[TowerTalk] Radiation Resistance

[EUGENE SMAR]

TowerTalkians:

     Here's a question the answer to which is mainly of academic, rather
than practical, interest.  But I'll ask it anyway:

     In several treatises on RF ground fields for vertical antennas
especially, (see any antenna handbook and
http://www.bencher.com/pdf_download.html#tech_notes  as examples),  the term
"antenna efficiency" has been defined as the ratio of radiation resistance
(the "good" antenna resistance) to the total feedpoint resistance (radiation
plus ground plus Ohmic loss resistances).   We all, then, go on to calculate
the efficiencies of our resultant vertical antenna systems as the product of
this "efficiency" and the power input at the feedpoint of the antenna.

     But is this the correct procedure?  As I imagine this Radiation
Resistance, I see three "resistors" in series:  the ground and Ohmic losses
plus this Radiation Resistance - a voltage divider circuit.  Taking the
above ratio looks like I'm calculating the voltage drop across that one
Radiation Resistance value in a three-valued voltage divider circuit.

     If this is truly what is being modeled when we say "Radiation
Resistance", ought we not to calculate the POWER efficiency of the actual
antenna as this ratio SQUARED?   If we are truly to use the ratio as-is (not
squared), then this value can be used HOW?  Efficiency of WHAT?  Not, in my
estimation,  the net power delivered into that Radiation Resistance and sent
on its way to ZL8.

     Looking at a simple example, if I calculate this ratio for my
hypothetical 80 meter vertical, I might come up with a figure of 1/3.  This
is 33.3% "efficiency" according to the above definition.  But I'm arguing
that I ought to SQUARE this percentage figure (33.3% X 33.3%) to come up
with a POWER efficiency of 11.1%.  I say this  because the power is applied
to all three "resistors" in my antenna model but gets radiated by only 1/3
of the total resistance for 1/9 of the applied power.

     In a practical antenna, we would all work toward lowering the ground
resistance component by adding radials and the efficiency is whatever it is
when we're done.  But I hope someone can explain what the term "efficiency"
in all of these articles really means and what can we do with the number.

73 de
Gene Smar  AD3F


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