# -*- coding: utf-8 -*- """ Helical Antenna Tutorial Tested with - python 3.4 - openEMS v0.0.33+ (C) 2015-2016 Thorsten Liebig """ ### Import Libraries import os, tempfile from pylab import * from CSXCAD import CSXCAD from openEMS import openEMS from openEMS.physical_constants import * ### Setup the simulation Sim_Path = os.path.join(tempfile.gettempdir(), 'Helical_Ant') post_proc_only = False 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_mesh_res = 3 gnd_radius = lambda0/2 # feeding feed_heigth = 3 feed_R = 120 #feed impedance # size of the simulation box SimBox = array([1, 1, 1.5])*2.0*lambda0 ### Setup FDTD parameter & excitation function FDTD = openEMS(EndCriteria=1e-4) FDTD.SetGaussExcite( f0, fc ) FDTD.SetBoundaryCond( ['MUR', 'MUR', 'MUR', 'MUR', 'MUR', 'PML_8'] ) ### Setup Geometry & Mesh CSX = CSXCAD.ContinuousStructure() FDTD.SetCSX(CSX) mesh = CSX.GetGrid() mesh.SetDeltaUnit(unit) max_res = floor(C0 / (f0+fc) / unit / 20) # cell size: lambda/20 # create helix mesh mesh.AddLine('x', [-Helix_radius, 0, Helix_radius]) mesh.SmoothMeshLines('x', Helix_mesh_res) # add the air-box mesh.AddLine('x', [-SimBox[0]/2-gnd_radius, SimBox[0]/2+gnd_radius]) # create a smooth mesh between specified fixed mesh lines mesh.SmoothMeshLines('x', max_res, ratio=1.4) # copy x-mesh to y-direction mesh.SetLines('y', mesh.GetLines('x')) # create helix mesh in z-direction mesh.AddLine('z', [0, feed_heigth, Helix_turns*Helix_pitch+feed_heigth]) mesh.SmoothMeshLines('z', Helix_mesh_res) # add the air-box mesh.AddLine('z', [-SimBox[2]/2, max(mesh.GetLines('z'))+SimBox[2]/2 ]) # create a smooth mesh between specified fixed mesh lines mesh.SmoothMeshLines('z', max_res, ratio=1.4) ### Create the Geometry ## * Create the metal helix using the wire primitive. ## * Create a metal gorund plane as cylinder. # create a perfect electric conductor (PEC) helix_metal = CSX.AddMetal('helix' ) 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=np.array([]) Helix_y=np.array([]) Helix_z=np.array([]) zpos = feed_heigth for n in range(Helix_turns-1): Helix_x = r_[Helix_x, coil_x] Helix_y = r_[Helix_y, coil_y] Helix_z = r_[Helix_z ,coil_z+zpos] zpos = zpos + Helix_pitch p = np.array([Helix_x, Helix_y, Helix_z]) helix_metal.AddCurve(p) # create ground circular ground gnd = CSX.AddMetal( 'gnd' ) # create a perfect electric conductor (PEC) # add a box using cylindrical coordinates start = [0, 0, -0.1] stop = [0, 0, 0.1] gnd.AddCylinder(start, stop, radius=gnd_radius) # apply the excitation & resist as a current source start = [Helix_radius, 0, 0] stop = [Helix_radius, 0, feed_heigth] port = FDTD.AddLumpedPort(1 ,feed_R, start, stop, 'z', 1.0, priority=5) # nf2ff calc nf2ff = FDTD.CreateNF2FFBox(opt_resolution=[lambda0/15]*3) ### Run the simulation if 0: # debugging only CSX_file = os.path.join(Sim_Path, 'helix.xml') if not os.path.exists(Sim_Path): os.mkdir(Sim_Path) CSX.Write2XML(CSX_file) os.system(r'AppCSXCAD "{}"'.format(CSX_file)) if not post_proc_only: FDTD.Run(Sim_Path, verbose=3, cleanup=True) ### Postprocessing & plotting freq = linspace( f0-fc, f0+fc, 501 ) port.CalcPort(Sim_Path, freq) Zin = port.uf_tot / port.if_tot s11 = port.uf_ref / port.uf_inc ## Plot the feed point impedance figure() plot( freq/1e6, real(Zin), 'k-', linewidth=2, label=r'$\Re(Z_{in})$' ) grid() plot( freq/1e6, imag(Zin), 'r--', linewidth=2, label=r'$\Im(Z_{in})$' ) title( 'feed point impedance' ) xlabel( 'frequency (MHz)' ) ylabel( 'impedance ($\Omega$)' ) legend( ) ## Plot reflection coefficient S11 figure() plot( freq/1e6, 20*log10(abs(s11)), 'k-', linewidth=2 ) grid() title( 'reflection coefficient $S_{11}$' ) xlabel( 'frequency (MHz)' ) ylabel( 'reflection coefficient $|S_{11}|$' ) ### Create the NFFF contour ## * calculate the far field at phi=0 degrees and at phi=90 degrees theta = arange(0.,180.,1.) phi = arange(-180,180,2) disp( 'calculating the 3D far field...' ) nf2ff_res = nf2ff.CalcNF2FF(Sim_Path, f0, theta, phi, read_cached=True, verbose=True ) Dmax_dB = 10*log10(nf2ff_res.Dmax[0]) E_norm = 20.0*log10(nf2ff_res.E_norm[0]/np.max(nf2ff_res.E_norm[0])) + 10*log10(nf2ff_res.Dmax[0]) theta_HPBW = theta[ np.where(squeeze(E_norm[:,phi==0])