^{1}

^{2}

^{1}

^{2}

Theoretical investigation conducted on gap-coupled annular ring microstrip antenna is found to exhibit frequency tunability with the gap. The various parameters of the antenna such as input impedance, VSWR, return loss, and radiation pattern have been investigated as a function of gap length and feed point. It is found that the various parameters of gap-coupled microstrip antenna depend heavily on the gap length and feed points.

The annular ring microstrip antenna
(ARMSA) has been studied for a long time by a number of investigators [

In the present work, the authors have endeavored to design a gap-coupled concentric ARMSA on the basis of equivalent circuit model. The inner ring is a feed, and the outer ring is a parasitic element. The effect of mutual coupling is also taken into account along with variation of feed point and gap between the rings. The gap-coupled ARMSA can be used for dual band operation and especially in mobile communication. The main focus is on the effect of the gap length and feed point on the radiation pattern of the gap-coupled ARMSA.

In the concentric ARMSA, the structure having physical
gap is shown in Figure

(a) Concentric annular ring microstrip antenna. (b) Parallel gap-coupled microstrip lines.

The input impedance characteristics
of the gap-coupled ARMSA can be analyzed. Figure _{1}_{1}

Two concentric annular ring antenna
having width _{12} mode. Total line capacitance is taken up as a
parallel plate capacitance (_{p}_{f}_{f}_{c}

(a) Odd mode capacitances of coupled microstrip lines geometry. (b) Even mode capacitances of coupled microstrip lines geometry.

The odd mode capacitance is as shown in Figure

The gap capacitance

The gap-coupled ARMSA can be represented as the two
parallel microstrip lines. The equivalent circuit for ARMSA can be expressed as
the parallel combination of _{1}_{1}_{1}_{2}_{2}_{2}_{1}_{1}_{1}_{2}_{2}_{2}

(a) Modified equivalent circuit of gap-coupled ARMSA for odd mode. (b) Modified equivalent circuit of gap-coupled ARMSA for even mode.

Due to the presence of the parasitic ARMSA, the input impedance
of gap-coupled ARMSA differs from the impedance of single ARMSA. The effective
dielectric constant is due to the fringing and various other factors which can
be calculated for even mode

The return loss can be calculated as

The radiation pattern of the ARMSA is due to the
superposition of the fields radiated by all the apertures. The radiation
patterns of the

The gap-coupled ARMSA is designed with the following specifications
as given in Table

Designing specification of annular ring microstripantenna.

Parameters | Value |
---|---|

Substrate material used | RT Duroid 5870 |

Effective relative permittivity | _{eff} |

Relative permittivity of the substrate | _{r} |

Thickness of dielectric substrate | |

Inner radius of the inner ring | _{1} |

Outer radius of the inner ring | _{1} |

Feed point of the inner ring | |

Inner radius of outer ring | _{2} |

Outer radius of outer ring | _{2} |

Figure

Peak value of input impedance, resonance frequency for upper and lower band for different feed locations.

Gap length | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

Feed point
| ||||||||||||

39.40 | 74.08 | 1.727 | 1.899 | 43.58 | 73.93 | 1.744 | 1.899 | 53.05 | 73.30 | 1.778 | 1.900 | |

37.55 | 72.15 | 1.754 | 1.934 | 43.62 | 72.06 | 1.771 | 1.934 | 55.90 | 71.75 | 1.806 | 1.934 | |

36.50 | 65.02 | 1.788 | 1.977 | 42.35 | 64.16 | 1.806 | 1.977 | 53.20 | 65.50 | 1.842 | 1.977 |

*

^{**}

^{#}

^{##}

Peak value of input impedance, resonance frequency for upper and lower band for different gap lengths.

Feed point | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

Gap length
| ||||||||||||

35.39 | 74.31 | 1.710 | 1.899 | 34.81 | 72.41 | 1.736 | 1.9337 | 33.61 | 65.33 | 1.770 | 1.9771 | |

43.58 | 73.93 | 1.744 | 1.899 | 43.62 | 72.06 | 1.771 | 1.9338 | 42.35 | 64.16 | 1.806 | 1.9771 | |

53.05 | 73.30 | 1.778 | 1.900 | 55.90 | 71.74 | 1.806 | 1.9343 | 53.20 | 65.49 | 1.842 | 1.9774 |

Variation of input impedance with frequency for different gap
length at

The return
loss of gap-coupled ARMSA is shown in Figures

(a) Variation of return loss with frequency for different feed
location at

The radiation pattern of the gap-coupled
annular ring microstrip antenna in the direction of elevation and azimuth plane
is shown in Figures

(a) Variation of radiation pattern

The new technique to gap-coupled with parasitic element shows the tunability in frequency and also dual band operation. Efficiency of an antenna can be used where large bandwidth and tunable frequency is required.