"Roy Lewallen" wrote in message
...
Frank wrote:

As I understand NEC; large errors can be introduced by junctions of
dissimilar wire diameters, and in particular when the wires are at 90
deg. Therefore, when you have designed your "GH" inductors, the rest of
the antenna should by constructed of the same diameter wire. This may be
difficult since Dan is using two coils of significantly different Qs. I
guess you could overcome this problem by varying the conductivity of the
inductor to obtain the desired Q. Also, since segmentation tends to be
relatively high in a helix, should segment length tapering be applied to
those segments adjacent to the helix?

Frank, VE6CB

It's difficult to give an absolute answer to these questions, but some
general comments and guidelines should help.

First, the error introduced by NEC-2 when wires of dissimilar diameter are
connected is generally small, unless the wires are grossly different. This
error can be minimized by making the segments as *long* as possible
adjacent to the junction, which of course is contrary to the general
principle that more segments are better. Even a small error can cause
major changes in the pattern when the dissimilar diameter wires are in a
parasitic element. EZNEC and a number of other programs have a built-in
method of avoiding this problem for certain antenna types, but plain NEC-2
doesn't. NEC-4 is relatively free of this problem, but it's quite
expensive for hobby use.

The Q of an inductor is determined by the inductance and the loss. The
loss is a function of the dielectric, wire resistance, and radiation
(which isn't really loss, but lowers Q as though it were). NEC type
programs automatically account for the radiation, and it's easy to include
wire loss. So assuming negligible dielectric loss, the programs should
predict Q fairly accurately -- except for proximity affect. Proximity
effect could be modeled in NEC by increasing the resistivity of the wires
in the coil. EZNEC currently allows only a single wire resistivity for the
whole model (although this will probably change in the next version).
However, since the overall loss will be dominated by the inductors, the
higher resistivity could be specified for the whole model without
sacrificing significant accuracy. Alternatively, a number of resistive
loads could be inserted in the inductors.

Segment length tapering usually isn't necessary with NEC based programs,
unless there's a source near a place where the segment length changes. An
average gain check should be run to determine if there's a problem. If
there is, segment length tapering is one tool which can be tried in
improving the average gain.

Roy Lewallen, W7EL

Thanks for the information Roy, all remarks noted and saved. Will see what
I can do to generate some realistic helical models.

Frank VE6CB