Above is a connection of half-wave dipole antenna. The book states that the current profile is same at any given time instant on both the radiators, each of quarter wavelength length.
Now my confusion is clearly the coaxial cable is connected to both radiators. Say I am sending the signal through centre wire and the braid of the coaxial cable is always at ground. Now at any given time instant, only left end of the cable is going to have non zero potential, while the right end is always at 0V. Then how come both radiators would have same current profile at any given time when the excitation is different for left and right radiators!
To be very specific, say at any given time instant the voltage at the left end is 2.3V, so standing wave is generated in the left radiator, but on the right radiator the voltage is zero (braid is at ground), so how any excitation would happen don the right radiator.
Support will be greatly appreciated. I am a total newbie in antenna systems.
Best Answer
It seems the confusion is due to the shield of the coax being "grounded".
Yes, this may be grounded at the transmitter end. However, the antenna only "sees" the difference in potential between the two points it is fed at. It doesn't matter to the antenna that you consider one of these to be "ground".
In the case of a true earth ground, it can matter to the radiation pattern when the antenna is less than a wavelength or above the earth. This is why you often see a balun between a self-contained antenna like a dipole, and a non-symmetric transmission line like a coax cable. This balun essentially becomes a single-ended to differential voltage converter at the RF frequency.
A quick hack that can still make a effective balun is to wrap the coax around something magnetic for a few turns. This increase the impedance of the coax to common mode signals, while having no effect on differential mode signals. At the transmitter, the shield is at a fixed voltage (ground) and the center conductor varies. After the balun, both can vary up and down arbitrarily together, and only the difference between the two has low impedance and can carry any meaningful power at the RF frequency.