What is the purpose of the electromagnetic field which surrounds a traveling-wave tube?

The Gunn diode oscillator

Theoretically electromagnetic radiation field strength varies in inverse proportion to the square of the distance, but when atmospheric attenuation effects and the absorption of the terrain are taken into account the attenuation can be as high as the inverse ___________ power of the distance.

A reflex klystron is oscillating at the frequency of its resonant cavity. If the reflector voltage is made slightly less negative, the