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function field = GetField_TD2FD(field, freq)
% function field = GetField_TD2FD(field, freq)
%
% Transforms time-domain field data into the frequency domain
% Autocorrects the half-timestep offset of the H-field
%
% example:
% freq = linspace(0,1e9,100); %target frequency vector (Hz)
% field = ReadHDF5FieldData('tmp/Ht.h5');
% field_FD = GetField_TD2FD(field, freq);
%
% openEMS matlab interface
% -----------------------
% author: Thorsten Liebig
%
% See also ReadHDF5FieldData
if (~isfield(field,'TD'))
warning('openEMS:GetField_TD2FD','field has no time domain data... skipping FD transformation...');
return
end
t = field.TD.time;
dt = t(2)-t(1);
clear field.FD
field.FD.frequency = freq;
for nf = 1:numel(freq)
field.FD.values{nf} = 0;
end
numTS = numel(field.TD.values);
for n=1:numTS
for nf = 1:numel(freq)
f = freq(nf);
field.FD.values{nf} = field.FD.values{nf} + field.TD.values{n}.*exp(-1i*2*pi*f*t(n)) * 2 * dt;
% t(n) is absolute time and therefore the half-timestep offset of
% the H-field is automatically compensated
% openEMS output: E-fields start at t=0
% openEMS output: H-fields start at t=delta_t/2
end
end
field.FD.DataType=1;
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