Analysis and Design of Gap-Coupled Annular Ring Microstrip Antenna

Abstract

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.

Binod Kumar Kanaujia 1 and Anil Kumar Singh 2

Recommended by Deb Chatterjee

1, Department of Electronics and Communication Engineering, Faculty of Engineering and Technology, M. J. P. Rohilkhand University, Bareilly 243006, India
2, Department of Electronics and Instrumentation Engineering, Faculty of Engineering and Technology, M. J. P. Rohilkhand University, Bareilly 243006, India

Received 23 December 2007; Revised 7 March 2008; Accepted 21 July 2008

1. Introduction

The annular ring microstrip antenna (ARMSA) has been studied for a long time by a number of investigators [1, 2] because of larger bandwidth as compared to the other conventional microstrip patch antennas [1]. The interest in designing of such microstrip antenna has increased because of light weight, easier to fabrication, conformability, and so forth. The inner and outer radii of the ARMSA control the mode separation.

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.

2. Theoretical Considerations

In the concentric ARMSA, the structure having physical gap is shown in Figure 1(a). The inner ring is fed coaxially, while the outer ring is a parasitic element. Now, this can also be shown as a parallel gap-coupled radiator using planar waveguide mode for inner ring and outer ring as shown in Figure1(b) [3]. The characteristic impedance of the two-gap-coupled concentric ARMSA radiator can be analyzed by applying the theory of coupled microstrip lines [4, 5] and coupled microstrip antenna [6].

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

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

The input impedance characteristics of the gap-coupled ARMSA can be analyzed. Figure 1(a) which shows two-gap-coupled annular ring antennas in which inner one is fed at point (x,0) by a coaxial cable (_a1 <x<_b1 ), where a₁ and b₁ are inner and outer radii of the inner ring and outer ring, respectively. The thickness of the substrate h is small as compared to the difference between the inner and outer radii of the inner ring.

2.1. Even and Odd Mode Capacitances

Two concentric annular ring antenna having width _W1 =_b1 -_a1 and _W2 =_b2 -_a2 of Figure 1(a) can be shown, as a gap-coupled microstrip line of Figure 1(b), by using planer waveguide model. The inner ring of annular ring antenna of Figure 1(b) is excited in TM₁₂ mode. Total line capacitance is taken up as a parallel plate capacitance (C_p ) and two fringing capacitances (C_f ) as shown in Figures 2(a) and 2(b) for even and odd modes, respectively. The even mode capacitance is the capacitance between two parallel running conductors with respect to ground. From Figure 2(b), it is given as _Ceven =_Cp +_Cf +^{Cf[variant prime]} , where _Cp =_{[straight epsilon]r[straight epsilon]0} (w/h) is the parallel plate capacitance between the strip and ground plane, and C_f is the fringing capacitance due to edge conductor [6]: [figure omitted; refer to PDF] where c=3×¹⁰⁸ m/s and Z_c are the characteristic impedance of the line and _{[straight epsilon]eff} is the effective dielectric constant of substrate [7]. Since there is another line due to the presence of parasitic element, there is some modification in fringe capacitance as [6] [figure omitted; refer to PDF] where A=exp(-0.1 exp(2.33-2.53 w/h)) .

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

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

The odd mode capacitance is as shown in Figure 2(a) and given by _Codd =_Cp +_Cf +_Cgd +_Cga , where _Cgd is the capacitance between two strip lines through dielectric region, and _Cga is the capacitance between two strip lines through air.

The gap capacitance [figure omitted; refer to PDF] where K(k) and K(^{k[variant prime]} ) are elliptic functions. Reference [5] defined as [figure omitted; refer to PDF] Since the two strips have the electric flux between the air dielectric region so capacitance [figure omitted; refer to PDF]

2.2. ARMSA Analysis

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 Y₁ , L₁ , C₁ and Y₂ , L₂ , C₂ , where the subscript 1 represents for inner ring and 2 for the outer parasitic ring. The value of Y₁ , L₁ , C₁ can be written as [8] [figure omitted; refer to PDF] where [figure omitted; refer to PDF] The values of L₂ , C₂ , and Y₂ are obtained for the outer parasitic ring using (6), (7) for the inner ring of the antenna. The equivalent circuit for gap-coupled ARMSA can be given as in Figures 3(a) and 3(b) for even and odd mode cases.

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

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

2.3. Impedance of Gap-Coupled ARMSA

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 ^{[straight epsilon]ree} and odd mode ^{[straight epsilon]re0} using even and odd mode capacitances [8] as [figure omitted; refer to PDF] where ^Cea and ^C0a are the even and odd mode capacitances for the gap-coupled ARMSA with air as dielectric. With the help of these dielectric constants and the equivalent circuit model (Figures 3(a) and 3(b)), input impedance for even mode, _Zin (e) , and for odd mode, _Zin (0) , are calculated separately by putting even and odd mode relative permittivity (given in (8), respectively. The input impedance of gap-coupled ARMSA is now written as [6] _Zin =_Zin (e)+_Zin (0) .

The return loss can be calculated as [figure omitted; refer to PDF]

2.4. Radiation Patterns

The radiation pattern of the ARMSA is due to the superposition of the fields radiated by all the apertures. The radiation patterns of the N apertures are [9] [figure omitted; refer to PDF] where _Eam and _Ebm are the electric field at the inner and outer peripheries of the m th ring, respectively. We are considering only two rings, therefore, the radiation pattern of the ARMSA is obtained by putting m=1 and 2 in the above equations.

3. Design Parameters

The gap-coupled ARMSA is designed with the following specifications as given in Table 1.

Table 1: Designing specification of annular ring microstripantenna.

Parameters	Value
Substrate material used	RT Duroid 5870
Effective relative permittivity	[straight epsilon] eff = 2.1936
Relative permittivity of the substrate	[straight epsilon] r = 2.32
Thickness of dielectric substrate	h = 0.159 cm
Inner radius of the inner ring	a1 = 3 cm
Outer radius of the inner ring	b1 = 6 cm
Feed point of the inner ring	c = 3.35 cm
Inner radius of outer ring	a2 = 6.05 cm.
Outer radius of outer ring	b2 = 9.05 cm.

4. Discussion of Results

Figure 4 shows the variation of input impedance with frequency for different gap length for particular feed location of 3.001 cm. It is observed that the lower band shows the different resonant frequencies of 1.727 GHz and 1.778 GHz for different gap lengths of 0.095 cm. and 0.11 cm, however, for an upper band of gap-coupled ARMSA, the resonance frequency is approximately the same for different gap length. This type of characteristic can be used for different applications. It is also observed that peak value of real part of the input impedance for lower patch increases with increasing gap length. The variation of input impedance with frequency for different feed location and gap length is also verified from Tables 2 and 3, respectively.

Table 2: Peak value of input impedance, resonance frequency for upper and lower band for different feed locations.

Gap length [arrow right]	S = 0.095 cm	S = 0.100 cm	S = 0.110 cm
Feed point[arrow down]	Plower * (ohms)	Pupper ** (ohms)	Flower # (GHz)	Fupper ## (GHz)	Plower (ohms)	Pupper (ohms)	Flower (GHz)	Fupper (GHz)	Plower (ohms)	Pupper (ohms)	Flower (GHz)	Fupper (GHz)
c = 3.001 cm	39.40	74.08	1.727	1.899	43.58	73.93	1.744	1.899	53.05	73.30	1.778	1.900
c = 3.101 cm	37.55	72.15	1.754	1.934	43.62	72.06	1.771	1.934	55.90	71.75	1.806	1.934
c = 3.201 cm	36.50	65.02	1.788	1.977	42.35	64.16	1.806	1.977	53.20	65.50	1.842	1.977

* _Plower : The peak value of input impedance for lower band.

^** _Pupper : The peak value of input impedance for lower band.

^# _Flower : The resonance frequency for lower band.

^## _Fupper : The resonance frequency for upper band.

Table 3: Peak value of input impedance, resonance frequency for upper and lower band for different gap lengths.

Feed point [arrow right]	c = 3.001 cm	c = 3.101 cm	c = 3.201 cm
Gap length[arrow down]	Plower (ohms)	Pupper (ohms)	Flower (GHz)	Fupper (GHz)	Plower (ohms)	Pupper (ohms)	Flower (GHz)	Fupper (GHz)	Plower (ohms)	Pupper (ohms)	Flower (GHz)	Fupper (GHz)
S = 0.09 cm	35.39	74.31	1.710	1.899	34.81	72.41	1.736	1.9337	33.61	65.33	1.770	1.9771
S = 0.10 cm	43.58	73.93	1.744	1.899	43.62	72.06	1.771	1.9338	42.35	64.16	1.806	1.9771
S = 0.11 cm	53.05	73.30	1.778	1.900	55.90	71.74	1.806	1.9343	53.20	65.49	1.842	1.9774

Figure 4: Variation of input impedance with frequency for different gap length at C = 3.001 cm.

[figure omitted; refer to PDF]

The return loss of gap-coupled ARMSA is shown in Figures 5(a)-5(b) for different value of gap length and feed locations. Return loss of lower band decreases with increasing the feed point, while it decreases with increasing the feed point for upper band. It is also observed that the bandwidth of lower band decreases with increasing the feed point, however, the bandwidth of upper band increases with increasing the feed point for a different gap length.

(a) Variation of return loss with frequency for different feed location at S = 0.095 cm. (b) Variation of return loss with frequency for different feed location at S = 0.11 cm.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

The radiation pattern of the gap-coupled annular ring microstrip antenna in the direction of elevation and azimuth plane is shown in Figures 6(a)-6(b), respectively. It is observed that the beamwidth of gap-coupled annular patch is lower than the individual annular patch but the side lobes of gap-coupled annular patch are higher than the individual annular ring patch because enhancement in radiation power due to parasitic element is not accompanied by an increase in ohmic loss of the system.

(a) Variation of radiation pattern _Eθ with angle for S = 0.095 cm. (b) Variation of radiation pattern _E[varphi] with angle forS = 0.095 cm.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

5. Conclusion

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.

References

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