[AMPS] Re: ARRL Pi net formulae

[Phil (VA3UX)]

>
> >
> >Tom Rauch wrote:
> >
> >>Rich Measures wrote:
> >>> €  For c. five years, the ARRL Handbook had the wrong Pi-L tank values -
> >>> -  despite being repeatedly told about the problem.  .
> >>
> >>Is it easy to explain what is wrong about the values?
> >>
> >If we're talking about the same formulae, they were approximations that
> >assumed a high loaded Q. They were probably good enough for designing
> >output tank circuits, but gave major errors in Q values for cathode
> >circuits with deliberately low loaded Qs.
>
> €  As I understand it, there was no error in the formulae, Ian.
> .......
>

The printed formulae themselves had no errors, but the formulae were incomplete and produced different pi net values than the complete formulae did.  The incomplete formulae were widely published in all kinds of technical literature for years - not just ARRL (EIMAC, Motorola, RSGB, some text books, etc all used them).  In 1983, Elmer Wingfield, W5FD (now SK ?), published an excellent article in QST for August showing why the standard published formulae produced less than optimum pi and pi-L net component values.  He included the complete "corrected" formulae in his article. Excellent but simple mathematical proof accompanys the article.

I found it annoying that while ARRL published this article in 1983, they failed to make use of the information for updating the Handbook, choosing instead to leave the old formulae in the Handbooks until 1995.  In 1995, they finally made the change.

The tabulated Pi and Pi-L net values in the pre-1995 Handbooks were hard or impossible to duplicate with the old standard formulae.  There appeared to be alot of inconsistency.

The 1999 Handbook pretty much summarizes the essence of W5FD's 1983 article.  I would encourage anyone curious or otherwise interested in this topic to get a copy of the original article.  After reading it, you will no longer accept such blanket "carved in stone" recommendations for input Pi networks such as "set XC1 to 25 ohms".  For anyone thats interested but doesn't like to dink around with calculators or algebra,  I have an easy to use Excel 5.0 spreadsheet that displays comparisons between the 2 sets of formulae (available for the asking).

Phil

>
> Rich...
>
> R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
>
>
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