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Coplanar Tapered Line (Closed Form): CPWTAPER



CPWTAPER models a section of coplanar waveguide either on an infinitely thick dielectric substrate or on a finite thickness substrate with optional backing ground and/or optional metallic cover. Signal strip width linearly changes along the section, and the section is symmetric (the local widths of gaps between the strip conductor and lateral grounds are equal). Infinitely thick dielectric substrate implies that a coplanar waveguide is ungrounded -- the backing ground plane (if present) is way below the signal strip and does not affect modeling results. You can switch to the substrate of finite thickness and add optional backing ground and metallic cover.



CPWTAPER$ is a coplanar iCell and only has an L parameter. See the “Intelligent Cells (iCells)” for more information

Name Description Unit Type Default
ID Name Text CP1
W1 Conductor width @ node 1 Length W[1]
S1 Gap width @ node 1 Length W[1]
W2 Conductor width @ node 2 Length W[1]
S2 Gap width @ node 2 Length W[1]
L Length of taper Length L[2]
CPW_SUB Substrate definition Text [3]
*IsSubInf Selector: Infinite substrate thickness/Finite substrate thickness Text Infinite substrate thickness
*Acc Accuracy (1 ≤Acc≤10) (effective only if finite substrate thickness is selected)   1

[1] User-modifiable default. Modify by editing under $DEFAULT_VALUES in the default.lpf file in the root installation directory.

[2] L is user definable in the default.lpf file under $DEFAULT_VALUES in the L value.

[3] If only one CPW_SUB is present in the schematic, this substrate is automatically used. If multiple CPW_SUB substrate definitions are present, the user must specify.

* indicates a hidden secondary parameter

Parameter Details

IsSubInf. IsSubInf is a secondary control parameter-selector that allows you to specify the technique that CPWTAPER uses for simulation. IsSubInf value "Infinite substrate thickness" (the default) applies closed form modeling of classic CPW presented at [1]. IsSubInf value "Finite substrate thickness" applies method of quasi-static Method of Moments presented at [2].

CPW_SUB. CPW_SUB supplies parameters for dielectric substrate, conductor thickness, and conductor metal properties. If IsSubInf = "Infinite substrate thickness" then CPWTAPER does not use the following CPW_SUB parameters: Hcover, Cover, Gnd, Hab, ER_Nom, H_Nom, Hcov_Nom, Hab_Nom, T_Nom, Acc. If IsSubInf = "Finite substrate thickness" then CPWTAPER still does not use the CPW_SUB parameters Hab, ER_Nom, H_Nom, Hcov_Nom, Hab_Nom, T_Nom, but uses all material parameters as well as parameters H, Hcover, Cover, Gnd, and Acc.

Acc. Parameter Acc is visible and used only if IsSubInf = "Finite substrate thickness". This parameter controls meshing density of Method of Moments (see [2]). Acc default value is 1. Bigger values of Acc may slightly increase simulation time. Good accuracy is provided at 1≤ Acc ≤3.

Parameter Restrictions and Recommendations

If IsSubInf="Infinite substrate thickness" then model issues a warning if one of the following conditions is violated: H<2(W+2S1) or H<2(W+2S2). This is because infinite thickness substrate approach [1] implies that H is large compared to W+S, so the warning alerts you that the substrate height is too small, initial model assumptions are not met, and accuracy may degrade. If IsSubInf="Finite substrate thickness" then model uses alternative implementation [2] and accounts for finite thickness of the substrate.


This element uses line types to determine its layout. By default, the layout uses the first line type defined in your Layout Process File (LPF). You can change the element to use any of the line types configured in your process:

  1. Select the item in the layout.

  2. Right-click and choose Shape Properties to display the Cell Options dialog box.

  3. Click the Layout tab and select a Line Type.

  4. Click OK to use the new line type in the layout.

See “Cell Options Dialog Box: Layout Tab ” for Cell Options dialog box Layout tab details.

CPW elements have special configurations for the defined line types. The center conductor geometry draws on all the layers defined in the line type. The spacing to the ground plane is then drawn on negative layers with the same name as all of the layers in the line type. You must then draw the same layers on the positive layer to complete CPW layout. See “Negative Layers ” for more information on setting up processes for positive and negative layers.

See “The Layout Process File (LPF)” for more information on editing Layout Process Files (LPFs) and to learn about adding or editing line types.

Implementation Details

This model is constructed of a cascaded series of constant width coplanar waveguide transmission lines. See the CPWLINE model for details on the model used for these transmission lines. The number of sections used is frequency-dependent and is constant as a function of the length divided by the guided wavelength. The model assumes a quasi-TEM mode of propagation and incorporates the effects of dielectric and conductive losses and dispersion.

Note that if IsSubInf = "Infinite thickness substrate" then the model implements a technique developed in [1]; this approach implies that H is large, hence, actual modeling results do not depend on H. If IsSubInf = "Finite thickness substrate" then the model implements a technique developed in [2]; this approach implies that H is arbitrary and substrate parameters Cover and Gnd may be used to install respectively, a metallic cover at the height Hcover (substrate parameter) and/or perfect infinite ground at the substrate back.


[1] W. Heinrich, "Quasi-TEM Description of MMIC Coplanar Lines Including Conductor-Loss Effects," IEEE Trans. Microwave Theory Tech., vol. MTT-41, January 1993, pp. 45-52.

[2] M.B. Bazdar, A.R. Djordjevic, R.F. Harrington, and T.K. Sarkar, "Evaluation of quasi-static matrix parameters for multiconductor transmission lines using Galerkin's method," IEEE Trans. Microwave Theory Tech., vol. MTT-42, July 1994, pp. 1223-1228.

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