Arguing about the "true" meaning of radiation resistance is pointless.
If you'll look through various respected texts and references, you'll
see that it's used by different authors in different ways. (I posted
some specific examples quite a long time ago on this newsgroup -- I'll
find and repost if anyone is interested.) Some authors refer it to a
current loop; others use it to describe the feedpoint resistance, at
resonance or not. In "folded" antennas, it can mean either the
"unfolded" resistance or the transformed feedpoint resistance. The only
common thread in usage is that it always represents a sink for the power
which is radiated, so this is the only "true" meaning of the term. The
greatest danger in being careless about usage is in blindly using some
formula such as a common one for efficiency, Efficiency = Rr / (Rr +
Rl). This works only if Rr and Rl are referred to the same point. For
example, if used for a folded dipole or unipole, both Rr and Rl must be
as measured at the feedpoint, where they're both transformed by the
"folding" process; or both defined as properties of the unfolded
antenna. Using the "folded" value of one and "unfolded" value of the
other leads to incorrect conclusions about efficiency -- conclusions
which been successfully used to sell antennas.

A given antenna doesn't have a single "inherent" or "fundamental" value
of radiation resistance -- it's different at every point along an
antenna, and the term can be legitimately used to describe the radiation
"loss" component at any point. So whenever there's a chance of
misunderstanding, it's important to say exactly what you mean by
"radiation resistance" -- that is, where on the antenna this equivalent
resistance is -- whenever you use the term.

Roy Lewallen, W7EL

Richard Harrison wrote:
Richard Cl;ark wrote:
"The radiation resistance of an antenna is NOT necessarily the same as
the drivepoint impedance of the antenna."

True, but for many resonant antennas they are identical.

Fundamentally, the radiation resistance is the value when inserted in
series with an antenna will consume the same power as that radiated.
Unless otherwise specified, the radiation resistance is referred to a
current maximum point in an ungrounded antenna, and to the base of a
grounded antenna. See 1955 Terman page 890 and 1950 Kraus page 143. They
agree.

All of ON4UN`s loaded antennas have maximum current at their drivepoints
and they are resonant, so their feedpoint resistance coincides with
their radiation resistance in all the instances diagrammed in the
current distribution chart for short loaded verticals. ON4UN starts with
1A current to the base of all antennas and the current declines from
that value. Its value is the cosine of the number of degrees from the
feedpoint in most cases.

Best regards, Richard Harrison, KB5WZI