ANALYTICAL STUDY OF RADIAL GAP
CAPACITANCE FOR PROBE FEED
RECTANGULAR MICROSRIP PATCH ANTENNA

Kamal Prakash P; ey; Rakesh Kumar Singh; Anil Kumar

ANALYTICAL STUDY OF RADIAL GAP CAPACITANCE FOR PROBE FEED RECTANGULAR MICROSRIP PATCH ANTENNA

Kamal Prakash Pandey¹, Rakesh Kumar Singh², Anil Kumar³

Associate Professor, Dept. of ECE, SIET, Jhalwa, Allahabad, India
Assistant Professor, Dept. of ECE, SIET, Jhalwa, Allahabad, India
Assistant Professor, Dept. of ECE, SHIATS-DU, Allahabad, India

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Abstract

In this paper impedance (i.e. coaxial feed probe) is matched with patch of the microstrip antenna. At resonant frequency 3.137GHz. the impedance is 82 ohm For a particular design parameters of rectangular microstrip patch antenna. When impedance is 50 ohm bandwidth is 140 MHz for the same design parameters. The radial gap capacitance is analysed for the parameters as: L= 71.7mm a= 0.5 mm, h=12 mm, εr =1, ZL =250 , b=50 mm. and W= 71.7mm the symbol are used as usual as in figure below.

Keywords

Microstrip Patch Antennas, Patch Impedance, Radial Gap Capacitor, Coaxial Prob-Feed

INTRODUCTION

Microstrip patch antennas are known to have a narrow bandwidth. Several techniques have been introduced to improve the bandwidth of microstrip patch antennas [1]–[3]. Most of these attempts to increase the bandwidth. There have been many efforts to reduce the cross polarization and achieve wideband performance [4]–[6]. One of the most common approaches to increase the bandwidth of a microstrip patch antenna. It has been shown that the bandwidth of a microstrip patch antenna is a linear function of its dielectric thickness [7], increasing the bandwidth of the patch antenna with a thicker dielectric substrate also increases the series inductance. The circuit model of a coaxial probe feed rectangular microstrip patch antenna and its equivalent is [8] shown below in figure1 and figure2. The input impedance of a patch antenna is modeled as an RLC resonant frequency. The series inductor represents the feeding probe, and is proportional to the thickness of the dielectric. The inductance produced by the thin probe is compensated by using a parallel plate radial microstrip line and a series radial gap capacitor[9].

COAXIAL PROBE FEED MICROSTRIP PATCH ANTENNA

The novel feeding mechanism introduced is used in this communication. A thin,1mm diameter, via is used as the feeding probe of a patch on a 12 mm thick dielectric. The inductance produced by the thin probe is compensated by using a parallel plate radial microstrip line and a series radial gap capacitor. The proposed antenna is illustrated in fig. A parallel plate radial microstrip line is characterized by its substrate thickness, h, dielectric constant εr, radius of the feeding probe a, and radius of the line b. Characteristic impedance and wave number[10] of such a parallel plate radial microstrip line can be calculated as

(1)

(2)

where

(3)

The input impedance of a loaded parallel plate radial microstrip line can be calculated using the Riccati differential equation[11] as:

(4)

This equation has a closed form solution for the parallel plate radial microstrip lines with characteristic impedance presented in as:

(5)

(6)

RADIAL GAP CAPACITOR

In the antenna structure shown in figure1, there is a radial gap capacitance between the parallel plate radial microstrip lines and the patch antenna. The radial capacitance is a function of gap size g, radius of the parallel plate radial microstrip lines a, b, dielectric constant ???? , and thickness of the dielectric h. The thickness of metallic sheet is assumed to be zero. One can use the calculation results of the capacitance between two coupled microstrip lines[12].

Theoretical analysis of patch antenna, driven by coaxial feed and parasitic patch placed at the outer radiating edge of the patch, is analysed using the circuit theory concept, the open end of driven patch coupled to parasitic patch is considered as a real open circuit having scattering field at its end, which is designated by equivalent terminal capacitance CL given by [2]

(7)

where c? is velocity of light, ?eff the effective dielectric constant, ?il the conductor extension length due to fringing field, ZL (=ZA)the characteristic impedance of annular ring patch[13] given by

(8)

where

n₀ = free space intrinsic impedance.

= 120 π ohms

(9)

(10)

in which

(11)

here W is the width of patch, (t) thickness of the annular ring patch, ?r is the dielectric constant of the substrate of a microstrip antenna, ?e ' is the effective dielectric constant of the substrate. A transverse gap in the strip conductor of a microstrip line usually form a symmetrical arrangement with conductors of equal width on either side of gap (Figure1c). The equivalent circuit of gap coupling capacitance (Cs) and plate capacitance (C1)[14-19]. The values of the coupling capacitance (Cs) and plate capacitance (C1) in the equivalent circuit of the symmetrical gap are derived using hybrid mode analysis [20] and are given by

(12)

(13)

(14)

(15)

(16)

The matching impedance (i.e. coaxial feed probe is matched with frequency) is shown in the figure3. Here for a particular design parameters of rectangular microstrip antenna is analysed and it is clear that at resonance the impedance is about 82 ohm and resonance frequency is 3.137 GHz and for matching the 50 ohm impedance the bandwidth is required about 140 MHz for the same design parameters.

The figure4 shown here, the gap capacitance, variation of gap for different with of the patch of rectangular microstrip antenna. Here the capacitance of gap is decreases non linear when gap varied. The capacitance is decreased as shown in figure obtained by the Matlab prompt windows, that are good result as seen in the conventional result [9]. The design parameters are as L= 71.7mm a= 0.5 mm, h=12 mm, εr =1, ZL =250 , b=50 mm. and W= 71.7mm the symbols are used as figure1 and figure2 above.

CONCLUSION

In this paper it is concluded that for analyzed impedance and capacitance of the coaxial probe feed microstrip patch antenna are best to other conventional feeding technique. Here for a particular design parameters of rectangular microstrip antenna is analysed and seen that at resonance; the impedance is 82 ohm and resonance frequency is 3.137 GHz and for matching the 50 ohm impedance the bandwidth is 140 MHz. The gap capacitance, of the patch of rectangular microstrip antenna is decreased with non-linear of the gap of radial parallel line.

Figures at a glance


Figure 1	Figure 2	Figure 3	Figure 4

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