Antenna is a very important component of communication system. The enormous growth of mobile and satellite communication systems along with wonderful use of radars opens a huge demand to new kind of antennas such as small antennas, multi frequency antennas, and broadband antennas [1] -[3] . Planar multi-resonators and stacked microstrip patch antenna techniques are combined to yield a wide bandwidth and higher gain. Several two-layered configurations have been discussed in [4] - [6] , in which only a single patch on the bottom layer is fed by a coaxial probe and second patch on the top layer and both are electromagnetically coupled to each other. Multi-band or reconfigurable antennas are suitable candidates for providing multi-functionality. This will result in significant reductions in antenna size and cost. The primary advantage of the proposed multiband antenna lies in its ability to support two separate applications at two different frequency bands with distinctly different radiation patterns, gain and polarization characteristics using a single radiating aperture [7] . Although multi-band antennas have the capability of serving multiple frequency bands with one antenna, they are considered a weak choice in comparison to reconfigurable antennas because of crosstalk from the neighbor bands. In [8] , we have to see that the multiband antenna can be obtained by using of slot couple technique in multiple patches. In 2012, M. A. Motin, et al. [9] , presented the multiband microstrip patch antenna for X, K and Ku band application. In 2014, Jianxing Li et al. [10] , represented the multiband probe-fed stacked patch antenna for GNSS application.

In the present paper, a multiband microstrip stacked patch antenna is introduced. The proposed structure is a planar structure having all the dimensions in mm. The proposed antenna is designed by using a substrate of FR4 having thickness 1.6 mm. This proposed antenna is having seven bands of operation. The entire investigation is based on equivalent circuit model. In [11] , it represents the multi-band antenna by using stacked patch with multi- slot loaded patch, which can be compared to the proposed design. The co-axial feed technique is used for the analysis of this antenna because it occupies less space and has low spurious radiation by using Teflon connector. The proposed antenna design can be used in mobile communication, radar and satellite communication. Details of the antenna are given in the next stage.

The design of the proposed multiband antenna is depicted in Figure 1. The upper patch is double U-shaped slot loaded stacked patch and the lower patch is the H-slot loaded fed patch. Due to the presence of the stacked patch antenna, there are two resonances associated with the two resonators [12] .

Microstrip patch with a dielectric cover is considered as a single patch with a semi-infinite superstore with re- lative permittivity equal to unity and the single relative dielectric constant () given as:

in which W_e is the effective width and is the effective dielectric constant of the structure [13] . The effective dielectric constant of the lower patch is given as

where,

h₁ = height between ground plane and lower patch;

W = width of the patch.

The equivalent circuit of the simple patch antenna is parallel combination of resistance (R₁), inductance (L₁) and capacitance (C₁) (Figure 2), whose values are defined as [14]

where

L = length of the lower patch;

y_o = Y-coordinate of the feed point.

c = velocity of light.

ΔL = fringing length for the lower patch.

Considering the top patch as a simple stacked rectangular microstrip patch, the values of resistance (R₂), inductance (L₂) and capacitance (C₂) can be given as

where

where

L₂ = length of the stacked patch;

W₂ = width of the stacked patch;

;

;

ΔL₂ = fringing length for the top patch.

The equivalent circuit of the fed patch is shown in Figure 3(a), in which and are the additional inductance and capacitance respectively, which originate due to introducing the two notches and R_H is resonance resistance after cutting the notches into the patch. The value of R_H can be calculated using Equation (7) and the, can be given as [15]

where

and

where C_s is the gap capacitance between two side strips [16] . Now the equivalent circuit of H-shaped patch is given as shown in Figure 3(b) in which “Z_N” is the impedance of the notch incorporated patch and is calculated from Figure 3(a), Z_P is the impedance of the initial patch and C_m and L_m are the capacitive and inductive coupling between two resonant circuits.

When a dual U-slots is cut into the stacked patch, current distribution changes which ultimately changes the resonance behavior of the patch. Due to this changing in the patch adds series inductance () and series capacitance () in the initial circuit of the stacked patch, which is shown in Figure 4, in which the resonance resistance R₂, and are given as [17] - [19]

where is the effective length [15] and can be given as

where

;

;

;

is calculated as gap capacitance and given by [18] .

The value of C_b and L_b are calculated as [19]

Similarly we can analyze circuit concept of second U-slot which is parallel and compact to first U-slot on the upper patch.

The equivalent circuit of the proposed multiband antenna can be given as shown in Figure 5, in which only capacitive coupling is considered and given as [20]

where

and is the coupling coefficient between two resonators.

Thus the total input impedance can be calculated from Figure 5 as

In which Z_H and Z_U are the impedances of initial and parasitic patches calculated from Figure 3(b) and Figure 4 respectively and Z_m is the impedance due to mutual coupling between fed patch and stacked patch.

All the simulation results of the proposed antenna are given as Figures 6-8. Discussion of results have been explain in terms of S₁₁ parameters, resonance frequencies, gains and radiation patterns.

The variation of reflection coefficient with frequency for the proposed multiband antenna is depicted in Figure 6; it is found that the antenna can be operated for seven band applications. In which both lower and upper resonance frequencies of each bands are (4.24 GHz and 4.50 GHz), (5.02 GHz and 5.25 GHz), (7.84 GHz and 8.23 GHz), (12.16 GHz and 12.35 GHz), (14.25 GHz and 14.76 GHz), (15.25 GHz and 15.51 GHz), (17.52 GHz and 17.86 GHz) with return loss −13.81 dB, −17.02 dB, −26.07 dB, −12.79 dB, −20.42 dB, −31.32 dB and −18.41 dB respectively. The bandwidth of each bands are 6%, 4.5%, 4.83%, 2.36%, 3.53%, 1.68% and 1.91%. From Figure 7, it is observed that proposed antenna gains are 2.80 dBi, 4.39 dBi, 4.54 dBi, 6.25 dBi, 10.26 dBi, 8.36 dBi and 9.91 dBi respectively. From both the figure it is clear that the antenna gain is high as a higher frequency of the proposed antenna so that it is very useful in radar application. In [4] , Ansari et al. has already presented the similar work for dual band application. But in the proposed antenna we have replace and changed the radiating patch in place of U-slot loaded patch and H-slot loaded parasitic patch. We have to see that, if changed

the patch, the results are obtained as a multiband antenna with good gain, which is more useful as compared to dual band application presented in 2008. Radiation pattern of the proposed antenna is shown in Figure 8. It is found that radiated power is good. This shows that the directivity improves by stacking dual U-shaped patch.

A stacked multiband proximity coupled microstrip patch antenna is presented. This antenna has a very simple structure printed on FR4 substrate. Multiband has achieved by using dual U-slot loaded stacked patch. The total volume of the antenna is 39.25 × 29.25 × 1.6 mm². The proposed antenna shows satisfactory multiband performance and good radiation pattern. Proposed antenna finds the application in C, X and Ku band which can be used for radar, VSAT, mobile and satellite communication.

Nagendra PrasadYadav, (2015) Double U-Shaped Slots Loaded Stacked Patch Antenna for Multiband Operation. Open Journal of Antennas and Propagation,03,9-17. doi: 10.4236/ojapr.2015.32002