Electromagnetic Waves as Voltage Signals The text says :
The source connected to the transmission line’s sending end may be any circuit generating an output voltage, such as a radar transmitter, an amplifier, or a computer terminal operating in transmission mode.
Doesn't radar emit EM waves and not voltage?
 A: Every voltage is associated with an electric field by
$$V_{ab}=\int_a^b\vec{E}\cdot{\rm d}\ell$$
where $a$ and $b$ are two points in space and $V_{ab}$ is the voltage between $a$ and $b$.
But this integral only has a well-defined value in the electrostatic limit, meaning when the electric and magnetic fields in the region are not changing. When we analyze lumped circuits with AC or time-varying voltages, we are only making an approximation of their behavior, but it can be a very good approximation.
So when we apply a voltage between two electrodes, we produce an electric field between those electrodes. Or if we apply a voltage to an antenna, we produce an electric field between the antenna and the other terminal of our voltage source (which we can connect to earth). 
If we apply an AC voltage to an antenna, we can stimulate an electromagnetic wave to be radiated by that antenna. 
But once that wave is propagating in space, the electrostatic potential (voltage) concept is not adequate to describe its behavior because the time-varying magnetic component of the wave is significant.

Doesn't radar emit EM waves and not voltage?

Yes, a radar transmitter emits EM waves. But it is possible to drive this emission using a circuit analyzed as a lumped circuit with voltages and currents.
Note: Antenna engineers extend the concept of potential with the vector potential to enable analyzing many wave propagation phenomena beyond what the electrostatic potential can describe.
