates the right handed circularly polarized (RHCP) wave with the broad impedance bandwidth of 35.4%, the axial ratio (AR) bandwidth of 38%, the maximum gain of 4.2 dBic, and the profile of 0.025 λ.4. Wide band low-profile circularly polarized antenna mounted on the large metal platformBased on the center fed circular microstrip structure, a wide band low-profile circularly polarized antenna mounted on the large metal platform symmetrically employs the modified elliptic slots, the radial slots, and the shorting vias. The modified elliptic slots and the radial slots are used to control the Ψ-polarization, while the shorting vias are employed to radiate the θ-polarization. The proposed antenna works at a pair of degenerate modes of the modified elliptic slots. With the similar currents distributions on the patch at both of the modes, the proposed antenna radiates the left handed circularly polarized (LHCP) wave. The measurd results show that the proposed antenna has the impedance bandwidth of 19.4%, the axial ratio (AR) bandwidth of 25.3%, the maximum gain of 6.5 dBic, and the profile of 0.083 λ. Because the radiation properties are not sensitive to the size of the large ground plane, the proposed antenna can be mounted on the outdoor large metal platform. 5. Compact dual-band dual-circularly polarized omnidirectional microstrip antenna Based on the center fed circular microstrip structure, a compact dual-band dual-circularly polarized omnidirectional microstrip antenna symmetrically employs two kinds of different curved slots to control the two resonance frequencies. The θ-and Ψ-polarization are provided by the central coaxial probe and the curved slots, respectively. The proposed antenna works at TM02 mode and TM02 mode, and radiates the left handed circularly polarized (LHCP) wave and right handed circularly polarized (RHCP) wave, respectively. Moreover, the ratio of the two resonance frequencies can be tuned by adjusting the size of the curved slots. The proposed antenna is simple in structure, cheap in fabrication cost, low in profile, and suitable for the indoor communication systems.6. Dual-band horizontally and circularly polarized omnidirectional microstrip antennaBased on the center-fed circular microstrip structure, a dual-band horizontally and circularly polarized omnidirectional microstrip antenna symmetrically employs the curved slots on the ground plane to control the horizontal polarization in the far field. Operating at TM01 and TM02 modes, the proposed antenna radiates the right handed circularly polarized (RHCP) and the horizontally polarized waves, respectively, with the useful bandwidths of 9.6% and 14.9%, respectively. The ratio of the two resonance frequencies can be greatly changed by adjusting the curved slots on the ground plane. According to the operation bands, the proposed antenna can be applied to the WLAN and WiMAX systems.7. Triple-band omnidirectional microstirp antenna Based on the center-fed circular microstrip structure, a triple-band omnidirectional microstirp antenna symmetrically employs shorting vias and open elliptic slots on the patch. The measured results show that the proposed antenna resonances at 2.4 GHz,3.5 GHz, and 5.7 GHz with the impedance bandwidths of 3.7% (2.39-2.48 GHz),5.8% (3.33-3.53 GHz), and 17.5% (5.2-6.2 GHz) which belong to the top-loaded monopole mode, the TE110 and the TE210 modes of the open elliptical slots, respectively. The top-loaded mode is excited by the introduction of the shorting vias. With the size of 0.48 λ×0.48 λ×0.048 λ, the proposed antenna can be used as the indoor ceiling antenna for its low-profile.8. Multi-band omnidirectional microstrip printed monopole antennaInserting two currents bended structures into the microstrip line, a multi-band omnidirectional microstrip printed monopole antenna is obtained. With the bandwidths of 6.2% (2.35-2.5 GHz),8.5% (3.4-3.7 GHz),3.6% (4.1-4.25 GHz), and 3.8% (5.15-5.35 GHz), the proposed antenna resonances at 2.4 GHz,3.5 GHz,4.2 GHz, and 5.2 GHz, respectively. The measured results show that the two currents bended structures which are added on the microstrip line effectively suppresses the E-plane pattern distortion caused by the appearance of the multi-direction currents on the microstrip line.
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