Several notes from very different source recently set up a theme question
for me. The theme seemed to be something like this: I'll take less gain
from my Yagi to be able to have a direct 50-ohm feed with no lossy
matching network.
The question that emerged is this: just what are the basic, uneliminable
losses of some of the sorts of common matching networks used to raise a
lower Yagi feed impedance to 50 ohms? The question involves the
electrical network principles, not the many ways in which we can poorly
construct the matching section/system.
For many network types, calculations are possible (and simplified by some
calculation aids readily available). I checked out Tee matches (which can
be physically modeled with the antenna), beta matches (for which L-circuit
analysis is available), and quarter wave matching sections (35-ohm RG82A/U
is available). My question is not who is right and who is not--my
effort was only to find out what the inherent network losses are. From
that point forward, I assume that a builder will put as much effort into
the physical design and construction of the network as he/she does into
the antenna proper.
However, the results may be surprising, in light of the bad reputation
matching networks have had from way back and from not having their numbers
crunched. Although 50-ohm direct feed antennas have a good and proper
niche in radio work, I would not myself give up antennas of higher gain
just because their lower feedpoint impedances required a matching network.
The results of my little exercise are in the first entry under ATUs and
Impedance Matching at the site: "Who's Afraid of a Little Matching?"
Guess that gives away the surprise ending. . .. Hope the notes are useful
to someone.
-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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