This work represents isolation of mimo antenna system with mushroom type electromagnetic band gap structure in order to reduce mutual coupling between antennas.

1. Multiple-Input Multiple-
Output (MIMO) System

2. Multiple-input multiple-output (MIMO) SYSTEM
➢ A single antenna design leads the way for investigation of MIMO system for this project.
➢ Two different single rectangular patch antennas have been placed in the same plane with a distance, l1.
➢ Same conditions what it was used during the simulation of a single rectangular patch antenna has applied in
order to investigate MIMO antenna characteristics.
➢ The port of right-handed antenna, Port 2, has been used to feed antennas as it is seen below figure.
MIMO Antenna system.

3. Multiple-input multiple-output (MIMO) SYSTEM
➢ As a result of simulations, it is seen that while the right-handed antenna radiates properly at 10 GHz,
S22, the left-handed one, S12, cannot.
➢ This situation can be explained by antenna to antenna mutual coupling.
• S1,2 and S2,2 parameters for MIMO Antenna system.

4. ➢ Mutual coupling can be described as energy absorption by an antenna’s receiver while the other nearby
antenna is operating.
➢ The energy supposed to be radiated is absorbed by nearby antenna, so it is very unpleasant situation.
➢ Hence, the antenna efficiency and performance are degraded because of the mutual coupling. This case
can be easily observed below figures.
Multiple-input multiple-output (MIMO) SYSTEM
• E-field and H-field distributions for MIMO Antenna system.

5. Multiple-input multiple-output (MIMO) SYSTEM
Various techniques can be applied to decrease the mutual coupling among adjacent
elements.
These techniques can be listed like:
• Metamaterial insulator
• Slotted Complementary split-ring resonator
• Cavity backed
• Substrate removal
• Defected ground structures (DGS)
• Defected Wall structure
• Employing Electromagnetic Band Gap (EBG) Structure among two sheets for microstrip
antennas [1].

6. Multiple-input multiple-output (MIMO) SYSTEM
➢ For instance, mutual coupling could have handled by two different techniques.
➢ Firstly, the antenna could be located further from each other, so l1 could be increased.
➢ However, as a result of this the cell size of the antennas is increased and caused another
undesirable situation. Today’s technology requires everything is as small as.
➢ Band gap structures placed between antennas can be used to reduce the surface waves.

7. Now, lets take a look at 2D periodic Structures

8. TWO DIMENSIONAL PERIODIC STRUCTURES
➢ A mushroom-like periodic structure has been investigated in terms of parameters and dispersion
characterization of surface waves propagation in this chapter.
➢ The propagation property of electromagnetic waves is described by wavenumber, k.
➢ The phase constant, β, is equal to k in a lossless condition.
➢ Phase constant, β, is a function of frequency, ω.
➢ For a plane wave in free space, the linear function relationship between these two parameters is:
β(ω)=k=ω√𝜇0 𝜀0
➢ If the phase constant is already obtained, it helps for the derivation of the group velocity, vg,
and the phase velocity.
𝑉𝑝 =
𝜔
𝛽
and 𝑉𝑔 =
𝑑𝑤
𝑑𝛽
➢ The wavenumber, k, can be barely explained for the surface wave propagates along an EBG
structure. Fortunately, it can be determined well with either eigen-value solver of full wave
simulation.

9. ➢ In other words, several different propagation constants can exist at the same frequency. It means
that each one will refer to a particular mode with a specific own phase velocity, group velocity and
field distribution.
➢ The relationship for β and ω has been plotted and this graphic is called as dispersion diagram.
TWO DIMENSIONAL PERIODIC STRUCTURES
➢ EBG structure has a periodicity, so a surface wave’s field distribution is periodic as well.
➢ The periodicity is related to phase delay decided by the wavenumber, k, and the structure’s
periodicity.
➢ Also, each surface wave mode could be disintegrated in an infinite series of space harmonic waves.

10. TWO DIMENSIONAL PERIODIC STRUCTURES
➢ It is presumed that the direction of either periodicity and propagation is on the x direction.
➢ Same group velocity has been shared ever though there is a difference on phase velocities for each space
harmonics.
➢ Furthermore, the summation of space harmonics is required to satisfy the boundary conditions. If they are
individual, they are not satisfying for the boundary condition of the periodic structure.
➢ To sum up, they are considered to be the same mode.
𝐸⃗ (𝑥,𝑦,𝑧)= ෍
−∞
+∞
𝐸⃗ 𝑛(𝑦, 𝑧)𝑒−𝑗𝛽 𝑥𝑛 𝑥
𝛽 𝑥𝑛 𝑤 = 𝛽 𝑥 𝑤 + 𝑛
*
*
➢ Another observation from marked equation is related to the Brillouin Zone.
➢ The dispersion curve 𝛽𝑥(𝜔) has periodicity along the β-axis with a periodicity 2𝜋𝑝.
➢ Thus, only a single period ( 0≤𝛽𝑥𝑛≤2𝜋𝑝𝑥 ) is enough while plotting dispersion
diagram.
➢ This single period range is called as Brillouin Zone and builds the main concept of
two-dimensional periodic structures.

11. TWO DIMENSIONAL PERIODIC STRUCTURES
➢ Variation of the surface wave phase is known as Brillouin Zone.
➢ This phase variation is marked like; Г to Х, Х to М and М to Г.
➢ A simple illustration of the dispersion diagram for two-
dimensional periodic configuration based on Brillouin Zone
definition can be seen.
• Brillouin Zone definition for a 2D-periodic structure
Г X
M
Г to Х Х to М М to Г
Phase_x 0 to 180 deg 180deg 0 to 180 deg
Phase_y 0 0 to 180 deg 0 to 180 deg
• Phase Shift of a Brillouin Zone on a Graph

12. ➢ The unit cell of the structure is investigated by periodic boundary circumstance.
➢ Along Brillouin Zone the phase shift is changed, so thanks to eigenvalue solver
frequencies of eigenmodes are obtained for each step.
➢ Band gaps arise in frequency interval where there are not any dispersion curves in the
slow-wave region.
➢ On the contrary, dispersion curves are presented under the light line due to the slow-
wave behaviour of surface waves. Below figure represents a sample of dispersion
diagram and Phase shift distribution along the graph.
• Phase Shift of a Brillouin Zone on a Graph
TWO DIMENSIONAL PERIODIC STRUCTURES

13. Model and Dimension
➢ The design of a mushroom type electromagnetic band gap structure starts with a design of a unit cell.
➢ Resonant circuit model has been used to investigate the size of the unit cell.
➢ Substrate thickness and permittivity were already given parameters before starting.
Substrate : ISOLA-IS680-345
Dielectric Constant : 3.45 (ISOLA-IS680-345 )
Substrate Tickness :0.76 mm (ISOLA-IS680-345 )
➢ For this reason, the design procedure has started with equation solving via MATLAB, in order to get an
optimistic result to start simulation process.
➢ The first obtained parameters which is required for EBG unit cell design.
➢ There is an important point which has to be put into consideration that the via radius effect
is neglected in these equations, therefore they are helpful at the beginning to have an idea
about geometry design.
Width Gap Via
3.5 mm 0.05 mm 0.3mm

14. Model and Dimension
➢ A mushroom type electromagnetic band gap structure is a
combination of a grounded ISOLA substrate and a periodic
square patch above.
➢ The via is located in the middle of the patch to connect
grounded substrate and patch.
via
Rectangular Patch
Isola Substrate
d=W+g
d

15. Model and Dimension
Geometry design has followed this step:
• Define template: Dispersion Diagram
• Define material for the substrate ISOLA-IS680-345.
• Load COPPER from material library
• Define brick for substrate
• Define ground plane
• Define brick for patch
• Define cylinder for via
• Insert substrate and via by Boolean function
• Background properties has been set in NORMAL material
type; the height of the air box has been set ten times the
substrate thickness.
• Boundary conditions are set to periodic for each side walls
surrounding of the calculation box. Meanwhile, the boundary
conditions are defined as electric conductor. It is known that
open boundaries are not supported through the Eigenmode
Solver of CST MWS, for this reason the boundary condition for
unit cell side walls should be set periodic.
• Frequency setting from 0 to 12 GHz.
Periodic Boundary
Et=0

16. Model and Dimension
!
• CST MWS 2014 provides an easy way for phase shift functionality.
• Phase shift has a relation automatically with PathPara parameters sweeping from
0.1 to 2.9 cover whole Brillouin Zone and the dispersion diagram can be obtained
immediately.
• In older version, phase shifting has to be completed for three different routes
respectively and the results have to be gathered to complete dispersion diagram.

17. Model and Dimension
W g h εr via
4.13 mm 1 mm 0.76 mm 3.45 0.3 mm
• Dimensions for the unit cell of a mushroom type structure.

18. Implementation of the mushroom type EBG Structure on MIMO Antennas
➢ Designed mushroom type electromagnetic band gap structure has been placed in the middle of two designed
rectangular patch antennas.
➢ The geometry design has been done through CST MWS with following steps:
• Define Template: Planar Antenna.
• Define material: ISOLA-IS680-345.
• Define material from library: Copper (Annealed).
• Define brick for Patch1.
• Define brick for quarter wave transformer for Patch1.
• Define transmission line for Patch1.
• Define brick for Patch2.
• Define brick for quarter wave transformer for Patch2
• Define transmission line for Patch2
• Add shapes of Patch1 by Boolean function
• Add shapes of Patch 2 by Boolean function
• Define wave guard port1 and Patch2
• Define brick for the patch of a mushroom type EBG Structure
• Define cylinder for via of a mushroom type EBG Structure
• Translate the cell to complete the isolation between antennas along
y, +x and-x direction. 3 columns along x direction is enough.
• Add shapes by Boolean function
• Insert shapes by Boolean function
• Define boundaries
• Define background
• Define frequency range: 4-12 GHz
• Define field monitor for E-field-field and far field.

19. ➢ Later on, mentioned design steps above, the last geometry has been obtained and first simulation has been
run. The goal of it is observing the effect of mushroom type Electromagnetic Band Gap structure on mutual
coupling.
Implementation of the mushroom type EBG Structure on
MIMO Antennas
• MIMO Antenna System • MIMO Antenna System with EBG

20. S2,2
S1,2

21. ➢ Mutual coupling problem has been solved by
isolation of mushroom type electromagnetic
structure between two patches antennas.
➢ Previous slide shows the mushroom type EBG
structure effect on antennas’ performance.
➢ The red line represents the case of isolation by
mushroom type EBG structure while blue line
shows first designed antenna without any isolation
➢ Upper lines on the graph represents the right-handed antenna
and its S-parameter, S2,2.
➢ It can be easily seen that isolation did not help to increase its
performance. On the contrary, it is decreased by 4dB.
Unfortunately, it was slightly expected result.
➢ Today’s technology in order to reduce mutual coupling can
cause such that result, fortunately the antenna still resonates
10 GHz with a suitable performance.
➢ Below lines on the graph shows the left-handed antenna
which was suffer from mutual coupling.
➢ Thanks to designed mushroom like electromagnetic band gap
structure a band gap occurs at 10 GHz and helps to reduce
mutual coupling. Earlier S1,2 parameter was -40 dB, now it
went below to 75.03 dB