Name Fakhar Elahi Reg Number 16PWELE4951 Microwave Engineering Lab 02
Objectives
To measure the frequency of Microwave source using cavity meter.
Apparatus :
Cavity meter, console unit, frequency source, fixed diode detector, load resistance, attenuator .
Cavity meter/resonator working:
It is used to measure the frequency of microwave signal. It is connected to a waveguide through a slot. It
absorb the power of signal when it is in resonance. Resonance occur when frequency of cavity meter is
become equal to frequency of signal passing through a waveguide. The frequency of cavity meter is
changing by changing the length of its cavity by its probe.
Procedure:
We observed the power of signal by using console unit . if power of
signal when it enters to waveguide of cavity is equal to power of signal
leaving to a waveguide then its mean cavity meter does not absorb the
power and resonance does’ t occur. Now we change the length of cavity
by its prob and change its frequency and observed the output signal
power. when output signal power is less then input signal power its mean that resonance occur and
cavity had absorb the some power of signal and at that stage frequency of cavity meter is become equal
to frequency of input signal.
Now we can find the frequency by graph between length of cavity and its frequency which is provided
by the manufacturer.
The cavity meter has two scale linear scale and circular scale. Lets Ls represent the reading of linear
scale and Lc represent the reading of circular scale the length of cavity can be calculated by formula;
L (in mm)= Ls + ( Lc * 0.01)
When we have L value then we can easily determined frequency from above curve. The frequency is
known as measure data.
Submission date October 15 2019 Section C Submitted to Dr Shahid Bashir 1
Name Fakhar Elahi Reg Number 16PWELE4951 Microwave Engineering Lab 02
Analytical method:
We can also find the value of f by using mathematical equation which we drive from simple formula
Types of wavelength;
Guided wavelength (𝝀𝒈) the wavelength of wave when it travels through a medium
Free space wavelength (𝝀𝒐) the wavelength of wave when it travels through a free space
Cut off wavelength (𝝀𝒄) it depends on the mode of cavity meter and size of cavity meter. Here
the mode of cavity resonator is mode E01 and 𝝀𝒄 =37.1mm
1 𝟏 𝟏
Relation among them is 𝝀𝒐𝟐
= 𝝀𝒈𝟐
+ 𝝀𝒄𝟐
Derivation of formula for frequency
L = n 𝝀𝒈 /2 where n=1,2,3……. Order of resonance
For n=1 L = 𝝀𝒈 /2
𝝀𝒈 = 𝟐𝑳
As we know that c=𝑓 𝝀𝒐 and f= 𝒄/𝝀𝒐 where c is speed of light
From relation put value of 𝝀𝒐 then
𝟏 𝟏
𝒇 = 𝒄(√( 𝝀𝒈 )𝟐 + ( 𝝀𝒄 )𝟐 c=3*10^8 𝝀𝒈 = 𝟐𝑳 𝒂𝒏𝒅 𝝀𝒄 = 𝟑𝟕. 𝟏𝒎𝒎
𝟏 𝟏
𝒇 = 𝟑 ∗ 𝟏𝟎^𝟖(√( )𝟐 + ( )𝟐
𝟐𝑳 𝟑𝟕.𝟏𝒎𝒎
Now by putting L value we find f
Block Diagram:
Console unit
Input output
Microwave Load
attenuator Cavity Fixed diode
source resistor
meter detector
Submission date October 15 2019 Section C Submitted to Dr Shahid Bashir 2
Name Fakhar Elahi Reg Number 16PWELE4951 Microwave Engineering Lab 02
Explanation of block diagram:
Console unit output terminal is connected to microwave frequency source to energize it the
input terminal is connected to diode detector to measure the power of signal.
Microwave source generate a signal in X band.
The attenuator is used to control the control the excessive current and for safety purpose. Keep
attenuator at 40 degree.
The diode detector is used to measure the current of wave.
Load is used as terminator.
Measurement :
Ls =17mm Lc= 20mm
L =(17mm) + (20mm*0.01)
L=17.2mm
When L=17.2mm then f= 10.54GHz
Now from graph this is measured value
From by putting the value of L in equation
𝟏 𝟏
𝒇 = 𝟑 ∗ 𝟏𝟎^𝟖(√( 𝟐𝑳 )𝟐 + ( 𝟑𝟕.𝟏𝒎𝒎 )𝟐
When L=17.2mm then f= 11.89GHz
this is analytical value
Submission date October 15 2019 Section C Submitted to Dr Shahid Bashir 3
