If charge is oscillating in a time-motion, it radiates even if the wire is straight.
In a radiation emmiting antenna, a (high-frequency) potential is applied to make the electrons in the antenna move in an oscillatory way.
According to the classical theory of electromagnetism, every charged particle that is accelerated emits an electromagnetic wave. The power carried by the electromagnetic wave is proportional to the thrid time derivative (and first derivative) of the position vector, P~x' x'''. With some mathematical manipulation, and under some rather general assumptions, the power emitted can be cast as P~(x'')^2 = a^2, i.e., the power carried by the electromagnetic wave is proportional to the square of the acceleration of the charged particle.
For the electrons moving oscilatory in the antenna, x~sin(omega t). Hence x'' ~ (omega ^2) [sin ( omega t)]^2, and the radiated power is P~ (omega ^2) [sin ( omega t)]^2, i.e., the higher the frequency, the higher the power dissipated as an electomagnetic wave.
Note 1: The phenomenon is reversible: an electomagnetic wave hitting an antenna, induces an oscillatory current in the antenna ("receiver antenna").
Note 2: The above theory can also be used to model the bremsstrahlung ("braking radiation"). Obviously, the charged particle's movement in this case is not oscillatory, though.
Acceleration of charge is a reason for radiation of electromagnetic wave. Oscillation
is always accompanied with acceleration. If an oscillation is harmonic one, then
the frequency of radiation coincides with the frequency of the harmonic oscillation.
In a radiation emmiting antenna, a (high-frequency) potential is applied to make the electrons in the antenna move in an oscillatory way.
According to the classical theory of electromagnetism, every charged particle that is accelerated emits an electromagnetic wave. The power carried by the electromagnetic wave is proportional to the thrid time derivative (and first derivative) of the position vector, P~x' x'''. With some mathematical manipulation, and under some rather general assumptions, the power emitted can be cast as P~(x'')^2 = a^2, i.e., the power carried by the electromagnetic wave is proportional to the square of the acceleration of the charged particle.
For the electrons moving oscilatory in the antenna, x~sin(omega t). Hence x'' ~ (omega ^2) [sin ( omega t)]^2, and the radiated power is P~ (omega ^2) [sin ( omega t)]^2, i.e., the higher the frequency, the higher the power dissipated as an electomagnetic wave.
Note 1: The phenomenon is reversible: an electomagnetic wave hitting an antenna, induces an oscillatory current in the antenna ("receiver antenna").
Note 2: The above theory can also be used to model the bremsstrahlung ("braking radiation"). Obviously, the charged particle's movement in this case is not oscillatory, though.
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