Re: NEC-LIST: Currents on TX line, with voltage source connected.

Nikhil -

Remember that in NEC the transmission line is just a special case of
the non-radiating network.

I presume that you have used a dummy segment to feed the transmission
line. That is, you have used a very short segment that is located far
away. When doing that, the voltage source will look into the dummy
segment in parallel with the transmission line.

The current shown in the output file under the
heading - - -Structure Excitation Data at Network Connection
Points - - - is that current flowing into the dummy segment and the
corresponding impedance is the very high capacitive reactance of the
very short dummy segment. The current shown in the output file under
the heading - - - Antenna Input Parameters - - - is the sum of the
current flowing into the dummy segment plus the current flowing into
the transmission line. In the latter case, the corresponding
impedance is the parallel impedances of the short dummy segment and
the transmission line.

The drive point impedance seen by the drive source is that shown under
the heading - - - Antenna Input Parameters - - - .

This is explained in detail on page 60 of my book titled "Basic NEC
with Broadcast Applications" available from Barnes and Noble,
Amazon.com, etc.

I hope this helps.

J.L. Smith

"Nikhil Mehta" <nikhil_at_rri.res.in> writes:
> Dear Folks,
>
> I have a small problem with the use of transmission lines in NEC2.
> Simply
> stated, I notice that if I have connected a voltage source to the
> same
> segment as a tx line, then current on that segment is different
> under
> "Structure Excitation Data at Network Connection Points" from that
> under
> "Antenna Input Parameters". Now the LPDA example given in NEC2
> manual
> says, and I quote:
>
> "The latter is the current through the voltage source and includes
> the
> current into the segment and into the transmission line."
>
> I am not able to make sense of above statement. My problem is in
> interpreting the electrical connection that NEC uses for such a
> case. What
> I want is simply to connect a voltage source to a balanced twin wire
> tx
> line.
> I am asking this because different currents means different
> impedances at
> that segment, and I fail to understand how impedance at a port can
> have 2
> different values.
>
> Kindly clarify the implicit electrical connection used in NEC.
>
> With regards,
> Nikhil Mehta
> Visiting Student
> Raman Research Institute
> Sadashivnagar Bangalore 560 080
>
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