ELEC0019 Interference, Diffraction and Polarization of Electromagnetic Waves
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ELEC0019 Interference, Diffraction and Polarization of Electromagnetic Waves
Note: This is a modified version of the Lab. script “Interference and Diffraction” adapted to be done without attending the laboratory. Completion of the tasks requires the student to study the topics in more detail using the references and the use of Matlab to perform the calculations and produce the plots.
Assessment is through a report that consists of the answers to the questions marked in this lab script. More details and guidelines to write this report are given at the end of this document.
Wave Propagation
Wave propagation is a central concept in physics and engineering. It is at the heart of most systems and devices in electronic engineering. Examples include among others, long, medium and short wave radio, microwave links, optical fibre systems, sonar, audio (acoustic) systems, optics and electron waves in semiconductor devices.
In this experiment we shall be concerned with electromagnetic waves, nevertheless many (but not all) of the results and conclusions will apply to other kinds of wave such as acoustic or electron waves. This experiment has two main parts that deal with important aspects of wave propagation: The first part is “Interference of waves”, that will investigate the superposition effect of two waves, and the second part, “Diffraction of waves” will concern the effect of superposition of multiple waves, which is commonly manifested by the “bending of wave paths at the edges of obstacles” . Additionally, two more sections include the polarisation of waves and “Antenna Arrays” . You will be asked to explain aspects of the theory and to show your theoretical calculations as part of the answers to the questions indicated in this script.
Part 1 Interference of waves
In this section we will be concerned with monochromatic waves, that is, waves of the same, single frequency, not made up of a superposition of multiple components of different frequency – like for example, a beam of sunlight.
Two waves at the same frequency will give rise to interference effects when they overlap.
This effect appears in numerous practical
applications. It is the basis of interference fringes in,
for example, optical interferometers and in the design
of antenna arrays. The figure illustrates the
phenomenon with water in the sea where waves and
their reflections from the coastline interfere. You
can also see Fig. 3.5 in the course lecture notes where
an interference pattern is generated by the
superposition of two waves propagating in different
directions.
In this experiment we investigate a particularly
simple case of interference of waves emitted from
two narrow aperture sources. The experiment is
essentially the microwave counterpart of Young’s
two-slit experiment in optics. Microwaves are
convenient for us because their wavelength is much
longer than those of optical waves, and the
interference pattern can therefore be measured with reasonable accuracy using a simple apparatus.
Consider the situation shown in Fig. 1.
Fig. 1 Two in-phase sources, S1 and S2, illuminating an observation plane.
Two sources, S1 and S2, with equal amplitude and phase illuminate an observation plane where we consider an arbitrary point at a distance x from the central point denoted by 0. The sources could be ‘line sources’ (along z) or ‘point sources’ generating cylindrical or spherical waves, respectively. In either case waves propagating away from a source will suffer some reduction in intensity (|E|2)1 with 1/r or 1/r2, respectively. If the distance from the sources to the point x=x, l1 and l2, are nearly equal ( (l1 − l2 )D << 1 ) we can approximate and neglect the difference in amplitudes at the observation plane. However, we cannot neglect the difference in phase. Hence, we can see that there will be constructive interference at some position x when the path lengths l1 and l2 differ by zero or an integer number of wavelengths i.e. when l1 – l2 = nλ . Similarly, there will be destructive interference at some position x when the path lengths l1 and l2 differ by an odd integer number of half wavelengths i.e. when l1 – l2 = (2n+1)λ/2. Thus, along the x-axis there will be a series of maxima and minima of intensity.
We can easily calculate a general expression for the intensity along the x-axis. The total field at x, on the observation plane without approximation is given by2:
E = e − jkl1 + e − jkl2 (1)
Here we have considered a 1/r-fall off of field (i.e. a 1/r2 fall-off of intensity), corresponding to a point source radiating in 3D space (spherical wave).
From Fig. 1 we have:
sin 91 = (d2 − x)l1
also: sin 92 = (d2 + x)l2
Then, since sin2 9+ cos2 9 = 1 we can write:
l1 = D cos91 + (d2 − x)sin91
l2 = D cos92 + (d2 + x)sin92
Now if l1 ≈ l2 ≈ D, we can approximate ET in (1) by:
ET ≈ (e −jkl1 + e −jkl2 )
and substituting (2):
ET ≈ (e− jk (d 2−x)sin91 +D cos91 + e− jk (d 2+x)sin92 +D cos92 )
Since 91 and 92 are small, we can make the following approximations:
sin 91 ≈ 91 ≈ (d2 − x)D and similarly, sin 92 ≈ 92 ≈ (d2 + x)D . Then, for the cosine terms (using the first 2 terms of the Taylor series), we have:
cos9 ≈ 1− 92 2 and cos9 ≈ 1− 9 2 2 .
Q1. The square of the magnitude of the electric field is known as the intensity. Show in your report that the approximations above lead to: ET 2 ≈ cos2 (4) From this expression, what is the distance between consecutive maxima or minima? |
Q2. Download and run the Matlab script Interference.m that calculates the intensity versus position x using eqn. (1) directly instead of using the approximate expression (4). Modify the script to include a calculation of the intensity as it varies with the displacement x of the observation point on the screen using eqn. (4) as well and plot this in the same figure as the results from eqn. (1). Comment on the approximations used in (1)-(4), compare the results and discuss and explain the differences |
Experiment 1.1: Interference of waves
For this part of the experiment, you will be given experimental data obtained using the set-up described schematically in the following diagram.
Fig. 2 Experimental arrangementfor the interference experiment. Note that the picture is not at scale. In the
practical situation D >> d. In thefigure, the receiver is shown at x = 0.
Two in-phase equi-amplitude ‘point’ sources are provided by the rectangular waveguide T- junction feeding two open-ended waveguides of equal lengths. The signals from both sources are received by the horn at the right and fed to a microwave detector and amplifier. The horn is moved along a ruler on the x-axis and the measured output is observed as the horn is moved.
The whole excursion along the x-axis is ±36 cm and the horn is kept aligned along they-axis throughout this experiment. The input is provided by a microwave generator connected through a coaxial cable to the waveguide T-junction. It generates a microwave signal of 10 GHz modulated in amplitude with a 5 KHz square wave signal. We need to measure the intensity of the total field received by the horn and this is done using the setup indicated in the figure. The dimensions in Fig. 2 are: D = 255 cm and d = 63 cm.
At the receiver end there is a horn followed by a “detector”, which is a piece of waveguide with a probe or a small antenna inside, connected through a microwave diode to the output coaxial cable. The signal received by the horn is very low and needs to be amplified before is sent to the meter. This is done by an amplifier circuit connected between the detector and the meter.
Q3. Explain the function of the detector diode. What is the frequency of the signal carried by the coaxial cable to the meter in the setup described in the figure above? What is the relation between the electric field intensity received by the horn and the magnitude of the signal (current) coming from the detector through the coaxial cable? It happens that the output of the detector is proportional to |E|2 i.e., is proportional to the field intensity; can you explain why? |
2022-08-26