Hi All,

I tried this post on the towertalk reflector. I should have tried it
first as I find more of you NEC guy hang out here (why is that
anyway!??!?).

I was modeling a Cushcraft 402CD loaded 40m yagi. I use Nec-Win Plus
and MultiNEC. I enabled stepped diameter correction in both programs
and noticed that in both programs a taper correction didn't occur. The
output from NEC-Win Plus during the taper correction seemed unhappy
because the element resonated around 11Mhz, but the simulation
frequency range was 7 Mhz.

How can I calculate a taper correction for a loaded element?

-Scott, WU2X

On Mon, 27 Apr 2009 12:05:37 -0700 (PDT), Scott M
wrote:

How can I calculate a taper correction for a loaded element?

Hi Scott,

It sounds like you have more problems than taper if your 40M antenna
is resonating at 11 MHz. The clue appears to be in the word "loaded
element."

73's
Richard Clark, KB7QHC

I meant without the loads (inductors) in place. Regardless if they
are in the model or not, the model isn't being converted to a
constant diameter. -Scott


On Apr 27, 4:54*pm, Richard Clark wrote:
On Mon, 27 Apr 2009 12:05:37 -0700 (PDT), Scott M

It sounds like you have more problems than taper if your 40M antenna
is resonating at 11 MHz. *

On Mon, 27 Apr 2009 14:20:05 -0700 (PDT), Scott M
wrote:

I meant without the loads (inductors) in place.

Hi Scott,

Then that detail is not essential.

Regardless if they
are in the model or not, the model isn't being converted to a
constant diameter. -Scott

This would surprise me if it did. Tapering involves the segments of
one wire being converted into wires of diminishing length.

73's
Richard Clark, KB7QHC

On Mon, 27 Apr 2009 12:05:37 -0700 (PDT), Scott M
wrote:

I enabled stepped diameter correction in both programs
and noticed that in both programs a taper correction didn't occur.

Hi Scott,

I went back to your original posting after pondering how your
question:
How can I calculate a taper correction for a loaded element?
wanders through several topics - such as above.

Taper and stepped diameter are not the same thing, although
tangentially related and asking for "how to calculate" jumps the
tracks entirely.

How about stripping out the extraneous details and simply describing
what your problem is?

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Mon, 27 Apr 2009 14:20:05 -0700 (PDT), Scott M
wrote:

I meant without the loads (inductors) in place.

Hi Scott,

Then that detail is not essential.

Regardless if they
are in the model or not, the model isn't being converted to a
constant diameter. -Scott

This would surprise me if it did. Tapering involves the segments of
one wire being converted into wires of diminishing length.

73's
Richard Clark, KB7QHC

"Tapering" is used to mean two different things. One is segment length
tapering, as Richard describes. It was essential in MININEC-based
programs to get accurate results from wires connecting at an angle, and
it's only occasionally useful with NEC based programs in other
situations. EZNEC has an automated segment length tapering feature left
over from its MININEC-based predecessor ELNEC. More detail can be found
in the EZNEC manual index under "Segment Length Tapering". The demo
version includes the full manual, and this feature.

The other meaning of "tapering", which is what the thread is about, is
diameter tapering, exemplified by a Yagi element made from telescoping
tubing. This tapering has a very significant effect on element length.
NEC-2 produces a small error whenever wires of different diameter are
connected, and this error is enough to seriously affect the pattern of a
Yagi made with diameter-tapered elements. An element of a single
diameter can be substituted which gives the same result as the
diameter-tapered element, but the method isn't intuitively obvious. The
length of the equivalent is different from the original, and the
diameter isn't an average of the diameters. The method was developed by
Dave Leeson W6QHS and described in his book "Physical Design of Yagi
Antennas" (ARRL). EZNEC and some other NEC-2 based programs incorporate
Leeson's method and automatically make the substitution when
appropriate. The method assumes a sinusoidal current distribution, so
EZNEC won't make the substitution if the element contains traps or
loading coils, or for other reasons isn't close to natural resonance.
But EZNEC also has a method of showing the user whether the substitution
is being made, what the substituted wire length and diameter are, and if
it's not being made, the reason why. More information is in the EZNEC
manual, indexed under "Stepped Diameter Correction".

It sounds like the OP's program normally does a similar substitution but
doesn't tell the user the reason when it doesn't. The software developer
should be able to explain the conditions under which the substitution is
applied.

Roy Lewallen, W7EL