Planewave behaving like a point source

[mikeb127]

Hi All,

Apologies if this is a stupid question, but I am trying to get a plane wave source to test some EM shielding enclosure designs, and I cannot get the source to stop behaving like a simple point source. My code is below:

`% OpenEMS FDTD Analysis Automation Script for Microwave Protection with HDF5 Handling
% To be run with MATLAB or GNU Octave.
% FreeCAD to OpenEMS plugin by Lubomir Jagos.

# For the purpose of shielding, we consider far-field to be d > λ/2π
close all
clear
clc

addpath('/home/michael/opt/openEMS/share/openEMS/matlab')
addpath('/home/michael/opt/openEMS/share/CSXCAD/matlab')

% Set current directory
if ~isdeployed
    mfile_name = mfilename('fullpath');
    [pathstr, ~, ~] = fileparts(mfile_name);
    cd(pathstr);
end

% Constants
physical_constants;
unit = 0.001; % mm
fc_unit = 0.001; % STL mm

% Switches
postprocessing_only = 0;
draw_3d_pattern = 0; % Enable 3D visualization

currDir = strrep(pwd, '\', '/');
disp(['Current directory: ', currDir]);

% OpenEMS options
openEMS_opts = '--verbose --debug-material'; % Added --debug-material for diagnostics

% Prepare simulation folder
Sim_Path = 'simulation_output';
Sim_CSX = 'PEC_conductor_only.xml';
if exist(Sim_Path, 'dir')
    rmdir(Sim_Path, 's');
end
mkdir(Sim_Path);

% Check STL file
#stl_file = [currDir, '/Cut001_gen_model.stl'];
#if ~exist(stl_file, 'file')
#    error('STL file %s not found. Ensure Cut_gen_model.stl is in %s.', stl_file, currDir);
#end

% FDTD setup
max_timesteps = 500000; % Increased to ensure multiple time steps
min_decrement = 1e-8; % Tighter criteria for better convergence
FDTD = InitFDTD('NrTS', max_timesteps,'EndCriteria',min_decrement);


% Boundary conditions
BC = {'PML_8', 'PML_8', 'PML_8', 'PML_8', 'PML_8', 'PML_8'};
FDTD = SetBoundaryCond(FDTD, BC);

% Coordinate system
CSX = InitCSX('CoordSystem', 0);
mesh.x = [];
mesh.y = [];
mesh.z = [];
CSX = DefineRectGrid(CSX, unit, mesh);

% Excitation: 40 GHz for microwave protection
f0 = 40e9;
FDTD = SetSinusExcite(FDTD,f0);

%% MATERIAL - PEC
#CSX = AddMetal(CSX, 'PEC');
#CSX = ImportSTL(CSX, 'PEC', 9900, [currDir '/Cut001_gen_model.stl'], 'Transform', {'Scale', fc_unit/unit});
#CSX = ImportSTL(CSX, 'PEC', 10000, [currDir '/Tube_gen_model.stl'], 'Transform', {'Scale', fc_unit/unit});

% Grid lines
mesh.x = (-11:.35:21);
mesh.y = (-11:.35:21);
mesh.z = (-81:.35:28);
CSX = DefineRectGrid(CSX, unit, mesh);

#start = [0, 0, -71];
#stop = [10, 10, -55];
#stop = [10, 10, 20];
start = [-6, -6, -71];
stop = [16, 16, -55];

# START EXCITATION OPTION 1
k_dir = [0, 0, 1]; # Moving along z-axis
E_dir = [0, 200, 0]; # E-y polarized field 200 V/m
CSX = AddPlaneWaveExcite(CSX, 'plane_wave', k_dir, E_dir, f0);
CSX = AddBox(CSX, 'plane_wave', 0, start, stop);
# END EXCITATION OPTION 1

# START EXCITATION OPTION 2
%% apply the excitation %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
#CSX = AddExcitation(CSX,'excite',1,[0 0 200]); # 200 V/m in z direction
#CSX = AddBox(CSX,'excite',0 ,start,stop);
# END EXCITATION OPTION 2

# START EXCITATION OPTION 3
%% apply the excitation %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
#CSX = AddExcitation(CSX,'excite',10, [0 200 0], 'PropDir', [0 0 1]); # 200 V/m E-y Field travelling in Z direction
#CSX = AddBox(CSX,'excite',0 ,start,stop);
# END EXCITATION OPTION 3

probe_pos = [5, 5, -5];
CSX = AddProbe(CSX, 'E_field', 2); # Probe type: E-field
CSX = AddPoint(CSX, 'E_field', 1, probe_pos);

CSX = AddProbe(CSX, 'E_field2', 2); # Probe type: E-field
CSX = AddPoint(CSX, 'E_field2', 1, [5,5,4]);

CSX = AddProbe(CSX, 'E_field3',2); # Probe type: E-field
CSX = AddPoint(CSX, 'E_field3', 1, [5,5,-60]);


% Run simulation
WriteOpenEMS([Sim_Path, '/', Sim_CSX], FDTD, CSX);
CSXGeomPlot([Sim_Path, '/', Sim_CSX]);

if postprocessing_only == 1
    disp('Running OpenEMS simulation...');
    try
        RunOpenEMS(Sim_Path, Sim_CSX, openEMS_opts);
    catch e
        warning('OpenEMS simulation failed: %s. Check %s for logs.', e.message, Sim_Path);
        return;
    end
end


U = ReadUI('E_field2', Sim_Path);

% Deconstruct readUI results for graph
E_x = U.TD{1,1}.val;
E_y = U.TD{1,1}.additional(1,:);
E_z = U.TD{1,1}.additional(2,:);
time = U.TD{1,1}.t;

E_magnitude = sqrt(power(E_x,2) + power(E_y,2) + power(E_z,2));

figure
plot( 1E10 * time, E_magnitude, 'k-', 'Linewidth', 2 );
hold on
grid on

title( 'Voltage vs Time' );
xlabel( 'Time (pS)' );
ylabel( 'E-field (V/m)' );
#legend( 'port 1');
legend( 'port 2' );
axis( [0 20000] );

For test purposes I have removed all non free space items from the simulation:

This particular scenario has the boundaries of the source close to the PML layers, but I have also tried a smaller source box further away and no differences

The output graphs of the different probes are:

Efield3 - In the source box:

Efield - -5mm from the origin:

Efield2 - 4mm from the origin:

Can anyone provide any guidance on how to resolve this so I have a relatively consistent E-field similar to a plane wave in free space throughout the simulation?

[thliebig]

I'm not sure I understand the question or task fully but here is a quick comment on how a plane wave source is supposed to behave.
You only have a plane wave inside the source box. Outside of it you should only see reflections from any objects inside the box.
This is usually used e.g. to calculate radar reflections (RCS) and such.
I hope this helps?

[mikeb127]

That's what I needed. When I extend the source box to cover the piece/Faraday cage under test it works correctly. For some reason I had it in my head that I would only need to define the 'launch point' of the plane wave and it would continue throughout the mesh in the relevant direction rather than needing to define the entire area for the plane wave.

Thanks for taking the time to answer.

[mikeb127]

@thliebig as a side note here: now that I have it working as expected would you be open to reviewing a PR with an example/tutorial for docs on EMI shielding test simulations? I'm happy to do a write up over the next few weeks if it is the sort of thing that could be useful.

[thliebig]

Feel free to post it maybe here for me and others to review, update etc.? And maybe then create a PR for it?
We could do it as a PR directly, but I have the feeling not many (others) look at them?