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[TowerTalk] Sevick (W2FMI) Baluns and Ununs

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Subject: [TowerTalk] Sevick (W2FMI) Baluns and Ununs
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Date: Thu, 12 Feb 1998 14:41:58 -0600

A short while back, a posting was made by someone referencing an IEEE web page 
where a paper on baluns and ununs could be found.  I forget the name/call of 
person who made the posting (thank you!!), but I did go to the web page and 
retrieve the paper.  It is titled "Design and Realization of Broadband 
Transmission Line Matching Transformers" and is authored by Jerry Sevick, W2FMI.

In this paper was a description of a 2:1 step-down balun (50 ohms to 25 ohms)
that caught my eye.  On page 5 of the paper, Sevick writes:

"By using a 2.25:1 or 2:1 step-down unun (Figure A) in series with a 22.22 or 
25-ohm 1:1 Guanella balun, excellent baluns are now available to match 50-ohm 
cable to Yagi beam antennas with impedances of 20 ohms to 30 ohms.  Both units 
can be wound of the same core [B5]."

Reference [B5] is to his book "Transmission Line Transformers", 1990, 2nd 
edition.  I believe that the ARRL no longer publishes this book.

This device appears to be ideal for an antenna impedance matching application 
that I have.  My homebrew 45' linear-loaded 40m rotatable dipole has a 
Z at resonance of around 20 to 25 ohms.  I am currently using a quarter-wave Q 
section of paralleled 75-ohm coax for the step-down impedance transformation, 
and by winding 12 turns around a 4" pvc form am hopefully achieving some 
choking action for the balun part.  While this scheme does work, it occured to 
me that perhaps a better method may be available.  I am also aware of the 
helical hairpin approach, and am considering that, too.

My question:  Does anyone have any data and/or comments about the Sevick 2:1 
step-down balun?  Anyone ever built or used one?  Any data comparing 
efficiencies with the narrow-band (Q-section and hairpin) approaches?

Many thanks in advance.


Charlie  N9CO

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