Any of the ARRL amateur radio handbooks will have the illustration of the
voltage (and current) upon the half wave wire. Maximum positive at one
end, near zero at the middle, (since the middle does not go below some low
impedance), and then changing polarity of the voltage as it rises to a
maximum at the other end of the half wave wire.
The current simultaneously starts at zero current flow at the end of the
wire, (the discontinuity), and rises to maximum current 1/4 wave along the
wire at its midpoint.
The current then declines back to zero at the other end. When we feed the
wire in the middle, we are feeding at the lowest impedance point (around 72
ohms in free space), and therefore, most current can flow, as compared to
the end discontinuity where the impedance is maximum thus limiting current
to the minimum.
Yes, to view the antenna action, the texts show a snapshot in time, of a
steady state action of current and voltage, the product of which is the
power imparted by the feedline to the antenna. However, of course, there is
a dynamic action going on to launch an electromagnetic wave into space
around the antenna. It is fairly easy to see that current reaching the
barrier of the open circuit end of the wire would result in an action that
results in launching an electromagnetic wave as the energy must travel
outward from the antenna. In the receiving case, inbound electromagnetic
waves cut across the wire, and in the classic action a current is induced in
the wire by the wave action moving past the conductor. In the half wave
folded dipole, the end is not open circuit, but changes direction thus
following the other Kraus definition of what it takes to impart antenna
radiation or reception, (a change of direction).
-Stuart K5KVH
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