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Experiment of Microwave Cavity





 Objective:Measurement of the Q-Factor of a Cavity.



 List of Equipments:


1. Microwave source with square wave modulation

2. Isolator

3. Variable attenuator

4. Slotted line

5. Tunable detector

6. VSWR meter.





The microwave cavities are basically of two types:

a) Transmission type

b) Absorption type


The transmission type cavity transmits maximum power at resonant frequency and absorption type absorbs the power at the resonant frequency. Cavities are used both as circuit elements and as measuring instruments.

The Q-factor of a cavity has the same meaning as the Q of a resonant circuit lower frequency. It is defined as

Energy stored in the cavity depends on the volume, and the power loss depends on the losses in the conducting walls of the cavity. Approximate value of the Q of the cavity is

 Q = V / AS

V = Volume of the cavity

A = Surface area

         S = Skin depth

The intrinsic or unloaded Q is greater than the total or loaded which includes the effect of coupling mechanism. The loaded of a cavity may be measured very simply by observing the shape of its resonance curve as a function of frequency. The of the cavity is determined from


                                                                  f0 Resonant frequency

                                                                  f1 & f2 the 3 dB points





                                             Fig 1. Experimental arrangement for measurement of Q factor of a Cavity


1. Set the frequency and amplitude of the source by slider and knob respectively. Do square wave modulation using modulator.

2. Press “Forward Direction” in isolator and adjust attenuator to minimum by slider.

3. Now in cavity window first set the ‘Resonant Frequency’ by using slider.

4. Move the slider given below to get the power at the resonance. Fix the value by pressing “Power (dB) at resonance” button.

5. Move the slider to get the first 3 dB point corresponding to the resonant frequency int.

6. In the same manner fix the "Second 3db" point.

7. Now to know the calculated bandwidth press “Bandwidth” button.

8. Press “Q-Factor” button to fix the Q factor.



1. Does the Q-depend on the coupling of the cavity?

2. What are the different types of coupling to cavity?

3. Give the equivalent circuit of a cavity at the resonant frequency, and also below and above the r

4. What are the dimensions of rectangular, cylindrical & coaxial cavity to achieve maximum Q? 

5. Where the fixed and variable cavities used?



1. S. Ramo, J.R. Whinney and I. VanDuzer, Fields and Waves in Communication Electronics, Third

2. E.L. Ginzton: Microwave Measurements, McGraw-Hill Book Company, Inc. New York, 1957.

3. Annapurna Das, Sisir K Das, Microwave Engineering, McGraw-Hill International Edition, Singapore

4. C. G. Montgomery, Techniques of Microwave Measurements, McGraw-Hill, New York, 1947.



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