RADIATION EFFICIENCY AND IMPEDANCE BANDWIDTH OF SINGLE CPW-FED SLOT ANTENNA ON CONDUCTOR-BACKED TWO-LAYER SUBSTRATE

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BACKGROUND AND OBJECTIVES

Slot antennas fed by coplanar waveguide (CPW) exhibit various attractive qualities including significantly wider impedance bandwidth than microstrip patch antennas. Fig. 1.1 shows a CPW-fed rectangular slot dipole on a single-layer substrate, which is the simplest kind of practical CPW-fed slot with a standing-wave-type field distribution. The basic operation of such a slot is roughly as follows: the resonant frequency can be adjusted by adjusting the slot half-length L, while input impedance and bandwidth can be varied through adjustment of the slot width W (of course, fine-tuning could entail joint adjustments of these dimensions). Narrower slots yield wider E-plane radiation beamwidths [1]; the E-plane coincides with the plane φ = 90◦ in Fig. 1.1. The fractional bandwidth (VSWR < 1.5) of such an antenna on an electrically thin substrate may exceed 10% [2], which is substantially greater than that of a rectangular microstrip patch on an electrically thin substrate. Recent years have seen a spate of articles introducing modified geometries aimed at enhancing the bandwidth even further, e.g., [2–5], or achieve dual-band operation [6, 7]. A recently introduced broadband CPW-fed circular slot antenna was reported to have a fractional bandwidth of 143% [8]. In the sub-millimeter and millimeter-wave ranges, CPW-fed slots have been used as feeds in dielectric lens antennas for single-pixel atmospherical and astronomical applications [9]; arrays of CPW-fed slots are anticipated to be used in space missions [10]. There are also advantages implicit in the use of CPW as feed transmission line: CPW offers easy integration of lumped circuit elements which takes place entirely in the plane of the transmission line, and exhibits less dispersion than microstrip at millimeter-wave and submillimeter-wave frequencies.

OVERVIEW OF THESIS

The remaining chapters of the thesis are organized as follows. In Chapter 2, radiation efficiency and impedance bandwidth of single matched CPW-fed slot dipoles on conductor-backed two-layer substrates are systematically investigated as a function of bottom substrate layer height.6 Curves for radiating efficiency and return loss are provided that could be used to determine the bottom layer height that would yield the optimum efficiency/bandwidth combination for a particular application. Chapter 3 investigates radiation efficiency and impedance bandwidth of matched broadside twin slot dipoles fed by CPW on a conductor-backed two-layer substrate as a function of distance between the slots, and height of the bottom substrate layer.7 Previous reports dealing with slots on other layered media configurations indicated that radiation efficiency can be improved by using, instead of a single slot, two broadside slots spaced half a wavelength of the dominant surface-wave mode apart. The effect of bottom substrate height on gain and directivity is also investigated. The radiation efficiency of twin slots on a conductor-backed two-layer substrate is compared to that of previously reported twin slot antennas on a single dielectric substrate and a back plane placed λ0/4 away from the etched side (λ0 is the free-space wavelength). Measured scattering parameter results are presented to validate bandwidth computations. In Chapter 4, general aspects of the mutual admittance between twin CPW-fed slots on a conductor-backed two-layer substrate with an air bottom layer are investigated using the full-wave moment-method-based simulator IE3D [46]. Two separate issues are addressed. First, a more comprehensive characterization of the mutual admittance between CPW-fed slots on a conductor-backed two-layer substrate than is currently available, is presented.

NUMERICAL METHOD AND RESULTS

Numerical investigations were carried out using the moment method-based electromagnetic simulator IE3D [46], which uses a mixed-potential integral equation formulation [50] and assumes laterally infinite layered substrates.2 In order to assess the program’s performance with respect to planar slot antennas, the return loss, gain and directivity as a function of frequency, and principal radiation patterns were computed for two CPW-fed antennas reported in the literature: a CPW-fed slot dipole on a conductor-backed two-layer substrate [16] and a broadband slot antenna (consisting of a slot dipole capacitively coupled with another slot) on a single-layer substrate without conductor backing [51]. (Radiation efficiency can be calculated directly as gain divided by directivity [52].) In both cases good agreement with published results were obtained.3 The geometry of a CPW-fed slot dipole on a conductor-backed two-layer substrate is shown in Fig. 2.1.4 In order to ensure a non-leaky transmission line, it is necessary (but not sufficient) for the top substrate layer to have a higher relative dielectric constant than the bottom substrate layer (²r1 > ²r2) [12, 13]. In simulations, dielectric constant values ²r1 and ²r2 were fixed throughout at respectively 2.2 and 1. For any slot dipole antenna used as a point of departure, the centre strip width w and slot widths s of the feed line were adjusted to yield a characteristic impedance of 50 Ω. These dimensions were kept constant when the effect of other dimensions of the structure, such as the height of the bottom substrate layer h2, was investigated. Notably, feed line characteristic impedances deviated negligibly from 50 Ω when h2 was varied as described below

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INFLUENCE OF INTER-SLOT DISTANCE

The effect of inter-slot distance d on radiation efficiency was explored by varying d using increments of arbitrary value. Fig. 3.3 shows radiation efficiency at 2.4 GHz for twin slots as a function of normalized distance d/λ0 (λ0 is the free-space wavelength at 2.4 GHz). For each distance value corresponding to markers in the graph, the slots were matched at 2.4 GHz within the twin slot configuration using identical half-lengths L and widths W for each slot (feed lines were also identical). Slots were fed 180◦ out of phase at the feed line ports to achieve in-phase aperture field distributions. The bottom substrate layer height h2 was fixed at 0.12λ0. This particular choice of h2 (i.e., h2 = 15 mm) was well below an upper limit of 40 mm motivated by an interest in antennas sufficiently thin to allow for use on airframes (cf. Chapter 2); it also precluded propagation of two-layer parallel-plate modes other than the TM0 mode [56]. The radiation efficiency value at d/λ0 = 0 shown in the graph of Fig. 3.3 pertains to a single slot matched at 2.4 GHz. Fig. 3.3 reveals that radiation efficiency at 2.4 GHz for the matched twin slots increased monotonically as distance d was increased to 0.45λ0 and then decreased as d was further increased to 0.94λ0. Maximum radiation efficiency occurred for the case d = 0.45λ0: 90% as opposed to 39% for the single slot. This value of d is close to half the wavelength of the two-layer parallel-plate TM0 mode (λTM0 ) at 2.4 GHz. Radiation efficiency at 2.4 GHz was the smallest for the case d = 0.94λ0: 28% as opposed to 39% for the single slot case.

CONCLUSIONS

Radiation efficiency and impedance bandwidth of matched broadside CPW-fed twin slot dipoles on conductor-backed two-layer substrates were investigated as a function of distance between the slots, and height of the bottom substrate layer h2. Radiation patterns were shown for optimally spaced twin slots, and the influence of h2 on directivity and gain assessed. Twin slots on a relatively low substrate having h2 = 0.12λ0 were shown to yield more than double the radiation efficiency of a single slot on the same substrate (i.e., 90% vs. 39%) when they were spaced close to half a wavelength of the two-layer parallel-plate TM0 mode apart. The bandwidth of this configuration (13%, VSWR < 1.5) was about a third less than that of the matched single slot (i.e., 18%). However, an inter-slot distance could be found that resulted in double the bandwidth of the maximally efficient case at the cost of a reasonable comprise with respect to radiation efficiency (73% vs. 90%).

CONTENTS :

  • CHAPTER 1 INTRODUCTION
    • 1.1 Background and objectives
    • 1.2 Overview of thesis
  • CHAPTER 2 RADIATION EFFICIENCY AND IMPEDANCE BANDWIDTH OF SINGLE CPW-FED SLOT ANTENNA ON CONDUCTOR-BACKED TWO-LAYER SUBSTRATE
    • 2.1 Introduction
    • 2.2 Numerical method and results
    • 2.3 Conclusions
  • CHAPTER 3 RADIATION EFFICIENCY AND IMPEDANCE BANDWIDTH OF BROADSIDE CPW-FED TWIN SLOT ANTENNAS ON CONDUCTOR-BACKED TWO-LAYER SUBSTRATES
    • 3.1 Introduction
    • 3.2 Numerical method
    • 3.3 Influence of inter-slot distance
    • 3.4 Influence of bottom substrate layer height
    • 3.5 Conclusions
  • CHAPTER 4 GENERAL ASPECTS OF MUTUAL ADMITTANCE OF CPW-FED TWIN SLOTS
    • ON CONDUCTOR-BACKED TWO-LAYER SUBSTRATES
    • 4.1 Introductory remarks
    • 4.2 Mutual admittance of first- and second-resonance CPW-fed twin slots on conductor-backed two-layer substrate
      • 4.2.1 Introduction
      • 4.2.2 Method
      • 4.2.3 Mutual admittance between broadside slots
      • 4.2.4 Mutual admittance between collinear slots
      • 4.2.5 Conclusions
    • 4.3 Effect of back plane distance on mutual admittance between CPW-fed slots on conductor-backed two-layer substrates
      • 4.3.1 Introduction
      • 4.3.2 Numerical method
      • 4.3.3 Results
      • 4.3.4 Conclusions
  • CHAPTER 5 RECIPROCITY-EXPRESSION-BASED APPROACH FOR MUTUAL ADMITTANCE BETWEEN CPW-FED SLOTS ON CONDUCTOR-BACKED TWO-LAYER SUBSTRATES
    • 5.1 Introduction
    • 5.2 Formulation of reciprocity-expression approach for mutual admittance between CPW-fed slots on conductor-backed two-layer substrate
      • 5.2.1 Mutual admittance between centre-fed slots radiating into half-space
      • 5.2.2 Mutual admittance between CPW-fed slots on conductor-backed two-layer
      • substrate
        • 5.2.2.1 External mutual admittance formulation
        • 5.2.2.2 Internal mutual admittance formulation
  • 5.3 Spatial-domain Green’s function for conductor-backed two-layer substrate
    • 5.3.1 HTM for xˆ-directed HMD against top conducting plate inside two-layer parallel-plate substrate
  • 5.3.2 HT E for xˆ-directed HMD against top conducting plate inside two-layer parallel-plate substrate
  • 5.3.2.1 HT E x of xˆ-directed HMD in homogenous medium
  • 5.3.2.2 HT E x of xˆ-directed HMD inside two-layer parallel-plate substrate with top conducting plate removed
  • 5.3.2.3 Evaluation of singular integrals for HT E x of xˆ-directed HMD against top conducting plate inside two-layer parallel-plate substrate
  • 5.4 Implementation and results
  • 5.4.1 Implementation strategy
  • 5.4.2 Results
    • 5.4.2.1 Substrate I: h1 = 0.05λd, h2 = λ0/6, ²r1 = 3.38, ²r2 =
    • 5.4.2.2 Substrate II: h1 = 0.1λd, h2 = λ0/6, ²r1 = 6.15, ²r2 =
    • 5.4.2.3 Substrate III: h1 = 0.013λd, h2 = λ0/6, ²r1 = 2.2, ²r2 =
    • 5.4.2.4 Effect of shift in two-port reference planes
    • 5.4.2.5 Experimental results
  • 5.5 Conclusions
  • CHAPTER 6 CONCLUSIONS AND FUTURE WORK

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MUTUAL ADMITTANCE BETWEEN CPW-FED SLOTS ON CONDUCTOR-BACKED TWO-LAYER SUBSTRATES

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