This document describes a physics lab experiment on microwaves. The experiment studied the directional characteristics and polarization of microwaves emitted from a horn antenna. Key aspects covered include: 1) Measuring the transverse and longitudinal electric field distributions in front of the horn antenna to determine the microwave wavelength and polarization. 2) Using a polarization grating and rotating it to measure the received signal and study polarization effects. 3) Calculating the microwave wavelength from the measured field distributions and determining the focal length of a lens used in the experiment.

1. Physics Lab VII
Directional characteristics and polarization of microwaves in
front of horn antenna
Submitted To
Dr. 𝑵𝒂𝒆𝒆𝒎 𝑨𝒏𝒋𝒖𝒎
Submitted By:
Group Leader: 𝑻𝒂𝒔𝒔𝒂𝒘𝒂𝒓 𝑰𝒒𝒃𝒂𝒍 (IU16S6BA030)
Coordinators: 𝑴𝒖𝒉𝒂𝒎𝒎𝒂𝒅 𝑯𝒂𝒎𝒎𝒂𝒅 (IU16S6BA020)
𝑴𝒖𝒉𝒂𝒎𝒎𝒂𝒅 𝑹𝒊𝒂𝒛 (IU6S6BA032)
𝒀𝒂𝒔𝒊𝒓 𝑨𝒍𝒊 (IU16S6BA006)
Semester: BS 7th
Session: Spring (2016-2020)

2. Contents:
 Abstract:
 Introduction:
 History:
 Gun diode and its working:
 Horn antenna:
 BNC connector:
 Voltmeter:
 Applications:
 Apparatus:
 Procedure:
 Observation and calculation
 Conclusions:
 References:

3. Abstract:
In this experiment we will study about microwaves, its polarization, setup which is
used for its propagation, how we can find its wavelength, measure the focal length
of lens which is used in this experiment and verification of Braggs Law using
relation as:
2𝑑𝑠𝑖𝑛𝜃 = 𝑛𝜆

4. Introduction:
Microwaves are a form of electromagnetic radiations with wavelength ranging
from one meter to one millimeter; with frequency between 300 MHz (100 cm) and
300 GHz (0.1 cm). Different sources define different frequency ranges as
microwaves. A more common definition in radio engineering is the range between
1 and 100 GHz. The boundaries between far infrared terahertz radiation,
microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used
variously between different fields of study.
To transmit microwaves signal a wave guides is used as a transmission lines. A
wave guide is a special type of transmission line that consist hollow metal tube or
pipe used to conductelectromagnetic waves through its interior. It can be at any
shape but the most popular shape is rectangular waveguides is perfectly shielded
hence no radiation loss. The attenuation of a hollow pipe is less and the power
capacity is greater than that of the coaxial cable of the same size at the same
frequency. .
In the electromagnetic wave spectrum, microwaves are located after the radio
waves.
History:
The existence of radio waves was predicted by James Clerk Maxwell in 1864 from
his equation. In 1888, Heinrich Hertz was the first to demonstrate the existence of
radio waves by building a spark gap radio transmitter that produced 450 MHz
microwaves, in the UHF region. The equipment he used was primitive, including a
horse trough, a wrought iron point spark, and Lay den jars. He also built the first
parabolic antenna, using a zinc gutter sheet. In 1894, Indian radio pioneer Jadish
Chandra Bose publicly demonstrated radio control of a bell using millimeter
wavelengths, and conducted research into the propagation of microwaves.
Microwaves:
Microwaves are a form of electromagnetic radiations with wavelength ranging
from one meter to one millimeter; with frequencies between 300 MHz (100 cm)
and 300 GHz (0.1 cm). Different sources define different frequency ranges as
microwaves. A more common definition in radio engineering is the range between
1 and 100 GHz.

5. Propagation:
Microwaves travel solely by the line sight paths; unlike lower frequency radio
waves they do not travel as ground wave which follow the contour of the Earth, or
reflect off the ionosphere (sky waves). Although at the low end of the band they
can pass through building walls enough for useful reception, usually rights of way
cleared to the first zone are required. Therefore, on the surface of the Earth
microwave communication links are limited by the visual horizon to about 30 – 40
miles (48 – 64 km). Microwaves are absorbed bymoisture in the atmosphere, and
the attenuation increases with frequency, becoming a significant factor (rain fade)
at the high end of the band. Beginning at about 40 GHz, atmospheric gases also
begin to absorb microwaves, so above this frequency microwave transmission is
limited to a few kilometers.
Applications:
Microwaves are used in following fields.
o Communication:
o Navigation:
o Radar:
o Radio astronomy:
o Heating and power applications:
o Spectroscopy

6. Polarization:
It is define as the actionof restricting the vibrationsof transverse
wave, wholly or partially inone direction.
It’s a property applying to transverse wavesthat specifiesthe
geometrical orientationof the oscillations.
Apparatus:
 Gunn diode
 Horn Antenna
 Voltmeter
 Analogue voltmeter
 BNC Leads
 Polarization Grating

7. Gunn Diode:
Gunn diodes are used to build Oscillators for generating microwaves with
frequency ranging from 10GHz to THz. It’s negative differential resistance device
also known as a transferred electrondevice (TED), is a form of diode a two-
terminal semiconductor electronic component, with negative resistance, used in
high-frequency electronic. It is based on the "Gunn effect" discovered in 1962 by
physicist J. B. Gunn. Its largest use is in electronic oscillator to generate
microwave, in applications such as radar speed microwave rely data link
transmitters, and automatic dooropeners. [2]
Its internal construction is unlike other diodes in that it consists only N-type doped
material, whereas most diodes consistof bothP and N-doped regions. It therefore
does not conductin only one direction and cannot rectify alternating current like
other diodes, which is why some sources do not use the term diode but prefer TED.
In the Gunn diode, three regions exist: two of those are heavily N-doped on each
terminal, with a thin layer of lightly n-doped material between. When a voltage is
applied to the device, the electrical gradient will be largest across the thin middle
layer. If the voltage is increased, the current through the layer will first increase,
but eventually, at higher field values, the conductive properties of the middle layer
are altered, increasing its resistivity, and causing the current to fall. This means a
Gunn diode has a region of the negative differential resistance in its current-
voltage characteristics curve, in which an increase of applied voltage, causes a
decrease in current. This property allows it to amplify functioning as a radio
frequency amplifier, or to becomeunstable and oscillate when it is biased with a
DC voltage.

8. Gun diode as a microwavedetector
The electronic band structure of some semiconductor materials, including gallium
Arsenide (GA As), have another energy band or sub-band in addition to the
valence and conduction bands which are usually used in semi-conductor devices.
This third band is at a higher energy than the normal conductionband and is empty
until energy is supplied to promote electrons to it. The energy comes from the
kinetic energy of ballistic electrons, that is that is, electrons in the conductionband
but moving with sufficient kinetic energy such that they are able to reach the third
band.
These electrons either start out below the Fermi level or are given a sufficiently
long mean free path to acquire the needed energy by applying a strong electric
field, or they are injected by a cathode with the right energy. With forward voltage
applied, the Fermi level in the cathodemoves into the third band, and reflections of
ballistic electrons starting around the Fermi level are minimized by matching the
density of states and using the additional interface layers to let the reflected waves
interfere destructively.

9. In GA As the mobility or drift velocity in the third band is lower than that in the
usual conduction band, so with a small increase in the forward voltage, more and
more electrons can reach the third band and current decreases. This creates a region
of negative incremental resistance in the voltage/current relationship.
Horn antenna:
A horn antenna or microwavehorn is used for the transmission and reception
of microwavesignals. It derivesits namefrom the characteristic flared
appearance. The flared portion can be square, rectangular or conical. The
maximum radiation and responsecorrespondswiththe axis of horn .The Hat
consist of a flaring metal waveguideshaped like a horn to direct radio waves
in a beam. Hornsare widely used as antennasat UHF and
microwaves frequencies above 300 MHz’s

10. They are used as feed antennas for larger antennastructuressuch as
parabolic, as standard calibration antennasto measurethe gain of other
antennas, and as directive antennasfor such devices as radar gun, automatic
door opener and microwaveradiometer. Their advantages are moderate
directivity, low standingwaveratio (SWR), broad bandwidth, and simple
construction and adjustment.
Working of Horn Antenna:
Horn Antenna provides a gradual transition structure to match the impedance of a
tube to the impedance of free space, enabling the waves from the tube to radiate
efficiently into space.
If a simple open-ended waveguide is used as an antenna, without the horn, the
sudden end of the conductive walls causes an abrupt impedance change at the
aperture, from the wave impedance in the waveguide to the impedance of free
space(about 377 ohms). When radio waves travelling through the waveguide hit
the opening, this impedance-step reflects a significant fraction of the wave energy
back down the guide toward the source, so that not all of the power is radiated.
This is similar to the reflection at an open-ended transmitting line or a boundary
between optical mediums with a low and high index of refraction like at a glass
surface. The reflected waves cause standing in the waveguide, increasing the SWR,
wasting energy and possibly overheating the transmitter. In addition, the small
aperture of the waveguide (less than one wavelength) causes significant

11. diffraction of the waves issuing from it, resulting in a wide radiation pattern
without much directivity.
BNC Connector:
The BNC (BayonetNeill–Councilman)connector is a type of coaxial RF (radio
frequency) electrical connectorthat is used in plane of coaxial connectors. BNC
connectorgot its name from its bayonet mount looking device. One of the benefits
of the BNC connectoris its close fitting connection. The connection is locked by
the BNC male connectorthat has a pin that fits into the main conducting wire. Its
closefitting connection uses s mount comparable to a knife (bayonet) that is
attached onto the end of a rifle.
A BNC connector connects variousradio frequencies up to 3GHZand voltages
under 500VDC and is used in electronic architectures such as audio, video
and networking. It features two bayonet lugs on the femaleconnector;
mating is fully achieved with a quarter turn of the couplingnut. BNC
connectorsare used with miniature-to-subminiaturecoaxial cable in radio,
television, and other radio frequency electronic equipment, test instruments,
and video signals.

12. Voltmeter and Analogue Voltmeter:
A voltmeter is an instrumentused for measuringelectronic
potential differencebetween two points in an electric circuit. Analog
voltmeter gives readingsby deflecting a pointer along an analog scale. Its
worksby passinga currentthrough a coil that is suspended between two
permanentmagnets. This coil of wireis known asmovingcoil since it moves
in relation to the permanentmagnetswhen a voltage is applied. This magnetic
field causes a correspondingdeflection of the pointer. This pointer deflection
will be in direct to the amountof voltage being applied to the movingcoil
wrappingthe pointer pivot. Once pointer oscillation has stopped, accurate
readingscan be measured.
Voltmeters are made in a wide range of styles. Any measurement that can be
converted to a voltage can be displayed on a meter that is suitably calibrated.

13. Polarization Grating:
A polarization grating is a thin film prismthat combines the functions of a hyper
efficient polarizer and a unique beam splitter into a thin film. A polarization
grating which is designed like a printed circuit on a board is used as a polarizer for
microwaves. Stripes of Tin-plated Copperprevent formation of an electric field
parallel to stripes due to their high conductivity. The electric field can only build
up perpendicularly to the metal stripes. [1]
How it works
PGs diffract incident light into two beams in the +_ first orders (no higher order
diffraction), with the output beams having oppositecircular polarizations. The
diffraction angle is determined by the design of the film.
Set Up
Measuring result may be distorted by reflection of the microwaves fromthe
vertical surfaceof objects close to the experimental setup. Choosethe direction
of horn antenna so that the reflecting surfaceis at a distance of least 4m. If
possible, usea microwaveabsorber to make a reflection free measuring chamber.
The experimental setup is as follow.

14.  For measuring distances, make an 800 mmlong rule by sticking together
scale paper or use a ruler.
 Attach the gun oscillator to the horn antenna with quick connectors.
 Align the horn antenna horizontally, screw the 245 mm long stand rod into
the corresponding thread and clamp it in a saddle base.
 Connect the gun oscillator to the output OUT via a BNC lead. Connect the
electric field probeto amplifier input and the voltmeter to the output DC
OUT of Gunn power supply.
 Set the E-field probe in front of the center of horn antenna.
Carrying out the experiment
Transverse Field Distribution:
 Set up electric field probein frontof the horn antenna at a distance xo=100
mm.
 Vary the position of E-field probe between y=-200, +200 mmin steps of 40
mm. For each case read the received signal and write down.
 Again repeat the measurementfor xo=200 mm.
LongitudinalFieldDistribution:
 Set up electric field probein frontof the center of the horn antenna (y=0
mm).
 Measurethe received signal between x= 100 mmand x=820 mm in steps of
40 mm.
Polarization:
 Firsthold the electric field probe in frontof center horn antenna vertically
and then horizontally, and measurethe received signal.
 Next set up the electric field probein frontof horn antenna (distance
approx. 300 mm), and place the polarization grating into the field between
the horn antenna and electric field probe.

15.  Rotate the horn antenna with Gunn oscillator into the vertical direction,
screw the stand rod into the corresponding thread and set up the horn
antenna at previous distancefromthe polarization grating and the electric
field probe.
 Again rotate the polarization grating from0o
to 180o
in steps of 10o
, each
time measurethe received signal and take it down.
Part 1
 Calculate the wavelength of microwaves:

16. No of observations: Intensity
(max)
w m2⁄
Di stance(mm) I n t e n s it y
( M in )
Distance (min) (cm)
1 8 2 1 1 . 5 3 5 1 2
2 8 4 1 2 . 4 3 4 1 3
3 9 5 1 3 . 3 3 2 1 3 . 8
4 8 9 1 4 . 2 2 5 1 4 . 9
5 6 6 1 5 . 4 2 2 1 6
6 5 5 1 6 . 5 2 0 1 7 . 3
7 5 6 1 8 1 9 1 8 . 4
8 5 4 1 9 1 7 1 9 . 3
Now take the difference between odd to odd and even to even frequencies.
Serial number Maxima I n t e n s i t y Difference
1 1… … . 3 82… … … 95 - 1 3
2 2… … . 4 84… … … 89 - 5
3 3… … . 5 95… … … 66 2 9
4 4… … . 6 89… … … 55 3 4
5 5… … . 7 6 6 … … . 5 6 1 0
6 6……...8 55… … … 54 1
Mean intensity =
−13−5+29+34+10+1
6
= 9.3 cm
Wavelength of microwaves =9.3 cm
Part 2:
Calculate the focal length of the lens.
No. of observation I n t e n s it y
( m a x )
D i s t a n c e
( m a x )
I n t e n s it y
( m i n )
D i s t a n c e
( m i n )
1 1 0 0 1 1 . 5 8 5 1 2
2 9 5 1 2 . 1 7 5 1 3 . 5
3 9 0 1 4 7 0 1 4 . 5
4 8 0 1 5 6 0 1 5 . 5

17. 5 7 5 1 8 5 0 1 8
6 6 5 1 9 . 5 5 5 2 0
7 6 0 2 2 45 23. 5
8 5 5 2 4 4 0 2 5
By adding the distance only of maximum intensity:
No ofobservations: Intens it y Distance
1 1 0 0 1 1 . 5
2 9 5 1 2 . 5
3 9 0 1 4
4 8 0 1 5
5 7 5 1 6
6 6 5 1 8 . 5
7 6 0 2 2
8 5 5 2 4
Mean focal length =
11.5+12.5+14+15+16+18.5+22+24
8
= 16.6875 cm
Part 3:
To verify the Bragg’s law.
nλ = 2dsin θ
No of observation Intensity
( m a x )
Distance
(2d)
Intensity
(min)
Distance
(min)
1 8 5 1 2 . 6 3 2 1 5 . 5
2 8 2 2 4 . 8 2 9 2 5 . 6
3 - - - - - - - - - - - - - - - - - - - - - - - - - -
To verify the Braggs law, we use the values of maximum distance.
2d1 = 12.6
2d2 = 23

18. 2d3 =-----
nλ = 2dsinθ
n =
2dsinθ
λ
For n1
n1 =
12.6∗sin45
9.3
=0.95=1
For n2
n2 =
24.8∗sin 45
9.3
= 1.89 ≈2
For n3
n3 =
∗sin45
9.3
=------
Overall conclusion
Noofobservations Distance
(2d )
Value of n Integer n
1 1 2 . 6 0 . 9 5 1
2 2 4 . 8 1 . 8 9 2
3 - - - - - - - - - - - - - - - - - - - -
Conclusions:
In this experiment we observe about the propagation of microwaves,
measure the Wavelength of microwaves, focal length of lens as well as
verification of Braggs Law .
References:
[1]: Hugh D. Young, Roger A. Freedman and A. Lewis University Physics with modern
Physics 13th edition Addison-Wesley
[2]: Lab manual
[3]:www.wikipedia.com