Because all microstrip antennas have to be matched to the standard generator impedance or load, the input impedance matching method for antenna is particularly important. In this paper a new methodology in achieving matching impedance of a planar microstrip antenna for wireless application is described. The method is based on embedding an Interdigital capacitor. The fine results obtained by using a microstrip Interdigital capacitor for matching antenna impedance led to an efficient method to improve array antenna performance. In fact, a substantial saving on the whole surfaces as well as enhancement of the gain, the directivity and the power radiated was achieved.
Among the various approaches to enhance antenna effectiveness, novel feeding structures are proposed; balanced feed for RFID applications [2], balun structures [3], proximity electromagnetically coupled microstrip feed [1]. Basically, it is important to have an efficient input impedance matching the antenna with the load, to obtain maximum radiated power, many methods have been in use: stub [4], progressive balun [4], combined effects of the insert microstrip line and the slits [5]. This study is carried out to evaluate the efficiency of using a microstrip Interdigital capacitor to guarantee both feeding and matching impedance. Inter digital capacitors find application in filter, hybrid couplers, Dc blocking circuits, tuning impedance matching network. In order to analyse the efficient results provided by using the microstrip Interdigital capacitor, we carried out a comparative study as well as a use of this structure for array antenna.
For planar antennas structures, the Method of Moments or that of Finite Element are quite popular. In order to streamline the antenna design process and generate accurate results before prototype construction, it is important to select an EM simulation program. The soft used has been MOMENTUM which is one of the tools in Advanced design System 2005(Agilent). Currently, a microstrip antenna consists of a dielectric substrate sandwiched between two conducting surfaces: the antenna plane and the ground plane. The simplified microstrip patch antenna is shown in Fig. 1 For this study an epoxy dielectric was used (h=1,52mm, єr = 4,32, metallisation layer thickness=35μm and substrate loss Tanδ=0,018), patch dimensions are chosen such that the antenna resonates on a desired frequency (W=35and L=29mm resonate on 2.45 GHz).
Basically, a matching juncture device is needed between the load and the feeding point in the patch antenna. This structure is shown in Fig. 2.
In recent years, microstrip antenna has been more and more used in wireless equipment such as wireless sensors, RFID tags and cellular phones. Planar patch antennas are recognized to be the most useful type. However, patch antenna also have some limitations; narrow band width, large ohmic loss in the feed structure of arrays, reduced gain (6dB), reduced efficiency, complex feed structures required for high performance array [1]. (Rin , Xin) the load impedance, (XS , XP) matching impedance and (R0 ,X0) antenna impedance. Several publications have surveyed many possible types of microstrip antenna feeding, among which are the microstrip coplanar feed [6], aperture-coupled microstrip feed [7], proximity-coupled feed [8] and gap-coupled feed [9]. Typically a gap-coupled feed is shown in Fig. 3. It is worth noticing that a narrow gap width provides an efficient coupling of power. In order to enable tuning capacitive effect, an Interdigital capacitor can be introduced between the feeding device and the patch antenna not just for coupling but also for matching impedance. Fig. 5 illustrates simulation of an antenna using the latter technique.
The dimensions of the patch are expressed by the following equations: f0 : Central resonance frequency taken 2.45 GHz . The previous equations ( 1), ( 2), ( 3) and (4) gave W=37.5 and L=32, an adjustment is operated when simulating antenna leads to L=28,95 mm and W= 35mm.
The antenna has a physical structure derived from micro strip transmission line, the microstrip antenna is modeled as a length of transmission line of characteristic impedance [10] Z0(Ω ) given by (5)(6). Characteristic impedance evaluation of microstrip is important to determine the width of the feeding line calculated using (6). The effective permittivity were calculated with (8) єeff=4,022. The width of a Z0=50Ω line is w=2,9438mm. In order to evaluate the input impedance of a resonant rectangular patch antenna, a simplified model considering the most important mode TM10 proposed by [11] is showed in Fig. 7 . Circuit element could be accurately calculated using empiric formulas.
On the other hand a simple analytical description of the rectangular planar patch antenna using transmission line model and models the patch as two parallel radiating slots [12] as shown in The slots admittance is given by (13).
Where: λ0 is the free-space wavelength z0= and k0=
Δ L is the slot width given by (17) The slots are identical having the same admittance given by ( 13) accept for fringing effect effects associated with the feed point on edge 1 [12]. For this typical design G1+ jB1= 0,00625+j0,0077
The input impedance is important to be evaluated. This parameter is important to determine the width of the microstrip feed line which allows the matching impedance between the patch and the load.
Where J0 Bessel function of the first kind.
For this typical design Zin= 225Ω In the case of feeding antenna with a quarter wavelength microstrip line, the line must have a width that satisfy a characteristic impedance Z0 calculated from
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