Performance Analysis of 2D-Ebg under Monopole Antenna

The artificial properties in two dimensional electromagnetic structures (2D-EBGs), such as PMC and Band Reject Region are investigated for a proposed structure of square shaped mushroom. The radiation characteristics of monopole antenna over this 2D-EBG is tested by considering two cases. During first case monopole antenna is made to operate within band rejection region. Second case monopole antenna made operate outside the band rejection range. The obtained results during first case is showing enhancement in operating band width and smoother radiation pattern. In second case the effect is null and 2D-EBG resembles like conventional plane reflector. The simulated results are presented.


INTRODUCTION
The periodic structures are became popular from the past decade because of its artificial characteristics, which are not possible to exhibit by natural materials. Based on the periodicity they are classified as one, two and three dimensional structures. One dimensional structures have ability to produce slow wave nature in one direction only. The best example of 1D-EBG's are corrugated surfaces. Two dimensional EBG structures have a ability to produce slow wave nature and has a band rejection range for EM surface waves in two directions. Most popular 2D-EBG's are mushroom high impedance surfaces proposed by Sevenpper in year 2009 [1]. The three dimensional EBG's can suppress EM surface waves in all three dimensions that means it can produce universal band gaps. Three dimensional EBG is proposed and designed in [2] In present paper square shaped mushroom 2D-EBG is designed. The design and optimization is given in [3][4]. The architecture consists of periodic arrangement of protrusions over dielectric substrate having a permittivity 2.55 and protrusions are connected to under lying by means of via. This structure exhibits two major properties of zero phase reflection between inward and reflected waves and band rejection range where this proposed structure does not allow any EM surface waves (neither TM not TE) to propagate through it. In order to measure these two properties an electromagnetic simulation software of Ansoft HFSS version 13 is used. This structure can be easily designed using PCB technology.

2D-EBG Design Specification
The proposed 2D mushroom EBG is designed with following specifications.  Figure 1. Geometry of HIS structure The figure 1 is depicting the two dimensional arrangement of unit cells. where a unit call can be defined as a specific model or architecture which is a sand which of dielectric substrate and metal.

Dispersion Diagram
In determining the dispersion characteristics of periodic structures, There is no need to consider and model the complete architecture, but instead on can consider a single unit cell. Hence in Electronic copy available at: https://ssrn.com/abstract=3630049 present case a unit cell is modeled in Ansoft HFSS software with the specification given in [5] by applying periodic boundary on four walls and a PML on the top to produce normally incident plane waves. The arrangement is shown in figure 2.

Reflection Phase
The figure 4 is showing the reflection phase characteristics of 2D-EBG. The reflection phase is of structure changes continuously from +180 0 to -180 0 verses frequency.

MONOPOLE ANTENNA
Monopole antennas are popular for wide range wireless communication applications. They are easy to fabricate and low cast. Present paper a monopole antenna is designed to operate at 6GHz placed over conventional ground plane and tested. Next the conventional ground plane was replaced with 2D-EBG structure whose specifications are described in Table 1. One can find a shift in operating point of monopole antenna. This effect is occurred due to change in impedance region. Now the dimensions of radiating antenna is altered by trial and error method and brought the operating point to 6GHz, that is within the stop band region of 2D-EBG. Then we found an enhancement in band width of monopole antenna. which are shown in figure 5. This indicates the suppression of surface waves avoided the multi patch propagation. One more advantage we found that the ripples in forward direction are significantly less, hence very less amount of power is wasted in back lobes. This 2D-EBG also providing a shielding to the components back to this. This also reduces the interference between neighboring elements in the complex environment. Now monopole antenna is designed to operate at 8GHz which is beyond the band gap range shown above, is tested over normal plane and HIS surfaces. The figure 7 is showing the radiation characteristics from which we can conclude that the characteristics of 2D-EBG in pass band region resembles similar to conventional reflector. Hence obtained characteristics are similar.

CONCLUSIONS
The dispersion and reflection phase characteristics of two dimensional electromagnetic band gap structure was analysed. The effect of 2D EBG on radiation parameters of monopole antenna are presented when comparing with convectional design. The obtained results are showing that the 2D EBG has EM surface wave suppression property enabling smoother radiation pattern and enhancement in operating bandwidth.