192 lines
5.3 KiB
Python
192 lines
5.3 KiB
Python
# -*- coding: utf-8 -*-
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"""
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Tutorials / helical antenna
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Tested with
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- python 3.4
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- openEMS v0.0.33+
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(C) 2015-2016 Thorsten Liebig <thorsten.liebig@gmx.de>
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"""
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import os, tempfile
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from pylab import *
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from CSXCAD import CSXCAD
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from openEMS.openEMS import openEMS
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from openEMS.physical_constants import *
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Sim_Path = os.path.join(tempfile.gettempdir(), 'Helical_Ant')
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post_proc_only = False
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## setup the simulation
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unit = 1e-3 # all length in mm
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f0 = 2.4e9 # center frequency, frequency of interest!
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lambda0 = round(C0/f0/unit) # wavelength in mm
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fc = 0.5e9 # 20 dB corner frequency
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Helix_radius = 20 # --> diameter is ~ lambda/pi
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Helix_turns = 10 # --> expected gain is G ~ 4 * 10 = 40 (16dBi)
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Helix_pitch = 30 # --> pitch is ~ lambda/4
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Helix_mesh_res = 3
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gnd_radius = lambda0/2
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# feeding
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feed_heigth = 3
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feed_R = 120 #feed impedance
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# size of the simulation box
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SimBox = array([1, 1, 1.5])*2.0*lambda0
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## setup FDTD parameter & excitation function
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FDTD = openEMS(EndCriteria=1e-4)
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FDTD.SetGaussExcite( f0, fc )
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FDTD.SetBoundaryCond( ['MUR', 'MUR', 'MUR', 'MUR', 'MUR', 'PML_8'] )
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## setup CSXCAD geometry & mesh
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CSX = CSXCAD.ContinuousStructure()
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FDTD.SetCSX(CSX)
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mesh = CSX.GetGrid()
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mesh.SetDeltaUnit(unit)
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max_res = floor(C0 / (f0+fc) / unit / 20) # cell size: lambda/20
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# create helix mesh
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mesh.AddLine('x', [-Helix_radius, 0, Helix_radius])
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mesh.SmoothMeshLines('x', Helix_mesh_res)
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# add the air-box
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mesh.AddLine('x', [-SimBox[0]/2-gnd_radius, SimBox[0]/2+gnd_radius])
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# create a smooth mesh between specified fixed mesh lines
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mesh.SmoothMeshLines('x', max_res, ratio=1.4)
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# copy x-mesh to y-direction
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mesh.SetLines('y', mesh.GetLines('x'))
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# create helix mesh in z-direction
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mesh.AddLine('z', [0, feed_heigth, Helix_turns*Helix_pitch+feed_heigth])
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mesh.SmoothMeshLines('z', Helix_mesh_res)
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# add the air-box
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mesh.AddLine('z', [-SimBox[2]/2, max(mesh.GetLines('z'))+SimBox[2]/2 ])
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# create a smooth mesh between specified fixed mesh lines
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mesh.SmoothMeshLines('z', max_res, ratio=1.4)
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## create helix using the wire primitive
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helix_metal = CSX.AddMetal('helix' ) # create a perfect electric conductor (PEC)
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ang = linspace(0,2*pi,21)
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coil_x = Helix_radius*cos(ang)
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coil_y = Helix_radius*sin(ang)
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coil_z = ang/2/pi*Helix_pitch
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Helix_x=np.array([])
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Helix_y=np.array([])
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Helix_z=np.array([])
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zpos = feed_heigth
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for n in range(Helix_turns-1):
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Helix_x = r_[Helix_x, coil_x]
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Helix_y = r_[Helix_y, coil_y]
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Helix_z = r_[Helix_z ,coil_z+zpos]
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zpos = zpos + Helix_pitch
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p = np.array([Helix_x, Helix_y, Helix_z])
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CSX.AddCurve(helix_metal, p)
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## create ground circular ground
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gnd = CSX.AddMetal( 'gnd' ) # create a perfect electric conductor (PEC)
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# add a box using cylindrical coordinates
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start = [0, 0, -0.1]
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stop = [0, 0, 0.1]
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CSX.AddCylinder(gnd, start, stop, radius=gnd_radius)
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### apply the excitation & resist as a current source
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start = [Helix_radius, 0, 0]
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stop = [Helix_radius, 0, feed_heigth]
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port = FDTD.AddLumpedPort(1 ,feed_R, start, stop, 'z', 1.0, priority=5)
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## nf2ff calc
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nf2ff = FDTD.CreateNF2FFBox(opt_resolution=[lambda0/15]*3)
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if 0: # debugging only
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CSX_file = os.path.join(Sim_Path, 'helix.xml')
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CSX.Write2XML(CSX_file)
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os.system(r'AppCSXCAD "{}"'.format(CSX_file))
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if not post_proc_only:
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FDTD.Run(Sim_Path, verbose=3, cleanup=True)
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## postprocessing & do the plots
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freq = linspace( f0-fc, f0+fc, 501 )
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port.CalcPort(Sim_Path, freq)
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Zin = port.uf_tot / port.if_tot
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s11 = port.uf_ref / port.uf_inc
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## plot feed point impedance
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figure()
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plot( freq/1e6, real(Zin), 'k-', linewidth=2, label=r'$\Re(Z_{in})$' )
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grid()
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plot( freq/1e6, imag(Zin), 'r--', linewidth=2, label=r'$\Im(Z_{in})$' )
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title( 'feed point impedance' )
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xlabel( 'frequency (MHz)' )
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ylabel( 'impedance ($\Omega$)' )
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legend( )
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## plot reflection coefficient S11
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figure()
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plot( freq/1e6, 20*log10(abs(s11)), 'k-', linewidth=2 )
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grid()
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title( 'reflection coefficient $S_{11}$' )
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xlabel( 'frequency (MHz)' )
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ylabel( 'reflection coefficient $|S_{11}|$' )
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## NFFF contour plots ####################################################
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## calculate the far field at phi=0 degrees and at phi=90 degrees
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theta = arange(0.,180.,1.)
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phi = arange(-180,180,2)
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disp( 'calculating the 3D far field...' )
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nf2ff.CalcNF2FF(Sim_Path, f0, theta, phi, read_cached=True, verbose=True )
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#
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Dmax_dB = 10*log10(nf2ff.Dmax[0])
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E_norm = 20.0*log10(nf2ff.E_norm[0]/np.max(nf2ff.E_norm[0])) + 10*log10(nf2ff.Dmax[0])
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theta_HPBW = theta[ np.where(squeeze(E_norm[:,phi==0])<Dmax_dB-3)[0][0] ]
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#
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# display power and directivity
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print('radiated power: Prad = {} W'.format(nf2ff.Prad[0]))
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print('directivity: Dmax = {} dBi'.format(Dmax_dB))
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print('efficiency: nu_rad = {} %'.format(100*nf2ff.Prad[0]/interp(f0, freq, port.P_acc)))
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print('theta_HPBW = {} °'.format(theta_HPBW))
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##
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E_norm = 20.0*log10(nf2ff.E_norm[0]/np.max(nf2ff.E_norm[0])) + 10*log10(nf2ff.Dmax[0])
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E_CPRH = 20.0*log10(np.abs(nf2ff.E_cprh[0])/np.max(nf2ff.E_norm[0])) + 10*log10(nf2ff.Dmax[0])
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E_CPLH = 20.0*log10(np.abs(nf2ff.E_cplh[0])/np.max(nf2ff.E_norm[0])) + 10*log10(nf2ff.Dmax[0])
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##
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figure()
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plot(theta, E_norm[:,phi==0],'k-' , linewidth=2, label='$|E|$')
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plot(theta, E_CPRH[:,phi==0],'g--', linewidth=2, label='$|E_{CPRH}|$')
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plot(theta, E_CPLH[:,phi==0],'r-.', linewidth=2, label='$|E_{CPLH}|$')
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grid()
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xlabel('theta (deg)')
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ylabel('directivity (dBi)')
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title('Frequency: {} GHz'.format(nf2ff.freq[0]/1e9))
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legend()
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### dump to vtk
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# TODO
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show()
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