Cecil, W5DXP wrote:
"Here`s a quote from _Antennas_by Kraus&Marthelka: "... the radiation
resistance may be thought of as a virtual resistance that does not exist
physically but is a quantity coupling the antenna to distant regions of
space via a virtual transmission line."

A problem comparing radiation to a transmission line is that everytime
line length increases a fixed amount, so does the loss. The loss is so
many db per mile. After a radio wave travels a million miles, the next
million miles only produces an additional 6 db loss. Thus, we are
looking at radio pictures from Mars. We couldn`t get them by the lowest
loss coax from Mars without distributed repeaters.

I can`t find my 1955 edition of Terman but as I recall he says that
radiation resistance is equal to the resistor, which if placed in series
with the antenna, would consume the same power that the antenna does.
Radiation resistance is a useful fiction. Radiation resistance is
usually the resistance at an antenna`s maximum current point. Fictional
or not, it can be measured with an impedance bridge.

My 1937 edition of Terman says:
"---fields produced by the antenna currents induce back voltages that
are equivalent to an impedance against which the applied voltage can be
considered as acting."

Terman also says: "In addition to the radiated energy, energy is also
lost in the antenna system as a result of wire and ground resistance,
corona, eddy currents induced in neighboring masts, guy wires and other
conductors, and dielectric losses arising from such imperfect
dielectrics as trees and insulators, located in the field of the
antenna. These losses can be represented in the same way as the radiated
energy, i.e., by a resistance which inserted in series with the antenna
will consume the same amount of power as that actually dissipated in
these various ways. The total antenna resistance is the sum, Rr + Rl, of
the radiation resistance Rr and the loss resistance Rl, and determines
the amount of energy that must be supplied to the antenna to produce a
given current.

The efficiency of the antenna as a radiator is the ratio Rr / (Rr + Rl)
of radiation to total resistance. This represents the fraction of the
total energy supplied to the antenna which is converted into radio
waves."

Terman also says:
"The radiated field (epsilon) varies directly as the current I, the
frequency f, the doublet length (delta)l, and the cosine of the angle of
elevation, and is inversely proportional to the distance d."

The strength of the radiation from an antenna at a point P is the sum of
the strengths of the fields of its elemental pieces carrying a current
I.

Current is not uniform throughout the usual antenna and neither is the
antenna`s impedance. The current is zero at the open ends of an antenna,
near zero at exact multiples of a half wave length distant from the open
end, while the current is maximum at points that are odd quarter wave
lengths distant from the open ends.

Best regards, Richard Harrison, KB5WZI