I can add a bit to the the difficulties of modeling linear loads in the
currently available software (NEC and MININEC).
NEC (both -2 and -4) have a weakness in modeling two different situations,
both of which enter into modeling linear loads. 1. NEC does not handle
angular junctions of wires having dissimilar diameters accurately. Most
linear load arrangements use smaller diameter wire for the linear load
than for the main element. 2. NEC does not handle accurately closely
spaced wires of different diameters. Most linear loads involve parallel
or near parallel wires of different diameters. Hence, modeling linear
loads as actually implemented by any antenna maker is not generally
feasible using direct modeling methods in NEC.
MININEC is not subject to either restriction. However, it requires the
use of a very high number of segments or length tapering as wires approach
angles ranging from slightly more open than a right angle to extreme acute
angles. This often limits the ability of the MININEC program to handle
complex elements (or arrays), since the most common implementations of
MININEC are still limited to a low maximum number of segments.
It is possible to model in NEC complex linear loading schemes so long as
the wires everywhere have the same diameter. These models do not reflect
actual manufactured elements, but do provide a means of studying the
currents that results from the various couplings that depend on wire
layout and load positioning. They raise some interesting questions of
their own. For example, consider a linear load that ostensibly begins at
mid-element and is brought toward the dipole center and then returns to
mid-element. Now consider a pair of linear loads that begin at the center
and proceed outward, turn around and return to center to join the
so-called main element. When one models all wires with the same diameter,
there is no electrical difference between the two systems, except for the
collinear or non collinear placement of the outer end of the element, and
the offset is relatively insignificant.
When linar loads are centered and the wires are equispaced from the main
element to follow, they answer very closely to stub reactance equations.
When the wires are not symmetrical with respect to main element, current
along the load wires differ and the requisite length differs from stub
transmission line calculations. As linear loads are moved outward,
currents on the wires differ, regardless of symmetry with respect to the
main element, and stub calculations also fail to coincide with requisite
modeled lengths. However, models of physical stubs (when no modeling
limitation is exceeded) do correspond within shop limitations with
constructed antennas. Using TL models can be misleading relative to both
linear load length and losses (since TL models are lossless).
Modeling stubs as physical wires tends to provide reasonably accurate
theoretical figures for such matters as bandwidth, source impedance,
current levels and phases, etc., but only for models that do not exceed
program limitations. However, every commercial implementation of a linear
load of which I am aware fails to model accurately due to exceeding one or
more limitations of existing programs.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://web.utk.edu/~cebik/radio.html
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