After writing an article for the QEX Magazine: “Designing Antenna Systems for Low Common Mode Current in Coaxial Feed Lines” (this article should be published in the July-August issue if not postponed again), I realized that there was a need for a calculator that could enable you to calculate the common-mode impedance of an RF choke (a.k.a. 1:1 balun or “line isolator”). Thanks to the effort of a few fellow hams, we have got some web resources showing the measurement results of such chokes. I used in the article the measurement results published by K9YC (http://k9yc.com/RFI-Ham.pdf) and GM3SEK (http://www.ifwtech.co.uk/g3sek/in-prac/inpr1005_ext_v2.pdf). Especially the publication of K9YC contains a lot of information. But the number of possible chokes designs is large and K9YC could not build and measure all of them. And I wanted to be able to find impedances of the chokes wound on various core types and having various turn numbers. That’s the origin of the calculator. From the very beginning, I realized that I could do myself only a very basic calculator based on the MS Excel spreadsheet. Something of very limited possibilities and not very easy to use. Fortunately, I know Dan, AC6LA, who is an indisputable Guru of Excel and Visual Basic. Who does not know the AutoEZ, Zplots, TLDetails or Moxon Rectangle Generator to name but a few of his software tools? When I asked him, Dan immediately agreed to team up with me and, after a few of weeks, we managed to create the calculator. The calculator enables you to plot |Z| vs. frequency and R vs. frequency for up to three different choke designs on the same chart. You can also find the choke common-mode impedance for any single frequency of interest. But that’s not all. The calculator enables you to find an equivalent circuit of the choke in the form of either one parallel RLC circuit or two parallel RLC circuits connected in series. Such equivalent circuit differs slightly from the calculated impedance but is very convenient to use when you want to model the choke in the antenna simulation programs like AutoEZ+EZNEC, 4nec2 or MMANA. Such model is valid for a very wide frequency band. You can find more info about modeling common-mode chokes in my article. On top of that, the calculator enables you to find an equivalent circuit of the real choke if you enter its measurement results in the workbook or paste the picture showing its |Z| vs frequency plot. I cannot present all the calculator details here (its help file is 13 pages long). Just follow this link https://sites.google.com/view/sp3l-...ous/toroidal-ferrite-core-inductor-calculator and download your copy of the calculator and its help file. Enjoy!