TL Card

This card connects a non-radiating transmission line between Feko geometry or other general non-radiating networks or transmission lines.

On the Source/Load tab, in the Loads / networks group, click the Transmission line (TL) icon.

Note: Loads and sources can also be connected on a transmission line terminal using the LN and AN cards respectively.

Parameters:

Remove all existing transmission lines: If checked, all previously defined transmission lines are deleted. All the other input parameters are ignored.
New transmission line: Defines a new transmission line, all previously defined transmission lines are replaced.
Add to existing transmission lines: An additional transmission line is defined.
Network name: The name of the transmission line.

Input port

The input port (start of the transmission line) can be connected to geometry or other non-radiating ports in a number of ways.

Wire segment label: The label of the segment to which the transmission line port must be connected. If more than one segment has this label, the transmission port is connected to the last segment with this label.
Wire segment position: The segment is determined by specifying the Cartesian coordinates of the segment centre. These values are in metres and are scaled by the SF card if Modify all dimension related values is checked.
Internal port: The network name and the network port number of another network port that has to be connected.
Edge between regions with multiple labels: The positive and negative labels define the positive and negative terminals of the port connection.
Edge connected to ground/UTD: The positive or negative labels that define the edge where the network port has to be connected to.
Edge of microstrip between two points: The points that define the edge of the microstrip line where the network port has to be connected to.
Vertex by segment label: The vertex is determined by specifying a segment label. Also select whether the start or end point of that segment should be used.
Vertex by position: The vertex is determined by specifying the Cartesian coordinates of the vertex.
FEM line port position: The input port is attached to a FEM line port. The position of the FEM line port is specified by the start point and end point.

Output port: Same as for Input port, but applies to the end of the transmission line.

Cross input and output ports: The positive port voltage is in the direction of the segment that it is connected to (from the start to the end point of the segment). Thus the input and output ports of the transmission line have unique orientations. If this item is checked the transmission line connecting the ports is crossed.
Calculate length from position: If checked, Feko determines the length based on the geometrical distance between the start and end points. Note that this feature is only available when both transmission line ports are connected to segments or vertices/nodes (the ports do not have to be the same type).
Transmission line length: The length of the transmission line in metres. This value is scaled with the scaling factor of the SF card.
Attenuation (dB/m): Losses of the transmission line in dB/m. Note that since the propagation constant is taken as the propagation constant of the medium in which the start and end ports are located, the attenuation specified by this parameter is added to any losses of this medium. This factor is not affected by scaling specified with the SF card. This means that should a scaling factor which reduces the length of the transmission line be added, the total loss through the line will be less. (The length is now less and the loss per distance remained the same.)
Velocity of propagation (%): The propagation speed through the transmission line relative to the speed of light.
Material label (dielectric): The label of the dielectric medium (as defined in the DI card) used as the background medium for the transmission line.
Real part of Z₀ (Ohm): Real part of the characteristic impedance of the transmission line in Ohm
Imaginary part of Z₀ (Ohm): Imaginary part of the characteristic impedance of the transmission line in Ohm. Note that the characteristic impedance only defines the ratio between the voltage and current of the two waves propagating along the line. It does not specify any losses.
Real part of shunt Y at port 1: Real part of the shunt admittance at the input port in Siemens. (This admittance is across the port, connecting the two wires of the transmission line.)
Imaginary part of shunt Y at port 1: Imaginary part of the shunt admittance at the input port in Siemens.
Real part of shunt Y at port 2: Real part of the shunt admittance at the output port in Siemens. (This admittance is across the port, connecting the two wires of the transmission line.)
Imaginary part of shunt Y at port 2: Imaginary part of the shunt admittance at the output port in Siemens.

Any load impedance defined over the transmission line port with the LZ, LS, LP, LD, L2, LE, CO or SK cards are placed in series with the port. Parallel admittances can be defined directly at the TL card.

To illustrate, a transmission line port connected to a wire segment is discussed, but this applies to connections at edges as well as nodes.

The figure below is an illustration of the placement for loads and excitations.

Figure 2. General placement of loads and excitations.

If a voltage source of type A1 or A3 is applied at one of the port segments, then this voltage source is assumed to be across the port (that is feeding the transmission line directly with an impressed voltage). If S-parameters are computed with respect to an excitation on a wire segment to which a TL card is connected, then the reference impedance is assumed to be in series with the source, but across the network port.

Figure 3. Load placement for S-parameter calculations.

Note that the propagation constant and thus also the propagation loss of the transmission line is the same as that of the medium surrounding the port unless an additional loss tangent is specified in the Losses field. If this is free space the transmission line will be lossless. For transmission lines with a propagation constant that is higher than that of the surrounding medium, such as coaxial cables filled with dielectric material, the length of the transmission line should be reduced.

The following guidelines apply to determining the surrounding medium of a transmission line:

When both ports of a transmission line are internal (not connected to geometry), the propagation constant of the background medium is used for the transmission line.
If one port of the transmission line is internal, the propagation constant of the medium at the other port (connected to geometry) is used.
Should both ports be connected to geometry, the medium of the input port (Port 1) is used for the propagation constant of the transmission line.
Additionally, if a transmission line is located inside a planar multilayer substrate, the following applies:
- If the transmission line is connected to geometry:
  - and lies inside a layer, the propagation constant of the medium of that layer is used.
  - and lies on the interface between two layers, the average medium between the two layers is used for the propagation constant.
- If the transmission line is not connected to geometry then either the upper or lower medium is used depending on which one is lossless, or should both be lossless, the one with the greatest propagation constant, $| β |$ , is used

Losses in the transmission line network (due to the shunt admittances or transmission line losses directly) are taken into account and will for instance reduce antenna efficiency or gain.