International Journal of Antennas and Propagation

International Journal of Antennas and Propagation / 2008 / Article

Research Article | Open Access

Volume 2008 |Article ID 792123 | https://doi.org/10.1155/2008/792123

Binod Kumar Kanaujia, Anil Kumar Singh, "Analysis and Design of Gap-Coupled Annular Ring Microstrip Antenna", International Journal of Antennas and Propagation, vol. 2008, Article ID 792123, 5 pages, 2008. https://doi.org/10.1155/2008/792123

Analysis and Design of Gap-Coupled Annular Ring Microstrip Antenna

Academic Editor: Deb Chatterjee
Received23 Dec 2007
Revised07 Mar 2008
Accepted21 Jul 2008
Published20 Aug 2008

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.

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 Figure 1(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].

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 (π‘₯,0) by a coaxial cable (π‘Ž1<π‘₯<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 π‘Š1=𝑏1βˆ’π‘Ž1 and π‘Š2=𝑏2βˆ’π‘Ž2 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 (Cp) and two fringing capacitances (Cf) 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 𝐢even=𝐢𝑝+𝐢𝑓+πΆξ…žπ‘“, where 𝐢𝑝=πœ€π‘Ÿπœ€0(𝑀/β„Ž) is the parallel plate capacitance between the strip and ground plane, and Cf is the fringing capacitance due to edge conductor [6]: 𝐢𝑓=12ξ‚ƒβˆšπœ€eο¬€π‘π‘π‘βˆ’πΆπ‘ξ‚„,(1) where 𝑐=3Γ—108 m/s and Zc are the characteristic impedance of the line and πœ€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] πΆξ…žπ‘“=πΆπ‘“ξ‚ƒξ‚€β„Ž1+𝐴𝑠tanh10π‘ β„Žξ‚ξ‚„βˆ’1ξ‚€πœ€π‘Ÿπœ€eff1/2,(2) where 𝐴=exp(βˆ’0.1exp(2.33βˆ’2.53𝑀/β„Ž)).

The odd mode capacitance is as shown in Figure 2(a) and given by 𝐢odd=𝐢𝑝+𝐢𝑓+𝐢gd+𝐢ga, where 𝐢gd is the capacitance between two strip lines through dielectric region, and 𝐢ga is the capacitance between two strip lines through air.

The gap capacitance 𝐢ga=12𝐾(π‘˜ξ…ž)πœ€0πΎξ€·π‘˜ξ€Έ,(3) where 𝐾(π‘˜) and 𝐾(π‘˜ξ…ž) are elliptic functions. Reference [5] defined as π‘˜=𝑠/β„Ž(𝑠/β„Ž)+2(𝑀/β„Ž),π‘˜ξ…ž=√1βˆ’π‘˜2,πΎξ€·π‘˜ξ…žξ€ΈπΎξ€·π‘˜ξ€Έ=⎧βŽͺβŽͺ⎨βŽͺβŽͺ⎩1πœ‹ξ‚ƒ2√ln1+π‘˜ξ…žβˆš1βˆ’π‘˜ξ…žξ‚„πœ‹,0β‰€π‘˜β‰€0.5,2√lnξ‚€ξ‚€1+π‘˜ξ‚/ξ‚€βˆš1βˆ’π‘˜ξ‚ξ‚ξ‚„,0β‰€π‘˜β‰€1.(4) Since the two strips have the electric flux between the air dielectric region so capacitance 𝐢gd=ξ‚€πœ€0πœ€π‘Ÿπœ‹ξ‚ξ‚†lncotβ„Žπœ‹π‘ ξ‚‡4β„Ž+0.65𝐢𝑓0.02√(𝑠/β„Ž)πœ€π‘Ÿ1+1βˆ’πœ€2π‘Ÿξ‚„.(5)

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] 𝐿1=πœ‡β„Žπœ‹π‘˜21𝐽𝑛,π‘šπ‘›ξ‚€π‘˜1π‘ξ‚π‘Œξ…žπ‘›ξ‚€π‘˜1π‘Žπ‘’ξ‚βˆ’π‘Œπ‘›ξ‚€π‘˜1π‘ξ‚π½ξ…žπ‘›ξ‚€π‘˜1π‘Žπ‘’ξ‚ξ‚„2,(6) where ξ€Ίξ€»=1𝑛,π‘š2π‘˜21ξ‚ƒξ€·π‘˜21𝑏21π½βˆ’1ξ‚ξ‚†π‘›ξ‚€π‘˜1𝑏1π‘’ξ‚π‘Œξ…žπ‘›ξ‚€π‘˜1π‘Ž1π‘’ξ‚βˆ’π‘Œξ…žπ‘›ξ‚€π‘˜1𝑏1π‘’ξ‚π½ξ…žπ‘›ξ‚€π‘˜1π‘Ž1𝑒2βˆ’4πœ‹2π‘˜21π‘Ž1π‘’ξ‚€π‘˜21π‘Ž21𝑒,πΆβˆ’11=πœ‡πœ€0πœ€π‘ŸπΏ1π‘˜21,π‘Œ1=πœ‹β„ŽπΈξ‚ƒξ‚€π‘ŽπΈπ‘ξ‚2𝐸𝑔(π‘Ž,π‘Ž)+𝑏𝐸𝑐2𝐸𝑔(𝑏,𝑏)βˆ’2π‘ŽπΈπ‘πΈ2𝑐.𝑦(π‘Ž,𝑏)(7) 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.

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 πœ€π‘’re and odd mode πœ€0re using even and odd mode capacitances [8] as πœ€π‘’re=πΆπ‘’πΆπ‘Žπ‘’,πœ€0re=𝐢0πΆπ‘Ž0,(8) where πΆπ‘Žπ‘’ and πΆπ‘Ž0 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 ξ‚€||Ξ“||𝑅=20log.(9)

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] 𝐸am where 𝐸bm and π‘š=1 are the electric field at the inner and outer peripheries of the mth ring, respectively. We are considering only two rings, therefore, the radiation pattern of the ARMSA is obtained by putting β†’ and 2 in the above equations.

3. Design Parameters

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


ParametersValue

Substrate material usedRT Duroid 5870
Effective relative permittivityΞ΅ eff  =  2.1936
Relative permittivity of the substrateΞ΅ r  =  2.32
Thickness of dielectric substrateh  =  0.159 cm
Inner radius of the inner ringa1  =  3 cm
Outer radius of the inner ringb1  =  6 cm
Feed point of the inner ringc  =  3.35 cm
Inner radius of outer ringa2  =  6.05 cm.
Outer radius of outer ringb2  =  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.


Gap length ↓ S  =  0.095 cmS  =  0.100 cmS  =  0.110 cm

Feed point 𝑃 l o w e r βˆ— 𝑃 u p p e r βˆ— βˆ— (ohms) 𝐹 l o w e r # (ohms) 𝐹 u p p e r # # (GHz) 𝑃 l o w e r (GHz) 𝑃 u p p e r (ohms) 𝐹 l o w e r (ohms) 𝐹 u p p e r (GHz) 𝑃 l o w e r (GHz) 𝑃 u p p e r (ohms) 𝐹 l o w e r (ohms) 𝐹 u p p e r (GHz) 𝑃 l o w e r (GHz)

c  =  3.001 cm39.4074.081.7271.89943.5873.931.7441.89953.0573.301.7781.900
c  =  3.101 cm37.5572.151.7541.93443.6272.061.7711.93455.9071.751.8061.934
c  =  3.201 cm36.5065.021.7881.97742.3564.161.8061.97753.2065.501.8421.977

* 𝑃 u p p e r : The peak value of input impedance for lower band.
** 𝐹 l o w e r : The peak value of input impedance for lower band.
# 𝐹 u p p e r : The resonance frequency for lower band.
## β†’ : The resonance frequency for upper band.

Feed point ↓ c  =  3.001 cmc  =  3.101 cmc  =  3.201 cm

Gap length 𝑃 l o w e r 𝑃 u p p e r (ohms) 𝐹 l o w e r (ohms) 𝐹 u p p e r (GHz) 𝑃 l o w e r (GHz) 𝑃 u p p e r (ohms) 𝐹 l o w e r (ohms) 𝐹 u p p e r (GHz) 𝑃 l o w e r (GHz) 𝑃 u p p e r (ohms) 𝐹 l o w e r (ohms) 𝐹 u p p e r (GHz) 𝐸 πœƒ (GHz)

S  =  0.09 cm35.3974.311.7101.89934.8172.411.7361.933733.6165.331.7701.9771
S  =  0.10 cm43.5873.931.7441.89943.6272.061.7711.933842.3564.161.8061.9771
S  =  0.11 cm53.0573.301.7781.90055.9071.741.8061.934353.2065.491.8421.9774

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.

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.

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.

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Copyright © 2008 Binod Kumar Kanaujia and Anil Kumar Singh. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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