Gene Fuller wrote:
Cecil Moore wrote:
Yes and, being open-ended, is a *standing-wave* antenna.
Contrary to what has been said here on r.r.a.a in the
past, Balanis says: "The current and voltage distributions
on open-ended antennas are similar to the standing wave
patterns on open-ended transmission lines. ... Standing
wave antennas, such as the dipole, can be analyzed as
traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by
traveling wave currents If and Ib in Figure 10.1(a)."

In other words, what Balanis says may be true, but so what?

The "so what" is the additional knowledge to be gained by
not choosing to ignore the underlying physics. When the
forward EM wave hits the end of the dipole, what happens?
Essentially the same thing that happens when a forward EM
wave hits the end of an open-circuit transmission line.
The H-field (current) goes to zero and the E-field (voltage)
doubles, i.e. the forward wave existing at that point is
completely reflected.

That explains why the feedpoint impedance of a 1/2WL dipole is
50-75 ohms instead of the physical characteristic impedance
of ~1200 ohms. The feedpoint impedance of a 1/2WL dipole is
a virtual impedance caused by destructive interference
between the forward and reflected voltages, Vfp = |Vfor|-|Vref|,
and constructive interference between the forward and reflected
currents, Ifp = |Ifor|+|Iref|, and Zfp = Vfp/Ifp

It is interesting to note the consistency of the arguments
here on r.r.a.a. Someone says, "'A' is true". Someone else
says, "No, 'A' is gobbledygook". After 'A' is proven to
be true, the argument shifts to, "OK, so what? Those grapes
are probably sour anyway." :-)
--
73, Cecil http://www.qsl.net/w5dxp