[TowerTalk] T-Network Calculator

[David Gilbert]

There are multiple online T-Network and L-Network calculators out there, 
but one thing that I've always found to be a limitation is that they 
only show a result for one frequency.  In an actual application, we 
might calculate a fixed network for a particular frequency but we 
wouldn't readily know what it does at other frequencies.  To address 
that, I once generated an Excel spreadsheet where you could enter the 
actual complex impedances for up to ten different frequencies and then 
see what various parameters (SWR, element voltages, element currents, 
and element power dissipation, etc) looked like at other frequencies 
when you built a matching network for just one of those frequencies.

With the availability of Codex, OpenAI's ChatGPT extension that converts 
chat instructions to code, I converted that spreadsheet to a web page 
and it is now sitting on my website at:

https://www.ab7e.com/T-Network.html

Here are the basics for using it:

1.  The topology assumes a series-shunt-series T-network configuration 
with any of the elements being either a capacitor or an inductor, but it 
provides the option to declare one of the series elements to be a Short 
in order to turn it into an L-Network.  You can change the type of 
element by clicking on its type label.  In the case of an L-Network, 
there is no need to specify a Q and whatever you enter there will be 
ignored.

2.  The application assumes that you can use a network analyzer like a 
NanoVNA or other unit to determine the complex impedance of your actual 
load at various frequencies.  The data for each frequency is considered 
to be a "Case", and while the default when you start the web page is 
only three cases you can add as many as you'd like (and later delete 
them if you want).  The only real limit to the number of cases is how 
much left-right scrolling you're willing to do. The "Add Case" button is 
at the far right of the page.

3.  Case 1 is special in that it represents the frequency and load 
impedance used for the calculation of values for the Network.  All other 
cases will use those network values to calculate their results.

4.   There are two calculation modes.  A T-Network is basically two 
L-Networks connected back to back, and as such if left open ended there 
are multiple possible solutions.  The way to address that is to specify 
a network Q, which basically determines the midpoint impedance that each 
L-Network transforms to.  So one calculation mode involves specifying a 
network Q and the app tries to satisfy that condition, but I also added 
the option to sweep through multiple possible Q values to find the one 
that results in the lowest currents and voltages for the network 
elements.  That provides the lowest component stress and lowest power 
dissipation, but it can sometimes result in very large or very small 
component values.  It's worth trying both modes, as well as different Q 
values for the fixed Q mode.

5.  You can restrict the range of values for each element in the 
network, and you can independently specify their Q.  The calculation 
will honor the boundaries you specify.  You can even specify a min and 
max value very close to each other in the situation where you might want 
to build part of the network using a particular component you might have 
on hand.   The application will do its best to satisfy whatever limits 
you give it.

6.  Although the application provides for automatic calculation of the 
network values for Case 1, you can manually change any of the network 
component values and the results information below will change 
accordingly.  The "Auto-Solve from Case 1" button will flash and try to 
get you to recalculate, but you don't have to do that for the output 
results to be valid.

7.  The default input values when you start the page are garbage ... 
purposely so.  Be sure to change:
     a.  The available source power at the top
     b.  The element types, min/max range, and Q
     c.   The frequency and complex load impedance for each case you are 
interested in.  Remember that the network will calculate based upon Case 1.

8.  There are loads, conditions, and network configurations that cannot 
provide a solution.  You will get an error message for those situations.


I you decide to give the application a try, please let me know if you 
find any data errors or functional glitches.  I've been using it in 
Firefox and it fires up in both Chrome and Edge, but I haven't tried it 
out in any other browser.

For the record, I didn't write a single byte of code for this 
application.  I merely gave Codex a starting point using the 
spreadsheet, and then had LOTS of text dialog with it to add features, 
fix problems, and refine the user interface.  These AI agents have 
opened up a really broad field for average hams to generate applications 
for our hobby that would otherwise be considered to have too narrow 
usage to make the cost of writing them worthwhile.

73,
Dave   AB7E