Bandwidth Enhancement of L-Probe Proximity-Fed Annular Ring Microstrip Slot Antenna A. K. Singh, (Research Scholar) Electronics Engineering, Indian School of Mines, Dhanbad, India. Email:
[email protected].
Ravi Kumar Gangwar
Binod K. Kanaujia
Electronics Engineering, Indian School of Mines, Dhanbad, India. Email:
[email protected]
Electronics and Commn. Engg., AIACTR, Gita Colony, Delhi, India Email:
[email protected]
Abstract— A parametric study and simulation of wide-band orthogonally slit cut annular ring microstrip antenna (SCARMSA) fed by an L-shaped probe is investigated using Modal expansion cavity model and circuit theory concept. The radiating patch is located about 8mm (~0.1λ) above the ground plane. The broadband characteristic of antenna is achieved by employing a thick substrate (~0.1λ) of 1.07 relative permittivity (Rohacell Foam). The annular ring patch is cut ortogonally with slot of 1mm width. The E- and H-plane radiation pattern are presented and VSWR is compared with the simulated result. Good agreement is obtained between computed and simulated results. The effects of the geometric parameters of the L-strip like length of the horizontal portion of the strip are investigated. An impedance bandwidth of about 36.76% and 33.23% (VSWR < 2) at 13.5 mm and 15 mm length of horizontal L-probe respectively.
Keywords- microstrip antenna; wide-band microstrip antenna; Lprobe proximity fed microstrip antenna; wireless communication;
I.
INTRODUCTION
With the definition and acceptance of the ultra wide-band (UWB) impulse radio technology in the USA [1]. Recently, the Federal Communication Commission (FCC)’s allocation of the frequency band 3.1–10.6 GHz for commercial use has sparked attention on UWB antenna technology in the industry and academia. Several antenna configurations have been studied for UWB applications [2–4]. The impedance bandwidth of a microstrip antenna depends primarily on both the thickness and the dielectric permittivity of the substrate. A thick substrate with a low dielectric permittivity can increase the bandwidth of the printed patch. Both these selections could be a solution of the problem of bandwidth enhancement[5]. Simultaneously these solution pose difficulties in integration of the antenna with other microwave circuits, and cause some other problems such as the surface wave propagation and the large inductive image part of the input impedance of the antenna, which makes its resonance unfeasible. Thus, a reasonable thickness should be considered in the selection of substrate and the bandwidth would be enhanced using additional techniques. The most common and effective of them are the loading of the surface of the printed element with slots of appropriate shape. The attractive features of annular ring microstrip antenna motivated the investigators [6-12]. The annular ring microstrip antenna is a popular antenna because of its small dimensions compared to other microstrip antennas resonant at the same frequency [13]. The L- probe proximity fed annular ring microstrip antenna is simple in structure and have been
investigated [13-14] for various ultra wide-band system and other communication systems. In this paper, the L-probe proximity fed annular ring microstrip antenna with orthogonally loading of the surface of the printed element with slots is investigated using Modal expansion cavity model and circuit theory concept. Characteristics including VSWR, and radiation pattern are considered. II.
THEORETICAL ANALYSIS
Geometry of the L-probe proximity fed annular ring microstrip antenna with orthogonally slit loaded is shown in Fig. 1. The broadband performance of the proposed antenna is achieved by employing a thick substrate. In our design, a Rohacell foam layer (εr= 1.07) of thickness 8 mm is used to support the radiating patch. Without L-probe, it is difficult to couple the energy from the microstrip line to the patch as the separation between them is too large. Therefore a step, which is designated as an L-probe. The horizontal part of the L-probe of y0 incorporated with the patch provides a capacitance to suppress the inductance introduced by the vertical part of the L-probe.
y0
top view
b a
ws X
side view
h2 h1
Patch
Z H L-probe Y ground plane
Fig. 1. Geometry of L-probe proximity-fed annular ring microstrip antenna with slit inserted.
The vertical part of L-probe is equivalent to a series combination of resistance (Rs) and inductance (Ls). The resistance Rs is because of finite conductivity of copper used. The expression for the resistance Rs and inductance Ls are given by [15]
2 h1 L s = 0 . 2 h 2 ln w s + t s
+ 0 . 2235
ws + ts + . 05 (nH) (1) h2
R s = 4 .13 × 10 − 3 h 2
fρ / ρ Cu
ε r +1
ws + t s
ε r −1
− 0 .5
12 H (6) 1 + W 2 2 where W=b-a, a and b is the inner and outer radius of the patch antenna respectively. If le is the extension in effective length due to fringing fields, then the fringing capacitance (Cf) can be calculated as [15-16] (7) C f = l e ε e cZ 0 +
Fig. 2 represents the equivalent circuit of the L-probe proximity fed microstrip antenna, due to L-probe, a series combination of resistance (Rs), inductance (Ls) and capacitance (Ctotal) in series with resonant element of patch as parallel component R, L and C. Hence, the input impedance can be given as
Z in = R s + j ω L s +
1 + Z inp j ω C total
(8)
where Ctotal is the equivalent capacitance because of capacitances C1, Cs1, Cf1, and Cf2 and is given by [16]
C total =
(C1 + 2C f 2 ) (C s1 + C f 1 ) (C1 + 2C f 2 + C s1 + C f 1 )
•
(2)
where ws is the width of the L-probe in mm, ts is the thickness of the prove in mm, h1 is the height of the vertical portion of the L-probe, ρ is the specific resistance of the strip (Ωcm), and ρcu is the specific resistance of the copper (1.72106×10-6 Ωcm). The horizontal portion of the L-probe and the patch are perfect conductors separated by a finite distance (h2) which provides a capacitance (C1) in series with vertical portion of the L-probe and this can be calculated by (3) C1 = ε r ε 0 y 0 w s h2 where εr is the relative permittivity of the material (Rohacell foam), and ε0 is the free space permittivity. Also there is a parallel plate capacitance (Cs1) between horizontal part of the L-probe and the ground plane and is given by (4) C s1 = ε r ε 0 y 0 w s h1 The open end of the L-strip which is under the patch will have fringing fields, which can be considered as a small increase in the length of the L-strip. This will have an extra capacitance (Cf). This effective increase in the length can be calculated as [15-16] w 0 . 412 h (ε e + 0 . 3 ) s + 0 . 264 h (5) le = ws (ε e − 0 .258 ) + 0 .8 h εe is the effective dielectric constant of the material and is given by
εe =
Cf2
(9)
Cf1 Rs
C1
Ls Cs1
Cf2
Zin
R
L
C
• Fig. 2. Equivalent circuit of L-probe proximity-fed annular ring microstrip antenna.
and Zinp is the input impedance of resonant element of patch as parallel component R, L and C is given by (10) 1 Z inp =
III.
jω C −
j 1 + R ωL
DESIGN DETAILS
Design paramour of the proposed antenna is as shown in table I. TABLE I. DESIGN PARAMOUR OF THE PROPOSED ANTENNA Parameter
Value
Inner radius of annular ring; a
17 mm
Outer radius of annular ring; b
8.5 mm
Substrate material used
Rohacell foam
Relative permittivity of Rohacell foam; εr
1.07
Length of the vertical L-probe; h1
6 mm
Height of patch from horizontal L-probe; h2
2 mm
Height of patch from ground plane; H
8 mm
Width of the L-probe; ws
1 mm
Length of the horizontal L-probe; y0
15 mm
IV.
DISCUSSION OF RESULTS
Variation of VSWR with frequency for different length of horizontal probe is plotted in Fig.3. The plot shows that the VSWR
