% % Tutorials / helical antenna % % Describtion at: % http://openems.de/index.php/Tutorial:_Helical_Antenna % % Tested with % - Matlab 2011a / Octave 3.4.3 % - openEMS v0.0.27 % % (C) 2012 Thorsten Liebig close all clear clc post_proc_only = 0; close all %% setup the simulation physical_constants; unit = 1e-3; % all length in mm f0 = 2.4e9; % center frequency, frequency of interest! lambda0 = round(c0/f0/unit); % wavelength in mm fc = 0.5e9; % 20 dB corner frequency Helix.radius = 20; % --> diameter is ~ lambda/pi Helix.turns = 10; % --> expected gain is G ~ 4 * 10 = 40 (16dBi) Helix.pitch = 30; % --> pitch is ~ lambda/4 Helix.wire_rad = 1; gnd.radius = lambda0/2; % feeding feed.width = 2; %feeding port width feed.heigth = 2; feed.R = 120; %feed impedance % size of the simulation box SimBox = [1 1 1.5]*2*lambda0; %% setup FDTD parameter & excitation function FDTD = InitFDTD( 30000 ); FDTD = SetGaussExcite( FDTD, f0, fc ); BC = {'MUR' 'MUR' 'MUR' 'MUR' 'MUR' 'PML_8'}; % boundary conditions FDTD = SetBoundaryCond( FDTD, BC ); %% setup CSXCAD geometry & mesh max_res = floor(c0 / (f0+fc) / unit / 20); % cell size: lambda/20 CSX = InitCSX(); mesh.x = [-SimBox(1)/2-gnd.radius -Helix.radius:Helix.wire_rad:Helix.radius SimBox(1)/2+gnd.radius]; mesh.x = SmoothMeshLines( mesh.x, max_res, 1.4); % create a smooth mesh between specified fixed mesh lines mesh.y = mesh.x; mesh.z = unique([-SimBox(3)/2 0:Helix.wire_rad:(Helix.turns*Helix.pitch+feed.heigth+Helix.wire_rad) (feed.heigth+Helix.wire_rad+Helix.turns*Helix.pitch)+SimBox(3)/2 ]); mesh.z = SmoothMeshLines( mesh.z, max_res, 1.4 ); CSX = DefineRectGrid( CSX, unit, mesh ); %% create helix using the wire primitive CSX = AddMetal( CSX, 'helix' ); % create a perfect electric conductor (PEC) ang = linspace(0,2*pi,21); coil_x = Helix.radius*cos(ang); coil_y = Helix.radius*sin(ang); coil_z = ang/2/pi*Helix.pitch; helix.x=[]; helix.y=[]; helix.z=[]; zpos = feed.heigth+Helix.wire_rad; for n=0:Helix.turns-1 helix.x = [helix.x coil_x]; helix.y = [helix.y coil_y]; helix.z = [helix.z coil_z+zpos]; zpos = zpos + Helix.pitch; end clear p p(1,:) = helix.x; p(2,:) = helix.y; p(3,:) = helix.z; CSX = AddWire(CSX, 'helix', 0, p, Helix.wire_rad); start = [Helix.radius-feed.width/2 -feed.width/2 feed.heigth]; stop = [Helix.radius+feed.width/2 +feed.width/2 feed.heigth+2*Helix.wire_rad]; CSX = AddBox(CSX,'helix',0,start,stop); %% create ground (same size as substrate) CSX = AddMetal( CSX, 'gnd' ); % create a perfect electric conductor (PEC) start = [0 0 -0.1]; stop = [0 0 0.1]; CSX = AddCylinder(CSX,'gnd',10,start,stop,gnd.radius); %% apply the excitation & resist as a current source start = [Helix.radius-feed.width/2 -feed.width/2 0]; stop = [Helix.radius+feed.width/2 +feed.width/2 feed.heigth]; [CSX] = AddLumpedPort(CSX, 5 ,1 ,feed.R, start, stop, [0 0 1], 'excite'); %%nf2ff calc start = [mesh.x(11) mesh.y(11) mesh.z(11)]; stop = [mesh.x(end-10) mesh.y(end-10) mesh.z(end-10)]; [CSX nf2ff] = CreateNF2FFBox(CSX, 'nf2ff', start, stop, 'OptResolution', lambda0/15); %% prepare simulation folder Sim_Path = 'tmp_Helical_Ant'; Sim_CSX = 'Helix_Ant.xml'; if (post_proc_only==0) [status, message, messageid] = rmdir( Sim_Path, 's' ); % clear previous directory [status, message, messageid] = mkdir( Sim_Path ); % create empty simulation folder %% write openEMS compatible xml-file WriteOpenEMS( [Sim_Path '/' Sim_CSX], FDTD, CSX ); %% show the structure CSXGeomPlot( [Sim_Path '/' Sim_CSX] ); %% run openEMS RunOpenEMS( Sim_Path, Sim_CSX); end %% postprocessing & do the plots freq = linspace( f0-fc, f0+fc, 501 ); U = ReadUI( {'port_ut1','et'}, Sim_Path, freq ); % time domain/freq domain voltage I = ReadUI( 'port_it1', Sim_Path, freq ); % time domain/freq domain current (half time step is corrected) % plot feed point impedance figure Zin = U.FD{1}.val ./ I.FD{1}.val; plot( freq/1e6, real(Zin), 'k-', 'Linewidth', 2 ); hold on grid on plot( freq/1e6, imag(Zin), 'r--', 'Linewidth', 2 ); title( 'feed point impedance' ); xlabel( 'frequency f / MHz' ); ylabel( 'impedance Z_{in} / Ohm' ); legend( 'real', 'imag' ); % plot reflection coefficient S11 figure uf_inc = 0.5*(U.FD{1}.val + I.FD{1}.val * feed.R); if_inc = 0.5*(I.FD{1}.val + U.FD{1}.val / feed.R); uf_ref = U.FD{1}.val - uf_inc; if_ref = if_inc - I.FD{1}.val; s11 = uf_ref ./ uf_inc; plot( freq/1e6, 20*log10(abs(s11)), 'k-', 'Linewidth', 2 ); grid on title( 'reflection coefficient S_{11}' ); xlabel( 'frequency f / MHz' ); ylabel( 'reflection coefficient |S_{11}|' ); P_in = 0.5*uf_inc .* conj( if_inc ); % accepted antenna feed power drawnow %% NFFF contour plots %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %find resonance frequncy from s11 f_res = f0; % get accepted antenna power at frequency f0 P_in_0 = interp1(freq, P_in, f0); % calculate the far field at phi=0 degrees and at phi=90 degrees thetaRange = unique([0:0.5:90 90:180]); phiRange = (0:2:360) - 180; disp( 'calculating far field at phi=[0 90] deg...' ); nf2ff = CalcNF2FF(nf2ff, Sim_Path, f_res, thetaRange*pi/180, phiRange*pi/180,'Mode',1,'Outfile','3D_Pattern.h5','Verbose',1); theta_HPBW = thetaRange(find(nf2ff.E_norm{1}(:,1)