There are several online T-Network and L-Network calculators out there,
as well as various standalone applications that do the same thing. My
problem with most of them, though, is that every time I change the
frequency or change the complex load values (or typically both), the
network gets recalculated for the new values. That isn't necessarily
the real world, though, where fixed networks might be used at the
feedpoint of an antenna or somebody just doesn't want to bother retuning
the network every time they change the frequency. What I wanted to see
was what happened to impedances, currents, and voltages when the network
didn't change but everything else did.
So I wrote a simple spreadsheet to show that sort of thing. It's
nothing special ... literally just the application of Ohms Law using
complex impedances ... but the calculations get messy enough that it's
much easier handled using the available formulas in Excel.
I'm pretty certain that applications like the excellent AutoEZ and EZNEC
can display a similar output for fixed tuner values based upon an
antenna model, but this spreadsheet is useful when you have an antenna
that isn't easy to model and you are able to obtain the actual feed
impedances as a function of frequency using one of the relatively
inexpensive complex impedance analyzers that are available now. Looking
at you, nanoVNA.
I've made the spreadsheet freely available for download from the files
page of the Arizona Outlaws Contest Club website. Go to:
http://www.arizonaoutlaws.net/ ; then click on "Downloads" under the
"More" drop down tab at the top of the page.
Using the spreadsheet should be pretty obvious, but the first sheet
("Intro") has some explanatory text. The sheet titled "AB7E" has the
calculator. To be clear, this spreadsheet does NOT give you the
optimized network values for any particular frequency and load, although
you can get there by trial and error if you so choose. You can easily
get the automatically optimized values elsewhere, such as from TLW, the
free transmission line calculator that comes free with the ARRL Antenna
Book. But once you load those values into the spreadsheet for one
frequency you can see what happens for other frequencies and loads.
The spreadsheet was written in Excel 2010 to handle ten columns of input
data, but a simple copy/paste will extend that to however many data
points you might want to use.
The spreadsheet is set up as a T-Network, but by simply setting one of
the capacitors to a very high value (like 50,000 pf) that element will
look like a short and the network becomes essentially an L-Network. You
could even do the same for a single series or shunt element using the
same idea ... make the other values high enough that they don't matter.
The spreadsheet assumes a 50 ohm non-reactive source impedance. I could
have written it for an arbitrary complex source impedance, but that
would require the assumption that the source is able to output the
stated power at that impedance. I didn't think that was necessarily
realistic, but if anyone thinks that would be especially valuable, drop
me a note.
If the spreadsheet turns out to be useful to you, hug somebody you
haven't hugged in a while.
73,
Dave AB7E
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