Please be careful when modeling a multi-band Yagi on NEC-2 (EZNEC/
NEC-Wires), even if there are element length correction factors to
compensate for NEC-2's inability to handle diapered-diameter elements.
The corrections are only applicable in a region around resonance--say +/-
15% or so. So software like EZNEC disables the correction factors for
elements that are not near resonance. Even if not disabled, the
correction factors would not yield accurate replacement monotaper elements
for those that are too long or too short relative to resonance.
The result model will often have only the elements near 1/2 wl for
the frequency in question corrected. Since all elements play a role
in the characteristics of a multiband Yagi, the uncorrected ones (or the
miscorrected ones) will skew the results away from being true to the
antenna in question. The skewing may be lesser or greater, depending upon
configuration and the role played in the performance by each
uncorrected/miscorrected element.
For multi-band tapered-diameter element Yagis, MININEC is still the most
accurate core, even though its feedpoint impedances are calculated as if
the antenna was over perfect ground (unless one is modeling in free
space). NEC-4 does a better job than NEC-2 without using any correction
factors--but it still has some limitations.
Even for beams without the multi-band problem, if you have
tapered-diameter elements, please use enough segments so the each segment
on a given element is reasonably close to the same length. The correction
factors calculate the equivalent length of each of the sections of the
original element and then stick them end to end. Accuracy is highest when
the segments along this substitute element are as equal as possible.
-73-
LB, W4RNL
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|