In trasmitting antennas the current is described with a standing wave (in resonant configuration). Since the current must be zero at both ends of the antenna (it could not be otherwise) the mechanical analogy should be the standing wave on a rope fixed at both ends.
Nevertheless if the length of antenna is $d=2 \lambda$ the configuration of current is
(For this and other configurations: see last pages of http://www.amanogawa.com/archive/docs/antennas1.pdf)
While the configuration of a rope fixed at both ends when $d=2 \lambda$ is
(of course it's not "current" here, but "dispacement")
I'm quite confused about this difference. I see that the reason of this difference is that the antenna in the first picture is connected in the central points to an AC current generator, and that connection "creates a condition of specular symmetry between the two branches".
But I do not see why the AC generator should provide a symmetrical situation at both sides necessarily.
On wikipedia: https://en.wikipedia.org/wiki/Antenna_(radio)#Resonant_antennas it is represented also the voltage wave for $d=\frac{\lambda}{2}$ (which is not the previous case).
It is clear that the voltage wave is not symmetric as the current wave is!
So my question is: why does the AC generator create a specular symmetric current wave for the two sides? And why is the voltage wave a "normal" standing wave (not specular) instead?