Dual U-Slot Loaded Patch Antenna with a Modified L-Probe Feeding

In this paper, a modified L-strip fed patch antenna is theoretically analyzed for wideband applications. Dual U-shaped slots are incorporated in the radiating patch and a maximum bandwidth of 16.93% (2.65 GHz to 3.14 GHz) is achieved. Further, when two parasitic elements are used, antenna bandwidth improves up to 25.89% (2.90 GHz to 3.77 GHz). The maximum gain of dual U-slotted patch and with parasitic elements is 8.23 dBi and 8.46 dBi respectively. Antenna parameters are calculated by CST Microwave studio and equivalent circuit model theory is presented. The proposed antenna is fabricated and the measured results compare well with the theoretical as well as simulated results.


I. INTRODUCTION
With the rapid development and attractive solution for various wireless communication systems and demands of their applications, compact and wideband antenna designs have been given immense priority [1], [2].In this process, various methods were used to improve the bandwidth such as by loading the slots of different size and shape, etching notches and introducing discontinuities in the radiating patch as well as in the ground plane [3], [4].There are several structures reported to improve the antenna characteristics such as E-shaped [5], [6], C-shaped, U-slot loaded and modified L-strip [7][8][9].Different feeding methods also increase the antenna bandwidth such as proximity feed patch, asymmetric CPW fed patch antennas [10], [11].Substrate with low dielectric constants, multilayer structures and use of air gaps between the dielectric layers increases the impedance bandwidth and gain of the microstrip antennas [12][13][14].Besides the fed patch, some parasitic inverted-L wire improves the radiation performance of patch antennas [15], [16].Apart from that some other types of parasitic element design such as slot type, shorted strip type etc [17], [18] can improve the antenna bandwidth and gain.
The present paper reports a radiating structure to study the antenna bandwidth, gain, efficiency and the radiation pattern.Dual U-slot is incorporated in the radiating patch such that one U-slot is lying within another U-slot forming dual U-slot loaded patch antenna (DUSPA).Further, to increase the bandwidth, two parasitic elements are used above this DUSPA.Both designs are fed by modified L-probe.All the calculations are made by using CST Microwave studio.Also, a theoretical analysis for the proposed antenna is developed based on cavity model.The proposed design is fabricated and various antenna parameters are measured.The details of antenna design and results are discussed in the following sections.

II. ANTENNA DESIGN AND GEOMETRICAL CONFIGURATION
The top view and the side view of the proposed antenna configuration are shown in Fig. 1.The radiating patch is printed on lower side of a substrate of thickness h 2 .On the other side a conducting strip of dimension l s × w s is printed.The relative position of the strip is at a distance 'p' from the edge of the patch.This strip is excited by center conductor of coaxial probe.The patch is suspended at a height h 1 from the ground plane.An inverted U-slot of dimensions L 2 × W 2 with width d 1 is etched within another U-slot of dimensions L 1 × W 1 with same width d 1 .This DUSPA which is printed on the lower side of the substrate of thickness h 2 , is energized by a conducting strip printed on the upper side of this substrate.The proposed antenna is fabricated on Rogers RT duriod substrate (dielectric constant 2.2) with ground plane dimensions (W g × L g ) 80 × 52 mm 2 (Fig. 2).Vertical Probe: Vertical probe can be analyzed as series combination of resistance R v and inductance L v , and can be given as [19].
where µ = permeability of the probe conductor, f = frequency in GHz, d p = diameter of probe and Horizontal Strip: A series combination of distributive resistance R s and inductive L s is developed due to horizontal conducting strip and can be given as [20].
here, w s = width of strip, t s = thickness of strip, ρ 0 = ratio of specific resistance of strip and copper.
The distributive capacitance C sp between horizontal strip and the radiating patch can be given as Since the open ends of the horizontal strip above the radiating patch will have fringing field, so the effective length of the strip is increased.The increment of length will cause some extra capacitance which is fringing capacitance and it can be calculated as here, in which   is effective dielectric constant for the material under conducting strip [19].The fringing capacitance C fp between open end of the strip and the radiating patch is calculated by putting the substrate height h = h 2 and the fringing capacitance C fg can be given by putting h = (h 1 + h 2 ).The fringing capacitance C pf1 between the parasitic elements and the strip is calculated by putting h = h 3 .The entire feeding acts as a series L-C resonant element and connected in series with the radiating patch.The impedance of this modified L-probe can be calculated using Fig. 3 as where, Analysis for DUSPA: The value of capacitance C 1 , inductance L 1 and resistance R 1 for a rectangular patch can be given as [20].
where, L = length of the patch, W = width of the patch, f 1 = resonance frequency,   = feed point location,  = 2 1 and Q r is the quality factor of the resonator.
where, c = velocity of light, f 1 = design frequency, ε e = effective permittivity of the medium in which, ε rs can be calculated as where, n is the number of stacked layers and ε r = relative permittivity of the substrate material.
A slot in the radiating structure can be analyzed using the duality relationship between the dipole and slot [21].The radiation resistance of an inclined slot in the patch is given by The input reactance of the inclined slot is given as [22] X r = 30cos 2 α [2S i (kL 1 ) + cos (kL 1 ){2S i (kL 1 ) − S i (2kL 1 ) − sin (kL 1 )} × {2C i (kL 1 ) − C i (2kL 1 ) − here, α is the inclination angle of slot with respect to x-axis, S i and C i are the sine and cosine integrals, d 1 = thickness of the slot, L 1 = length of the slot.Impedance for this inclined slot is given by [22] Now, U-slot in a patch is analyzed by assuming two slots along the y-axis as the vertical slot of length 'L 1 ' at angle α = 0 0 and a slot along x-axis as the horizontal slot of dimension 'W 1 ' at angle α = 90 0 .
The input impedance of the vertical slot can be calculated by using equations ( 9), ( 10) and (11) as: here, Z V1 is calculated by putting α = 0 0 .Similarly the impedance of the horizontal slot can be calculated as: here, Z H1 is calculated by putting α = 90 0 .Thus equivalent circuit for U-slot in the patch is given by Fig.

4.
Fig. 4. Equivalent circuit of U-slot in the patch.
Thus, the equivalent circuit for modified L-probe fed patch can be given as shown in Fig. 5.The input impedance of DUSPA is calculated by using Fig. 5 as: in which, Z U1 and Z U2 are the impedances of two U-slots in the patch and can be calculated using Fig. 4 and Z P is the impedance of the rectangular patch and can be calculated as:   The accurate equations for the coupling capacitance C g and the plate capacitances C P1 of the microstrip gap can be calculated from the hybrid mode analysis [23], [24].Now using the equivalent circuit as shown in Fig. 7, the total input impedance of DUSPA with parasitic elements can be calculated as: Z LU +Z M +Z Para (16) and Z g = Z P1 + 1 jωC g here, L M , C M are the mutual coupling inductance and capacitance between two radiators and Z P1 is the impedance of parasitic elements.
Using equation ( 16), we can calculate the reflection coefficient, VSWR and return loss of the proposed antennas.
where, Z 0 = characteristic impedance of the coaxial feed (50  ) and     The simulated return loss is compared with the theoretical and the measured results of DUSPA and DUSPA with parasitic elements respectively (Fig. 14).From the graph it is clear that simulated and theoretical results of both the antennas are agreeing quiet well with the measured one.Radiation efficiency is calculated for both the antennas and found quite acceptable (above 97.0%) for entire operating frequency band (Fig. 16).The radiation patterns of the proposed antennas are measured using anechoic chamber.Fig. 17 shows the measured and simulated radiation patterns for DUSPA at 2.80 GHz and 3.09 GHz.The cross polarization level is quite low at φ = 90 0 than that at φ = 0 0 .A good agreement between the measured and simulated results is observed.Radiation patterns for DUSPA with parasitic elements are plotted at 3.09 GHz, 3.45 GHz and 3.63 GHz (Fig. 18).The cross polarization level at φ = 90 0 is again quite low than that at φ = 0 0 for all three resonant frequencies.This is primarily because of the feed location which is along y-axis.Also, the inherent asymmetry property of probe feed which generates higher order modes and hence increases the cross-polarization level.In the measured results, some ripples are observed below ground plane due to reflection of radiation by the conducting strip.The simulated radiation pattern is compared with measured results however, some mismatch is observed in radiation pattern due to fabrication inaccuracy and numerical methods used in simulator.The dimension of conducting strip can be optimized to further improve the antenna characteristics.
Antenna bandwidth can also be controlled with inner and outer U-slot dimensions.This antenna is

FurtherFig. 1 .
two parasitic elements of dimensions L p × W p separated by gap D are placed at thickness h 3 from the conducting strip.These parasitic patches are excited by electromagnetic coupling with DUSPA.A detail design specification is given in Table-1.Geometry of the proposed antenna (a) Top view (b) Side view.

( c )
Analysis of the parasitic elements:The parasitic elements are excited through the electromagnetic coupling with DUSPA.Each parasitic element is considered equivalent to parallel combination of resistance R p , L p and C p .These two parasitic elements are coupled with each other by the gap coupling and the equivalent circuit is given in Fig.6.The equivalent circuit of the gap can be given as a π-circuit, consisting of the gap coupling capacitance C g and the plate capacitances C P1 .Now two radiating structures (DUSPA and parasitic elements) are coupled through the electromagnetic coupling.

Fig. 7 .
Fig. 7. Equivalent circuit of the modified L-probe fed DUSPA with parasitic elements.

Fig. 8 Fig. 8 .Fig. 9 .Fig. 10 .
Fig.8shows the simulated return loss obtained from CST Microwave studio for different values of dual U-slot width (d 1 ).From the graph it is observed that entire operating band shifts towards higher side for increasing value of d 1 , however, antenna bandwidth decreases with increasing value of d 1 .The bandwidth of the antenna is calculated for return loss < -10 dB and found maximum (16.67%, 2.66 -3.14 GHz) at d 1 = 1.0 mm and below d 1 = 1.0 mm the antenna exhibits dual band nature.

Fig. 11 .
Fig. 11.Variation of return loss with frequency for different values of L p .

Fig. 12 .
Fig. 12. Variation of return loss with frequency for different value of gap 'D'.

Fig. 13 .
Fig. 13.Variation of return loss with frequency for different values of W p .
Fig. 15 depicts the simulated and measured gain for both the antennas.The simulated peak gain for DUSPA is 8.23 dBi at 2.8 GHz while for DUSPA with parasitic elements peak gain is 8.46 dBi at 3.0 GHz.For DUSPA, the maximum gain variation is 0.63 dBi for the entire band of operation (2.75-3.10GHz) while for DUSPA with parasitic elements it is 0.96 dBi for the entire band of operation (2.82-3.75GHz).