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Create coplanar waveguide transmission line


Use the txlineCPW object to create a coplanar waveguide (CPW) transmission line. You can also use the txlineCPW object to create an infinite-bottom conductor or ground plane CPW transmission line. The cross-section of a CPW transmission line is shown in this figure. The physical characteristics of the CPW transmission line include conductor width (w), conductor thickness (t), slot width (s), substrate height (d), and permittivity constant (ε).

Cross-section of a coplanar waveguide and conductor-backed CPW transmission line



txline = txlineCPW creates a default CPW transmission line object.


txline = txlineCPW(Name,Value) sets Properties using one or more name-value pairs. For example, txline = txlineCPW('SlotWidth',0.0046) creates a CPW transmission line with a slot width of 0.0046 meters.


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Name of the CPW transmission line, specified as a string scalar or a character vector.

Example: 'Name','cpw1'

Example: txline.Name = 'cpw1'

Data Types: char | string

Physical width of the conductor, specified as a positive scalar in meters.

Example: 'ConductorWidth',0.0200

Example: txline.ConductorWidth = 0.0200

Data Types: double

Physical width of the slot, specified as a positive scalar in meters.

Example: 'SlotWidth',0.0008

Example: txline.SlotWidth = 0.0008

Data Types: double

Physical height of the conductor or dielectric thickness, specified as a positive scalar in meters.

Example: 'Height',0.000835

Example: txline.Height = 0.000835

Data Types: double

Physical thickness of the CPW transmission line, specified as a positive scalar in meters.

Example: 'Thickness',0.000008

Example: txline Thickness = 0.000008

Data Types: double

Relative permittivity of the dielectric, specified as a positive scalar.

Example: 'EpsilonR',8.8

Example: txline.EpsilonR = 8.8

Data Types: double

Loss angle tangent of the dielectric, specified as a nonnegative scalar.

Example: 'LossTangent',1

Example: txline.LossTangent = 1

Data Types: double

Conductivity of the conductor, specified as a nonnegative scalar in Siemens per meter (S/m).

Example: 'SigmaCond',2

Example: txline.SigmaCond = 2

Data Types: double

Physical length of the CPW transmission line, specified as a positive scalar in meters.

Example: 'LineLength',0.0200

Example: txline.LineLength = 0.0200

Data Types: double

Infinite-bottom conductor or ground plane, specified as a numeric or logical. When you specify 0, you create a transmission line without any conductor backing.

Example: 'ConductorBacked',1

Example: txline.ConductorBacked = 1

Data Types: double

Stub transmission line termination, specified as 'NotApplicable', 'Open', or 'Short'.

Example: 'Termination','Short'

Example: txline.Termination = 'Short'

Data Types: char

Type of stub, specified as 'NotAStub', 'Series', or 'Shunt'.

Example: 'StubMode','Series'

Example: txline.StubMode = 'Series'

Data Types: char

This property is read-only.

Number of input and output ports, returned as positive scalar.

Data Types: double

This property is read-only.

Terminals of the CPW transmission line, returned as a cell array of strings.

Data Types: char | string

Object Functions

sparametersCalculate S-parameters for RF data, network, circuit, and matching network objects
groupdelayGroup delay of S-parameter object or RF filter object or RF Toolbox circuit object
noisefigureCalculate noise figure of transmission lines, series RLC, and shunt RLC circuits
getZ0Calculate characteristic impedance with and without dispersion for transmission line
circuitCircuit object
cloneCreate copy of existing circuit element or circuit object


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Create a coplanar waveguide transmission line using these specifications:

  • Conductor width : 45 mu m

  • Slot width : 50 mu m

  • Height of substrate : 525 mu m

  • Thickness : 1 mu m

  • GaAS permittivity or Epsilon R : 2.5 F/m

  • Conductivity : 3.33e7 S/m

cpwtxline = txlineCPW('ConductorWidth',45e-6,'SlotWidth',50e-6,'Height',525e-6,...
cpwtxline = 
  txlineCPW: CPW element

              Name: 'CPW'
    ConductorWidth: 4.5000e-05
         SlotWidth: 5.0000e-05
            Height: 5.2500e-04
         Thickness: 1.0000e-06
          EpsilonR: 2.5000
       LossTangent: 0
         SigmaCond: 33300000
        LineLength: 0.0100
       Termination: 'NotApplicable'
          StubMode: 'NotAStub'
          NumPorts: 2
         Terminals: {'p1+'  'p2+'  'p1-'  'p2-'}

Calculate the S-parameters of the transmission line at 20 GHz.

sparam = sparameters(cpwtxline,20e9);

This example uses RF PCB Toolbox to calculate an electromagnetic (EM) solver S-parameter model of a conductor-backed coplanar waveguide (CPW) transmission line.

Behavioral Modeling

Select the FR4 dielectric and silver metal the metal and dielectric libraries, respectively, of RF PCB Toolbox.

dFR4 = dielectric('FR4');
mSilver = metal('Silver');

Design an infinite-bottom conductor-backed CPW transmission line at 6 GHz and Z0 of 75Ω.

tx = txlineCPW('ConductorBacked',true, ...
    'LineLength',0.0145,'ConductorWidth',2.6482e-4, ...
    'SlotWidth',2e-4,'Height',6.35e-4, ...
    "EpsilonR",dFR4.EpsilonR,"LossTangent",dFR4.LossTangent, ...
    "SigmaCond",mSilver.Conductivity,'Thickness',2e-7, ...

Calculate the S-parameters using a behavioral model from RF Toolbox with the reference impedance of 50Ω. The behavioral model is also called an analytical or closed-form model.

freq = (1:5:66)*100e6;
Srf = sparameters(tx,freq,50);

EM Modeling

Create an RF PCB Toolbox coplanarWaveguide object.

tx_em = coplanarWaveguide(tx);

View the conductor-backed CPW transmission line.


Figure contains an axes object. The axes object with title coplanarWaveguide element contains 32 objects of type patch, surface. These objects represent Conductor1, feed, Substrate1.

Calculate the S-parameters using the EM solver from RF PCB Toolbox.

Sem = sparameters(tx_em,freq,50);

Compare S-parameters

Compare the S-parameters from the behavioral and EM solver models.

hold on
title('Behavioral vs. EM solver modeled S-parameters')

Figure contains an axes object. The axes object with title Behavioral vs. EM solver modeled S-parameters contains 8 objects of type line. These objects represent dB(S_{11}), dB(S_{21}), dB(S_{12}), dB(S_{22}).


[1] Garg, Ramesh, I. J. Bahl, and Maurizio Bozzi. Microstrip Lines and Slotlines . 3rd ed. Artech House Microwave Library. Boston: Artech House, 2013.

Version History

Introduced in R2020b

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Behavior changed in R2021b