A Letter From Dr. Jerry Sevick, W2FMI

Dear Friends:

The following is a little history of my work on baluns and ununs that you might find interesting. Incidentally, I coined the word unun (unbalanced to unbalanced transformer) since it was a logical counterpart to the word balun (balanced to unbalanced transformer).

In looking at the transmission line transformer as a broadband matching network for matching 50 ohm coaxial cable to the low impedances of short ground-mounted vertical antennas, many questions came to mind that were not answered in the amateur or the professional literature. They included: Are ratios other than 4:1 and 16:1 available? How do transformers with rods compare with those with toroids? What role does permeability play on bandwidth and efficiency with simple test equipment? How does powdered-iron compare with ferrite? What are the important design considerations for these transformers? What are their failure mechanisms?

In order to answer many of the questions above, a series of measurements were made at Bell Labs on many different forms of ununs and baluns. From the extremely accurate data provided by Bell Labs, for the modeling techniques introduced by Ruthroff (also of Bell Labs), and from my analysis of higher-order windings (trifilar, quadrifilar, etc), I was able to answer most of the questions and decided to write a book on my findings. ARRL published the first edition in 1987.

From the feedback from the first edition, it became apparent that more practical designs were needed for the radio amateur and the professional practitioner. This lead to the second edition which contained over 100 different designs. However, from the feedback from the second edition it now became evident that the ferrite rods and toroids I used were not available in small numbers. They came as samples from various manufacturers who were unwilling to supply small quantities of their products.

By joining forces with James Lau, I was able to re-design practically all of the transformers in my second edition with components that became readily available. Furthermore, by careful standardization and redesign, I was able to optimize the designs on a cost/performance basis. Also, since my second edition, I was able to publish articles on many new designs. I am sure more will be added in the future.


Jerry Sevick, W2FMI

353 West Grove Avenue, Orange, CA 92865, U.S.A.

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