Design and Analysis of Multi layer High Impedance Surface

Design and Analysis of Multi layer High Impedance Surface

P.Saleem Akram 1, Dr.T.Venkata Ramana 2

1Associate Professor, Dept of ECE, Nalanda Institute of Engineering & Technology, Sattenapalli, AP, India,

2Professor, Dept of ECE, GITAM University, Visakhapatnam, AP, India,

Abstract

In the recent years High Impedance structures are drawing lot of interest in electromagnetic and antenna community. Due to compact sizes of electronic devices, required radiating element to be placed closed vicinity to electronic and biological systems hence designed antenna should have maximum radiated gain, with minimum near filed coupling with environment. This is achievable with periodic electromagnetic structures. By the Engineered electromagnetic surface textures, the properties of the metal surface are altered to perform required functions, such as change of surface impedance, to manipulate the propagation of surface waves or to control the reflection of phase. My present paper concentrating on two dimensional HIS, Multi layered mushroom type metal protrusions. They are analyzed as resonant LC circuits; These materials provide high impedance boundary conditions for both polarizations and for all propagation directions.

Key wards- HIS, Rectangular Patch, Surface wave.

1. Introduction

   In latest Meta-materials there are so many models up to now most of them are single layered structures but here we are considering a two layered HIS and the design is simulated by HFSS these Meta-materials have made improvements in radiation and BW and some other parameters of antennas used in Real time here we further more can develop these parameters by introducing two

   layered HIS. Basically monopole antenna is widely used in mobile communications here we are applying this structure to improve the gain of the antenna and reduce the return loss without altering the BW.

2. Design Consideration

   1. High Impedance surface

      The design of HIS structure consisting of metal patches on one side, connected by metal via to a solid conducting sheet. One period of HIS structure can be explained equivalent to parallel LC filter. This LC circuit controls the center frequency. By the applied voltage parallel to the surface causes charges to built up on the ends of metal plate. This fringing electric fields between adjacent metal patches resembles capacitance effect. The inductance is proportional to thickness of ground plane

      Fig 1: HIS Unit cell, its equivalent

      The surface impedance is Z= JwL/(1-w2LC). Which is inductive at low frequencies, thus supports transverse magnetic waves to propagate. This is capacitive at high frequencies so it supports transverse electric waves to propagate. When this structure interact with electromagnetic wave, currents are induced in top metal

      plates and spreads throughout the surface such as width of patch, via and ground, develops electric fields at the ends of plates results capacitance, magnetic fields develop current through via and ground plane results inductance.

      The electric flux developed is

      = (2V/)(cosh-1[(w+g)/g]).

      Where w= patch width, g= gap width,

      C= Wo(1+r) cosh-1((2W+g)/g)/

      sheet inductance is depends on height of substrate and permeability L=µoµrh. From the filter theory the resonance frequency of structure is the inverse of square root of the product of inductance and capacitance

      f= 1/(2LC)

      High impedance ground planes have mainly two important characteristics 1) this reflects field with zero degree of phase shift at resonance frequency, even when radiating element placed much closed to reflector prepared with same conducting material. The phase of reflection coefficient is +180 degree for low frequencies, and then it gradually decreases and becomes 0 degree at resonance frequency and reaches to -180 degree for high frequencies.

      2) This provides stop band characteristics for surface wave propagation this improves the antenna radiation pattern hence reduce the multipath and smoothen the radiation pattern.

   2. Multi Layer High Impedance Surface

      The multilayer High Impedance surface consists of dielectric substrate with a ground plane and two arrays of electrically small patches separated with dielectric film and shifted relatively to each other. Here the surface impedance is depends on circuit parameters like capacitance, inductance and equivalent resistance due to

      dielectriclosses.

      Fig 2: MHIS unit cell, its equivalent

      The equivalent circuit is obtained by dividing the structure into two parts such as substrate part and patch array part. Analyzing the impedance in this region we can obtain inductance and capacitance. Representing the ground plane with inductance L and series equivalent resistance R develops due to losses in dielectric. The dielectric substrate with a ground plane is considered as a shorted (Practical line of finite length) transmission line section the input impedance of transmission line at a distance h.

      Z(h)= Z0[ ZL+Z0Tanh(h)]/[ Z0+ ZLTanh(h)] Where Z0 =µ0µr2 /0r2

      Z0 Is the characteristic impedance of transmission line.

      ZL Is load impedance

      =Jk0 µr2r2 is complex propagation constant k0 is wave number,

      r2 relative permittivity,

      µr2 relative permeability of substrate medium.

      Since load end of transmission line is shorted, the load impedance will be zero.

      Then the input impedance will be Z(h)= Z0[ ZL+Z0Tanh(h)]

      Z(h)= Jµ0µr2 /0r2Tan(hK0 r2)

      Since the distance h is much smaller than the wave length, we will use Tailor series for expansion of tangent. Consider up to third term and neglecting all higher order terms. Grouping real and imaginary parts the substrate input impedance is

      Z(h)=Jk0 h[1+ (hK0)2 r2/3] + (hK0)3r2Tan2 [ 1/3+2(hK0)2

      r2/5]

      This is equivalent to Zsubstrate= JwL +R after equating real and imaginary parts we get inductance and resistance. Where Tan2 is loss tangent of substrate.

      The input impedance at two arrays of upper and lower patches is determined by parallel plate capacitance Cpp with dielectric losses is determined by Rpp.

      Cpp=(Wd0r1/g)[1+g/d+gln(d/g)/d][1+g/W+gln(W/g

      )/W]

      Rpp=g/(w 0r1tan 1A)

      g is gap between upper and lower patches

      A is overlapping area between upper and lower patches

      Tan1 is loss tangent of thin dielectric film between upper and lower patches

      The fringing capacitance between lower patches is

      Cf = Wo(1+r) cosh-1((W+g)/g)/

      total equivalent capacitance of array of patches is Ct=0.5Cpp+ Cf

      Effective impedance of patch array is Zpatch=2Rpp/(1+2JWC Rpp)

      Total input impedance of multi layer High Impedance Surface is

      ZMHIS= ZpatchZsubstrate/(Zpatch+Zsubstrate)

      For normally incident wave the reflection phase of surface is

      = (ZMHIS-0 )/ (ZMHIS-0 )

      The analysis of electromagnetic structure using lumped parameters is valid as long as the wave length is longer than size of individual.

      Parameter	Value
      Patch width	3.5cm
      Patch Gap	0.2cm
      Sub Height	0.5cm
      Reflecting Freq	2.4GHz
      Surface Impedance	2.513
      Inductance	6.28×10-5H
      Capacitance	7×10-11 F

      Table 1: Design Calculation

   3. Software Design

   Here antenna was designed in Ansoft HFSS software. Ware ground patches width is 3.5cm which are kept at a distance of 0.5cm from planar conducting ground, with a band gap of 2.4GHz. The medium between them is filled with air dielectric with dielectric constant 1.0006.

3. RESULTS

   1. Return loss

      This can be defined as ratio of power at receiving end due to incident wave to the power reflected by load. It is the two dimensional curve.

      Fig 3: Return loss curve Vs frequency

      In the above fig 3 it can be seen that the loss is reduced when using the Multi layered HIS in place of single layered HIS. And for both the BW is same.

   2. Radiation Pattern:

      Fig 4: The radiation pattern for Single and Multi layered HIS

      In the above figure 4 the radiation is enhanced for multi layered HIS there is no uncovered area at center of the antenna that mean it is radiating without any uncovered parts near to radiating element.

   3. Gain (Polar Plot)

   It is the measure of figure of merit of antenna

   Fig 5: Total Gain in 3D View for Single layer and Multi layered HIS

4. DISCUSSION

   By observing the results starting from return loss, radiation pattern, gain, and band width. All the parameters was enhanced in multilayer HIS comparison with normal structure HIS.

5. CONCLUSION

   The use of multi layered structures in place of normal mushroom structure HIS antenna parameters like radiation pattern, return loss were enhanced without altering the BW. Especially when using the HIS there is uncovered area of radiation at center area which is almost covered with radiation when HIS is replaced by multi layers HIS.

6. References

1. C.Balanis, Antenna theory, Analysis and Design 2nd ed., John Wiley and Sons, New York (1997)

2. D.Sevenpiper High Impedance EM surfaces Ph.D Dissertation University of California, Loss Angeles, 1999.

3. Remski R Analysis of photonic band gap surfaces using Ansoft HFSS Microwave Journal, September 2000.

4. Rehmat Samii.Y and F.Yang, Electromagnetic Band Gap Structure in Antenna Engineering Cambridge University Press 2009.

5. G.Gnanagurunatham and U.G.Udofia,Performance Analysis of mushroom like EBG structure integrated with a microstrip patch antenna Proceedings of IEEE Asia- Pecific Conference on Applied Electromagnetic (APACE), 2010

6. C.A.Balanis Antenna Theory Analysis and Design 3rd ed., John Wiley & Sons 2005.