[TowerTalk] T-Network Calculator Spreadsheet

[David Gilbert]

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