>I've been reading some older LPDA articles by Peter Rhodes, K4EWG,
>running from 1973 to 1992, in various QST articles and articles in ARRL
>Antenna Compendiums. Seems he's always claimed that logs have
>substatially more gain than recent computer models seem to show. I've
>also read K9LA's article in the latest Antenna Compendium where he talks
>about the "2 gain tables" for Log Periodics, and how older numbers were
>"optimistic" by about "2 dB", but I just find it interesting that the
>ham community didn't adapt those numbers as "gospel" until about 5 years
>ago. I wonder if we're not throwing out 20 years (yes, I know the log is
>older than that, I'm going back to K4EWG's first articles in QST) of lab
>data for 5 years of computer modeling.
K9LA was referring to information on gain calculations that appears almost
verbatim to his account in the 3rd Edition of the Antenna Engineer's
Handbook (known as Johnson and Jasik). The techical sources for the
revision of the gain calculations are given in both AEH and in the K9LA
article. It was not modeling that revised the numbers, but a correction
of the basic theory behind the calculations to correct what most
engineering research sources call an error in the original method. Lab
work in fact confirms the corrected gain calculations as more nearly
correct. Most material published in amateur circles has only reports of
lab work, but little if any at its core--only the individual antennas
built that are claimed to "work well."
There is nothing in an LPDA of standard design that presses any limit of
NEC-2 (if elements are of uniform diameter) or NEC-4, so the software
calculations will be as accurate for LPDAs as for Yagis. It is also well
to remember that standard LPDA design calculations contain some
oversimplifications, such as a single formula for element length
calculation based on a single length to diameter ratio that is so low that
it is rarely attained below UHF. Now, the gain calculations in LPDA
theory are also simplifications based on certain assumptions about the
uniformity of patterns throughout the passband range of the
antenna--assumptions that do not hold up in practice except approximately.
While NEC cannot design an LPDA, the foundations for the calculations it
performs are far more sophisticated than those used in LPDA theory.
Therefore, its reports of performance parameters for any given frequency
will be more accurate than those used to make design forecasts.
Note that the fundamental outputs of NEC are a series of tables giving the
results of complex calculations that are directly traceable to Maxwell's
Laws, along with sundry correctives of importance that have a solid
antenna engineering research base, for example, the precise wire end
effect on the performance of a linear wire. The pictures of patterns, SWR
curves, and the rest are grapical translations from the calculated tables.
No one should for a moment think that because the software bears the label
"modeling" that it is imprecise or even less precise than other
calculation techniques. It has limits within which effective modeling can
occur. Unfortunately, many calculation schemes are around that do not
announce their limits and leave the impression that they have none.
Little could be further from the truth.
In the end, any claim of distrust about the mathematical anaysis of
antennas--which is what NEC is all about--would have to show in what way
the calculational scheme used has hit or gone over a limit of accuracy if
that claim is to be credible. There are a number of ways in which this
can be done, and some have been used to set--and to correct--limitations
of the thin linear wire foundations of NEC. However, this is the same
process used to improve some of the LPDA design equations--including the
ones involved with gain estimates.
In the end, effective range testing remains the final A-B test for
physical antennas. I would still love to see the eventual establishment
of a test range (hopefully permanent) to which any and all amateur
antennas--commercial or home brew--could be brought for testing against a
single standard dipole for any frequency desired (meaning many dipoles for
many frequencies, but constant for any given frequency) with the latest,
most sophisticated measurement equipment applied to the most rigorous
methodological standards and procedures.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / Tel: (423) 938-6335
1434 High Mesa Drive / \/ \/\ ----/\---
Knoxville, Tennessee /\ \ \ \ / / || / http://www.cebik.com
37938-4443 USA / \ \ \ \ || e-mail: cebik@utk.edu
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