Cable Library - Coaxial Cables

Cables: Cable Library Coaxial Cables

The dialog box is also available via Cable Navigation Tree: Library Cables. The icons on the top of the dialog box enable actions which are explained below. The meaning of each icon is also explained by tool-tips. To see a tool-tip just move the mouse-pointer over the corresponding icon:

 

 

All available Coaxial cables are listed in the column on the left side of the dialog box. When selecting an item in the list the corresponding definition is shown on the right side. Each cable requires a unique name that has to be assigned in the field Name. The underlying unit size for geometry definition can be selected with the Units pull-down menu. Changing the unit for an existing object does not affect the size of the cable but just displays the original size in the corresponding unit.

 

Coaxial cables consist of a conducting core, wrapped into an inner insulator, shielded with a screening conductor, and protected by an outer insulator. This configuration is reflected by the list inside the Content frame in the middle of the dialog. By selecting one of these four items an appropriate dialog frame will appear on the right side enabling to set the corresponding parameters.

 

Wire

The shape of the wire is defined by its Shape type, which is always of type Circle. The Circle can be defined either by its Diameter, Radius or Area. The Material button enables to choose the corresponding material.

 

Insulator Inside, Insulator Outside

The definitions for both objects are similar to the Wire definition dialog.

 

Screen

The shape of the Screen is defined by its Shape type, which is either Circle or Wrap. A Circle describes the screen's outer circumference, the inner border of the screen is automatically defined by the outer border of the Insulator Inside. The Circle itself can be defined either by Diameter, Radius or Area.  A Wrap is a coating onto the Insulator Inside and can be defined by a Thickness. The Material button enables to choose the corresponding conductive material for the screen.

Screen type provides the choice between a Braided shield or a Solid shield. For a Solid shield no further definition has to be made. The screen will be interpreted as a solid tube filled with the assigned conductive material. The screening effect of a solid shield is exclusively based on the diffusion effect of the magnetic field inside the conductor. The effect depends on the thickness of the screen and its conductivity, both are already known with the Shape type and Material settings. For a Braided shield, two further screening effects are visible: The aperture coupling effect and the porpoising effect (see References). All three effects contribute to a special characteristic of the screen: the transfer impedance. The transfer impedance of a braided shield can be defined with three different Impedance models:

 

Simplified model:

Defines the transfer impedance with a  Transfer-resistance and a Transfer-inductance.

 

Kley's mode (see References):

Defines the transfer impedance by four design characteristics of the braid:

The product of the three parameters from above specifies a length. This length is not allowed to exceed the double of the screen's circumference, which is implicitly defined by the Insulator inside setting (see figure of dialog box above). The fourth characteristic specifies the weaving style and can be expressed by one of the following three parameters:

 

The relationship between the three parameters can be expressed in algebraic formulas. With:

 

D: diameter under the braid (defined by the isolator below)

d: diameter of a single filament

N: number of filaments in each carrier

C: number of carriers

F: fill factor

a: braid angle

P: picks per unit length

O: optical coverage

 

F = N . P . d / sin a;

O = 2 . F - F2;

tan a = 2 p . (D+2d) . P/C;

 

Measured results:

The Edit Measurements... button enables the import of an ASCII file which contains a list of frequency points and corresponding complex (or real) impedance values (in a comma separated form). In order to filter noise out of the measured data two parameters are available: The field Neighbors considered for smoothing specifies how many neighbors in front and behind a certain frequency point shall be used for filtering. The field Repetitions specifies the number of filter runs that should be performed. If the number is set to zero no filtering will be performed.

 

There is an Impedance preview function which enables the visualization of the defined transfer impedance over a certain frequency range. There are always two curve plots. The red curve is based on the original data given via the Simplified model settings,  the Kley's model settings or the Measured results import. The yellow curve displays the approximation based on a rational polynom fitting. This fitting is mandatory. It grants for the model's causality and enables its usage in either frequency- or time domain simulations.

 

  

 

Reference

"Shielding Effectiveness of Braided-Wire Shields", E.F. Vance, IEEE Transactions on EMC, Vol. 17, No. 2, May 1975

"An Improved Model for the Transfer Impedance Calculation of Braided Coaxial Cables", S.Sali, IEEE Transactions on EMC, Vol. 33, No. 2, May 1991

"Optimized Single-Braided Cable Shields", T. Kley, IEEE Transactions on EMC, Vor. 35, No. 1, February 1993

"EMC Analysis Methods and Computational Models", F.M. Tesche, M.V. Ianoz, T. Karlsson, John Wiley & Sons, 1997

 

 NOTE:

Screening conductors are treated like any other conductor. They carry signals and have got terminals at either side regardless if they are solid or braided shields.