These usual electronic vacuum tubes fails to operate above 1 GHz because of two reasons -appearance of the stray reactance due to the lead wire inductance and the inter-electrode capacitance also the electron transit time from the cathode to the grid becomes comparable to the time period of the sinusoidal signal.
Due to considerable transit time of the electrons in the cathode-grid space the grid potential may accomplish a negative half cycle by the time the electrons which started from the cathode during the positive half cycle of the grid, reach the grid .This makes the electrons fluctuate back and forth in the cathode grid space resulting in the reduction of competence. On the other hand the stray reactance cause an increase in the valid part of the input admittance which burden the input circuit and reduce the operating efficiency of the tube so because of these effects concepts of microwave tube design are dissimilar.
Microwaves tubes are normally known as Klystrons, travelling wave tubes(TWT) and Magnetrons which vary from all conventional electronic vacuum tubes in that transit time is utilized for microwave amplification.
The theory uses an electron beam on which space charge waves interrelate with electromagnetic fields in the microwave cavities to transmit energy to the output circuits of the cavity otherwise interact with the electromagnetic fields in the slow wave structures to give the amplification through transfer of energy.
TWT’s and Klystrons are the linear beam also known as “O”-type tubes in which accelerating electric field is in the same route as the static magnetic fields used to focus the electron beam .Magnetrons are known to be the crossed field devices where the static magnetic field is perpendicular to the electric field.
In a number of the low power applications, solid-state devices have substituted electrons beam devices because of the advantages of their light weight , small size, high reliability and low cost being included into microwave integrated circuits.
Two types of tubes:-
a)Linear beam tube (O type)
b)Crossed field beam tube (M type)
Uses of Microwave Sources (microwave tube devices) are:
- Radio astronomy
- Heating and power application
QUESTIONS AND ANSWERS
Q) What is the need of designing microwaves tube ?
Ans) The conventional electronic vacuum tubes fails to operate above 1 GHz because of two reasons i)The electron transit time from the cathode to the grid becomes analogous to the time period of the sinusoidal signal and ii)Appearance of the stray reactances due to the lead wire inductances and the inter-electrode capacitances.
Q)What are the advantages of electron beam devices over solid state devices?
Ans) Advantages are:
- low cost
- light weight
- Small size
- high reliability
Q) Explain how microwave devices are used in heating and power applications?
Ans) A microwave oven passes microwave radiation through food causing dielectric heating primarily by absorption of the energy in water. Microwave ovens became well-known kitchen appliances in Western countries in the late 1970s, subsequent expansion of reasonably priced cavity magnetrons. Water in the liquid state possesses many molecular interactions that enlarge the absorption peak. In the vapour phase, isolated water molecules absorb at around 22 GHz, almost ten times the frequency of the microwave oven.
Microwave heating is used in industrial processes for drying and curing products.
To produce plasma for such purposes as plasma-enhanced chemical vapour deposition(PECVD) and reactive ion etching many semiconductor processing techniques use microwaves.
Q) What are types of microwaves Sources?
Ans: There are two types of microwave sources:
i)Linear beam tube(O type)
ii) Crossed field beam tube(M type)