On 5/11/12 7:29 AM, firstname.lastname@example.org wrote:
> I would like to change tubing diameters on an antenna that I have
> previously designed. Is there a way do this and maintain the same
> element resonate frequency using EZNEC+? I tried "Show Stepped Dia
> Correction" thinking it would be easy see how much longer or shorter the
> new element is, however there seems to be no way to make the new and old
> correction diameters identical. It would be nice if I could set a
> standard taper correction size. Any suggestions?
Isn't that taper correction an empirical formula? (from Dave Leeson W6NL
nee W6QHS) It converts a specific taper schedule into an equivalent
constant diameter and length, with the calibration done by comparison
against a real antenna.
So, if you have Dave's book (or the algorithms in there), you can
probably figure it out.
It's possible that someone has done a numerical model to generate other
cases (without needing actual range tests to confirm), but I haven't
Joe Reiser (W1JR) has a write up on Yagi Antenna Design in
Communications Quarterly (google for it) which describes all the various
correction schemes that have been used. Some were empirical, some are
based on theory (W6NL's), etc. Others have their own schemes for taper
correction, and they all typically work better for some more restricted
problem space (e.g. K1FO UHF designs).
You could also model the tube, not as a single wire, but as a circular
array of wires, which overall are tapered. The usual rule of thumb from
Jerry Burke on modeling a "big" thing with "small" things is that the
surface area of the wires should match the surface area of the thing
So if you have a 2 cm diameter tube (surface area = 6.28 sq cm/cm
length), and you want to model it with 8 smaller wires, the smaller
wires should be 1/4 cm in diameter, spaced on a 2cm diameter circle.
Lots of segments in the resulting model, of course.
Some sort of program or spreadsheet is handy to generate the gazillion
segments, or make strategic use of the GM card to replicate and rotate
If you have access to NEC4, it does a better job with discontinuities in
the model, particularly if you have very short segments joining others
(NEC4 uses a different set of basis functions than NEC2 for each of the
segments, so the numerical precision problem is less)
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