Taking into account the situation of a main segment line and an attached secondary track to this one, it is possible to consider the placement of a ideal transformer with the ratio n:1 (dashed line in figure 2). At very low frequencies one can say that the connection to the main arm is direct realized, the conditions being: frequency is going to zero and n is going to zero.

The capacitance to the ground take into consideration the excess or the deficit of electrical charge in the spatial zone of T junction, which is due to the residual potential, potential depending on the charge distributions existing at T₁, T₂, T₃ reference planes.

Evaluation of the static excess capacitance can be done with full-wave electromagnetic simulation methods or with analytical methods, as the linear sources with reverse charge method, for instance. One considers XOZ the plane of T-branch.

 is the potential corresponding to a charge distribution  for an OX infinite signal line configured as a microstrip topology and

the potential corresponding to a charge distribution with reversed polarization at the point x = W₁ (figure 3). The potential according to an electrical charge in the domain xÎ(W₁,+¥) is, using the superposition principle:

 

                         (1)

Fig. 3 Coordinate axis for evaluation