[TowerTalk] Need help with series coax impedence transformer

[L. B. Cebik]

> I am trying to match a 26 ohm load to a 50 ohm line at 28.45 Mhz with
> .66 velocity factor RG-213 and RG-11.
> Does anyone have a computer program for this???
> 
Chuck,

The Regier series matching equations have been incorporated into HAMCALC
from VE3ERP for a long time under the program name "Series Section 
Matching Transformer."  I am not certain of downloading sites, but
distribution of the program is free.  The latest version (30) is available
directly from Murph, but unfortunately, no one is sure how long the
Canadian postal workers strike will last.

I ran your problem through the HAMCALC version using the specified
impedances and line types.  It is a close run, as you will see in a
moment.  RG 213 was listed as 50 ohms and RG-11 as 75 ohms, both with .66
VF numbers.  The 50 ohm length from the antenna came out 6.78' and the 75
ohm matching section came out 3.34' long (followed, of course, by the
50-ohm run to the shack).  SWR is 1.9:1.

If we change only a little bit, say to a 52-ohm coax and a 73-ohm matching
line, then the program burps, indicating an SWR > 2:1 and recommending
changes.

You may wish to consider alternative matching methods.  For example, a
beta match (also called a hairpin match) would achieve the requisite
transformation efficiently, as would a 2:1 balun transformer.  You can
even parallel 1/4 wl lengths of 75-ohm coax (for a 35-ohm transmission
line) and effect the 2:1 match that way.  Undoubtedly, this is not a
complete list of possibilities.

I suggest the alternatives, because with the components you have on hand
(50 and 75 ohm lines) the series match system is close to its limits for a
low SWR match in your situation.  The Regier series match system is indeed
extremely flexible, but not without its own limits.  In the original
professional journal articles, from which he summarized the QST article,
Regier shows that a number of common matching schemes, including the 1/4
wl section, are special cases of his general formulas.  Very interesting
reading for those interested in the finer points of transmission lines.

Hope this is useful to you and others.

-73-

LB, W4RNL


L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
1434 High Mesa Drive      /  \/  \/\     ----/\---     (Hm) (423) 938-6335
Knoxville, Tennessee     /\   \   \ \   /  / || /      (FAX)(423) 974-3509
37938-4443     USA      /  \   \   \ \       ||              cebik@utk.edu
         URL:  http://funnelweb.utcc.utk.edu/~cebik/radio.html




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