Guide Wavelength Frequency of a Signal in a Rectangular Waveguide

EXPERIMENT NO. 1 OBJECTIVE: To measure guide wavelength & frequency of a signal in a rectangular waveguide TEST SETUP: Klystron power supply , Klystron Tube 2k25, Klystron mount, Isolator, frequency meter, Variable Attenuator, Detector mount, wave guide stand , Slotted line section with tunable probe,VSWR meter, BNC cable etc.

THEORY: The measurement of frequency or wavelength is one of the primary requirements in most microwave measurements. Frequency is the most fundamental quantity because the Frequency of oscillation is the same at all parts of the micro wave system under steady- state conditions. Wavelength on the other hand depends upon the configuration of the electric and magnetic fields as determined by the geometry of the measuring device. The resonant mode excited by the possible frequency is called the fundamental mode or the dominant mode. A better way to measure frequency is with a calibrated resonant cavity. A resonant cavity wave meter is the analog of the tuned resonant circuit , in general there are two primary types (1) Transmission cavity, which pass only the signal to which they are tuned and (2) Absorption cavities, which attenuate (by absorption) only the frequency to which they are tuned. An absorption type is preferred for laboratory Frequency measurements. First the power level is adjusted to give a full scale reading on the output VSWR meter then the wave meter is tuned slowly until there is a dip in the power level. The frequency may then the read from the dial of the meter. For dominated mode TE 10 mode rectangular wave guide the following relation is in use:1/ λ 0 2= 1/ λ g 2+1 λ c 2 Where λ 0 is free space wave length λ g is guide wave length

λ c is cut off wave length For TE 10 mode λ c = 2a where ‘a’ is broader dimension of wave guide.

Klystron Power Supply

Reflex Klystron

Isolator

Frequency meter

Attenuator

Detector mount

V.S.W.R. Meter

Slotted line section

C.R.O.

Figure1: Microwave bench set up for Frequency & Wavelength measurement

PROCEDURE:1. Set up the components & equipments as shown in figure1. 2. Set up attenuator at minimum attenuation position 3. Keep control knobs of vswr meter as given below:Range -50db Input switch - crystal low impedance Meter switch - Normal position Gain - Mid position 4. Keep control knobs of Klystron power supply as given below :Beam voltage - off Mod switch - AM Beam voltage switch - Full anticlockwise Reflector voltage - Full clockwise AM amplitude knob - Full clockwise AM frequency knob - Mid position

5. Switch on the Klystron power supply, VSWR meter and cooling fan switch. 6. Switch on the beam voltage switch and set beam voltage at 300V with beam voltage knob. 7. Set the Reflector voltage to get some deflection in VSWR meter 8. Maximize the deflection with AM amplitude and frequency control knob of supply.

9. Tune the plunger of mount for maximum deflection 10.Tune the reflection voltage knob for maximum deflection 11. Tune the probe for maximum deflection in VSWR meter 12.Tune the frequency meter knob to get a ‘dip’ on the VSWR meter and note the frequency from frequency meter. 13. Move the probe along with the slotted section The deflection in VSWR meter will vary .Move the probe to a minimum deflection to get accurate reading and note the probe position . Again move the probe to next minimum position and note again.

OBSERVATIONS: Beam voltage =

…………………………..

OBSERVATION TABLE: -

λg = Sr. Repeller 1st 2nd No. voltage Maxima Maxima 2(d2d1) (d1) (volt) (d2)

1/ λo = √1/ λg2+1/λc2

calculate fo = C/ λo

Frequency meter reading (fo)

Measurement of distance either between two Maxima or two Minima.

CALCULATION:1. Calculate the guide wavelength as twice the distance between two minimum Position measure the waveguide inner broad dimension ‘a’ which will be around 22.85 to 22.86 mm for X band 2. Calculate the frequency, F= C/λo, where C = Velocity of light 1/ λo = √1/ λg2 +1/λc2 3. Verify with frequency obtained by frequency meter Where, C= 3 × 108 meter per second C= 3 × 1010 cm per second 4. Measurement of guide wavelength (λg) = ---------λg/2=Distance between two Maxima or two Minima λg= 2 ( d 2 -d 1 ) d 2 = Value of 1st maxima position from load. d 1 = Value of 2nd maxima position from load 5. Measurement of λc= 2a for TE 10 mode.

6. Measurement of wavelength (λ o ) by 1/ λo = √1/ λg2 +1/λc2 7. Measurement of Frequency f o by formula fo = C/ λo for rectangular waveguide

RESULT:Frequency measured with frequency meter = ………………….GHz Frequency measured by Slotted line method = ………………… GHz

POINTS FOR DISCUSSION: 1. Range for X band frequency spectrum. 2. What do subscripts m,n indicate in TE m,n mode? 3. TE 1,0 is dominant mode for rectangular waveguide not TE 0,1. 4. Type of waveguide which is preferred in microwave

measurements & Why? 5. Why Absorption type wave meter is preferred for frequency measurement ?