Important Questions and Answers: Microwave Tubes and Measurements

MICROWAVE TUBES AND MEASUREMENTS 

1)What is the role of slow wave structure in TWT?

Slow wave structure are a special circuits that are used in microwave tubes to reduce the wave velocity in a certain direction so that the electron beam and the signal were interact

2)Compare M and O type tubes

M type Tubes

Static magnetic field is perpendicular to the electric field

Magnetron is the M type Tube

Electron travel in curved path

O type Tubes

Static magnetic field is same direction to the electric field

Klystron and TWT are the O type tubes

Electron travel in linear path

3)Compare TWT & Klystron

Klystron Amplifier

Linear beam or O type tubes

Uses a cavities for input and output Circuits

Narrow band device duo to use of resonant cavities

TWTA

Linear beam or O type tubes

Uses non resonant wave circuit

Wide band device because use of non-resonant circuit

4) what are the limitation in conventional vacuum tubes

The concentional tubes such as triode, tetrodes, pentodes can be used as amplifiers and oscillators more efficiently. But these conventional tubes can not used as amplifier or oscillator at high frequency (>1000MHZ) because at higher frequencies output drops off

The factors of contributing of output at UHF are

ü Circuit resistance

a)     Inter electrode capacitance

b)    Lead inductance

ü Transit time effects

ü Cathode emission plate heat dissipation area

ü Power loss due to skin effect, radiation and dielectric loss

ü Gain band width product

5)Define convection current in TWT

The convection current induced in the electron beam is by the axial electric field. When the space charge effect is considered, the electron velocity, charge density, current density and the axial electric field will perturbate about their averages or DC values.

i= jβe I0E1/2vo(jβe-γ2)

6)Define SWR?

standing-wave ratio (SWR) is a mathematical expression of the non-uniformity of an electromagnetic field (EM field) on a transmission line such as coaxial cable. Usually, SWR is defined as the ratio of the maximum radio-frequency (RF) voltage to the minimum RF voltage along the line.

SWR = Vmax/ Vmin

=1+Г/1-Г

7.Name the possible errors VSWR measurements?

ü The signal source give a signal frequency. any spurious signals present leads to the shift in maxima and minima and hence error is resulted in the readings

ü There should not be any undesired reflection. for this matched load can be used as shown. These undesiresd reflection cause peaks and nodes to shift position in the standing wave pattern and VSWR thus measured will not be accurate.

ü For higher VSWR near 10, the minimum voltage will be small and there will be deformation in the pattern because of the high coupling at voltage maximum resulting in errors

ü Depth of penetration of probe into slotted wave guide introduces errors

8)What are the errors in impendence measurements

ü Scalar or sensitivity errors

ü Vector errors or phase errors

9)What is the significance of VSWR measurements

ü VSWR and the magnitude of voltage reflection coefficient (Г) are very important parameters which determine the degree of impendence matching

ü VSWR and Г are also used for measurement of load impendence by the slotted line method

ü When a load ZL ≠ Zo is connected to transmission line, standing waves are produced

10)           What is Bolometer?

It is a power sensor whose resistance change with changed temperature as it absorb the microwave power. It is a short thin metallic wire sensor with positive temperature coefficient of reistance.

11) Define insertion loss?

It is defined as difference in power arriving at the terminating load .with or without The network in circuit

Insertion loss(db)=10 log(po/pi)

12) What is radiation pattern?

Radiation pattern is a representation of radiation characteristics of an antenna which is a function of elevation angle azimuth angle for a constant radial distance and frequency

12)What is spectrum analyzer?

Spectrum analyzer is a broad band super heterodyne receiver which is used to display a wave in frequency domain additionally, power measurements, side bands can also be observed.

13)What is the principle by which high power measurements could be done by calorimetric method?

The measurement involves conversion of microwave energy into heat, absorbing this heat in a fluid (usually water) and then measuring the temperature rice of the fluid.