A 555 timer IC is used a a-stable multivibrator as shown in the given figure (A). The threshold input is now connected to the trigger input (Pin 2), the resister R1, R2 and capacitor C1 for timing network which set the frequency of the oscillator.
The PUT stand for Programmable Uni Junction Transistor, it is actually a type of Thyristors and not like the UJT at all. The only similarity to a UJT is that the PUT can be used in the same oscillator to replace the UJT.
Waveguide is the type of transmission line which carries microwave frequencies from source to load. The energy is in the form of electric field and magnetic field which are perpendicular to each other the electric field and magnetic field inside.
Transmission line is defined as the path of carrying alternating electrical energy from source to load. For example the wire used between T.V antenna and television set or the wire used between transmitter antenna and transmitter are known as transmission