[TowerTalk] LPDA gain figures

[alsopb]

LB,

Unfortunately there is a crowd out there which
believes in testimonials more than physics and
real world data.  It is evident from the comments
here.

Nothing new.  Madison Avenue couldn't sell without
testimonials.

If I'm going to bet, it would be on Maxwell's
Equations.

73 de Brian/K3KO
L. B. Cebik wrote:
> 
> >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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