NEC-LIST: Combining antenna modeling with real-world feedline calculations

The latest version of the freeware MultiNEC program can incorporate
real-world feedline calculations into your antenna model. You have a choice
of approximately 50 different types of coaxial and parallel wire lines and
you may add, remove, or change the feedline at any time. The feedline
calculations are done external to the normal NEC or EZNEC calculations for
the model.

The advantages of doing separate calculations for a feedline, as opposed to
using the NEC TL facility, include: no need to add a separate wire to the
model to serve as the anchor for the station end of the line; no need to
re-calculate the entire model just to change feedline parameters such as
characteristic impedance or length; and the ability to include the effects
of line loss in the calculations.

Note that this function models only the impedance transformation effects of
a non-radiating transmission line attached to the antenna source point.
Transmission lines that serve as an integral part of the model, such as
those used with phased arrays, must still be modeled with the NEC TL
facility.

MultiNEC now also includes a Smith chart sheet. You can see an example at
www.qsl.net/ac6la/adhoc/mnsmith1.gif (~30 KB). The antenna in this case is
everybody's-favorite-to-talk-about 102 foot doublet of #12 copper at 50 feet
above real ground, calculated with a frequency set of 3.5 to 4.0 MHz, 7.0 to
7.3 MHz, and 10.1 to 10.2 MHz in 0.1 MHz steps and with frequency steps
between the bands of 0.25 MHz. (Frequencies in a MultiNEC sweep need not be
evenly spaced.) The wire was divided into 53 segments, about 50 segments
per wavelength at the upper end of the sweep range. In the above Smith
chart example the blue trace shows the R and X values at the antenna source
and the red trace shows the impedances as transformed with 40 feet of
Wireman 554 ladder line. (Parameters for Wireman line courtesy Wes Stewart,
N7WS.) The chart is shown with a 50 ohm center and with the green dot
marker placed at the 3.5 MHz point. You can change the Smith prime center
if desired. The screen shot at www.qsl.net/ac6la/adhoc/mnsmith2.gif shows
the same impedance points but on a chart with a 360 ohm center (the Ro value
of the feedline) and with the marker on the 10.2 MHz point.

The latest MultiNEC release has several other enhancements, including the
ability to show 3D radiation plots using the EZNEC, NecVu (Nittany
Scientific), or 4nec2 viewers. For more details and a free download file
see www.qsl.net/ac6la/. MultiNEC requires Excel 97 or later.

73,
Dan AC6LA

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