%
% Stripline to Microstrip Line Transition
%
% Description at:
% <http://openems.de/index.php/Tutorial:_Stripline_to_MSL_Transition>
%
% Tested with
%  - Octave 4.0
%  - openEMS v0.0.35
%
% (C) 2017 Thorsten Liebig <thorsten.liebig@gmx.de>

close all
clear
clc

%% Setup the Simulation
physical_constants;
unit = 1e-6; % specify everything in um

line_length     = 15000; % line length of strip line and microstrip line
substrate_width = 6000;
air_spacer      = 4000;  % air spacer above the substrate

msl_width               = 500;
msl_substrate_thickness = 254;

strip_width               = 500;
strip_substrate_thickness = 512;

connect_via_rad =  500/2;
connect_via_gap = 1250/2;

substrate_epr    = 3.66;
substrate_kappa  = 1e-3 * 2*pi*2.45e9 * EPS0*substrate_epr; % substrate losses

f_max = 10e9;
resolution = 250;
edge_res   = 25;
feed_shift = 2500;
meas_shift = 5000;

%% Setup FDTD Parameters & Excitation Function
FDTD = InitFDTD();
FDTD = SetGaussExcite( FDTD, f_max/2, f_max/2);
BC   = {'PML_8' 'PML_8' 'PEC' 'PEC' 'PEC' 'MUR'};
FDTD = SetBoundaryCond( FDTD, BC );

%% Setup CSXCAD Geometry & Mesh
CSX = InitCSX();
edge_mesh  = [-1/3 2/3]*edge_res; % 1/3 - 2/3 rule for 2D metal edges

mesh.x = SmoothMeshLines( [-connect_via_gap 0 connect_via_gap], 2*edge_res, 1.5 );
mesh.x = SmoothMeshLines( [-line_length mesh.x line_length], resolution, 1.5);
mesh.y = SmoothMeshLines( [0 msl_width/2+edge_mesh substrate_width/2], resolution/4 , 1.5);
mesh.y = sort(unique([-mesh.y mesh.y]));
mesh.z = SmoothMeshLines( [linspace(-strip_substrate_thickness,0,5) linspace(0,strip_substrate_thickness,5) linspace(strip_substrate_thickness,msl_substrate_thickness+strip_substrate_thickness,5) 2*strip_substrate_thickness+air_spacer] , resolution );
CSX = DefineRectGrid( CSX, unit, mesh );

% Create Substrate
CSX = AddMaterial( CSX, 'RO4350B' );
CSX = SetMaterialProperty( CSX, 'RO4350B', 'Epsilon', substrate_epr, 'Kappa', substrate_kappa );
start = [mesh.x(1),   mesh.y(1),   -strip_substrate_thickness];
stop  = [mesh.x(end), mesh.y(end), +strip_substrate_thickness+msl_substrate_thickness];
CSX = AddBox( CSX, 'RO4350B', 0, start, stop );

% Create a PEC called 'metal' and 'gnd'
CSX = AddMetal( CSX, 'gnd' ); 
CSX = AddMetal( CSX, 'metal' ); 

% Create strip line port (incl. metal strip line)
start = [-line_length -strip_width/2  0];
stop  = [0            +strip_width/2  0];
[CSX,port{1}] = AddStripLinePort( CSX, 100, 1, 'metal', start, stop, strip_substrate_thickness, 'x', [0 0 -1], 'ExcitePort', true, 'FeedShift', feed_shift, 'MeasPlaneShift', meas_shift );

% Create MSL port on top
start = [line_length  -strip_width/2 strip_substrate_thickness+msl_substrate_thickness];
stop  = [0            +strip_width/2 strip_substrate_thickness];
[CSX,port{2}] = AddMSLPort( CSX, 100, 2, 'metal', start, stop, 'x', [0 0 -1], 'MeasPlaneShift', meas_shift );

% transitional via
start = [0, 0, 0];
stop  = [0, 0, strip_substrate_thickness+msl_substrate_thickness];
CSX = AddCylinder(CSX, 'metal', 100, start, stop, connect_via_rad);

% metal plane between strip line and MSL, including hole for transition
p(1,1) = mesh.x(1);
p(2,1) = mesh.y(1);
p(1,2) = 0;
p(2,2) = mesh.y(1);
for a = linspace(-pi, pi, 21)
  p(1,end+1) = connect_via_gap*sin(a);
  p(2,end)   = connect_via_gap*cos(a);
endfor
p(1,end+1) = 0;
p(2,end  ) = mesh.y(1);
p(1,end+1) = mesh.x(end);
p(2,end  ) = mesh.y(1);
p(1,end+1) = mesh.x(end);
p(2,end  ) = mesh.y(end);
p(1,end+1) = mesh.x(1);
p(2,end  ) = mesh.y(end);
CSX = AddPolygon( CSX, 'gnd', 1, 'z', strip_substrate_thickness, p);

%% Write/Show/Run the openEMS compatible xml-file
Sim_Path = 'tmp';
Sim_CSX = 'strip2msl.xml';

[status, message, messageid] = rmdir( Sim_Path, 's' ); % clear previous directory
[status, message, messageid] = mkdir( Sim_Path ); % create empty simulation folder

WriteOpenEMS( [Sim_Path '/' Sim_CSX], FDTD, CSX );
CSXGeomPlot( [Sim_Path '/' Sim_CSX] );
RunOpenEMS( Sim_Path, Sim_CSX );

%% Post-Processing
close all
f = linspace( 0, f_max, 1601 );
port = calcPort( port, Sim_Path, f, 'RefImpedance', 50);

s11 = port{1}.uf.ref./ port{1}.uf.inc;
s21 = port{2}.uf.ref./ port{1}.uf.inc;

plot(f/1e9,20*log10(abs(s11)),'k-','LineWidth',2);
hold on;
grid on;
plot(f/1e9,20*log10(abs(s21)),'r--','LineWidth',2);
legend('S_{11}','S_{21}');
ylabel('S-Parameter (dB)','FontSize',12);
xlabel('frequency (GHz) \rightarrow','FontSize',12);
ylim([-40 2]);