NEC-LIST: about the transmision line of the LPDA

hi,friends,
   I am thinking over a simple question and still uncertain about the use
 of the TL card in the NEC-2. The log-periodic dipole antenna(LPDA)
 is fed by a transmission line, which we can represent in the nec files
 easily by a "TL" card. As we know ,the transmission line has an effective
 characteristic impedance (Zeff) when it is used to connect dipoles
 of the LPDA. Zeff is different from the original characteristic impedance(Z0)
 of the transmission line. I think we should use Z0 in the "TL" card,
 otherwise it is too complicated to computed the Zeff, a approximate value.
However, I don't understand it more. When a network is used in moment
 method, should we use the linear superposition principle to keep the
 independence of the each part?
 Thanks a lot.
                                               sincerely Xiaolin Zhang
                                                             2/10/09

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