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Physical Specification, Floating Shield (Closed Form): COAX



COAX simulates a lossy coaxial transmission line with isolated shield terminals. Losses are determined via physical dimensions and material parameters of metal and dielectric filling.


Name Description Unit Type Default
ID Element ID Text CX1
Di Inner diameter Length 2.9027 mm
Do Outer diameter Length 10.287 mm
L Length Length 0 um
ER Relative dielectric constant   1
TAND Dielectric loss tangent   0
RHO Metal bulk resistivity normalized to copper   1

Parameter Details

Di,Do. This parameters define inner and outer diameters of internal cross-section of coaxial line. Defaults values are set to dimensions of RG-213 50 ohm coax.

Implementation Details

Implementation is based on modeling the coax as TEM line and determination of primary per-unit-length RLGC parameters. These RLGC parameters are used to obtain complex characteristic parameters of dominant propagating TEM mode (characteristic impedance and propagation constant). Model does not account for possible propagation of higher order modes.

See descriptions of model TLIN4 for details of implementation of models with floating ground.

Circuit Model Synthesis

COAX supports synthesis of physical parameters based on electrical specifications using the Transmission Line Calculator. To open the Transmission Line Calculator, right-click a transmission line element in a schematic and choose Synthesize.

To perform transmission line synthesis:

  1. In the Electrical property grid, select Target check boxes for desired electrical parameters and enter a corresponding value.

  2. In the Physical property grid, update frequency and substrate parameters if needed, then select Synthesize check boxes for transmission line physical parameters to synthesize based on the targets.

  3. Click the Synthesize left arrow to run the synthesis program. The values in both property grids update with the synthesized results. An analysis is also performed with the final physical values. If synthesis cannot achieve the target values, it shows how close it came.

  4. Click OK to update the selected transmission line element with the synthesized physical parameters. Expressions are overwritten with the new, evaluated values. You can click the Undo button on the program toolbar to revert all parameters in the schematic document to their pre-synthesized state. Parameters from substrate elements are never updated since typically substrate elements are used by more than one transmission line element. Click Cancel to close the dialog box without setting the parameters into the element.

Recommendations for Use

Extreme care should be used with this element as it is meant to work in concert with additional elements which relate the voltages at both ends of the transmission line to the global ground.

NOTE: Because the model definition does not include interactions with the global ground, unusual and unexpected results can occur if other components are not used to relate the voltage on both sides of the transmission line to global ground.


This element does not have an assigned layout cell. You can assign artwork cells to any element. See “Assigning Artwork Cells to Layout of Schematic Elements” for details.


[1] I. Bahl, P. Bhartia, Microwave Solid State Circuit Design, John Wiley & Sons, New York, NY, 1988.

[2] M. Gunston, Microwave Transmission-Line Impedance Data, Noble, Atlanta, 1997

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