Electromagnetics, Antennas & Propagation Summer Examinations 2018
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04 21489 EE4D
04 24083 MSc
Electromagnetics, Antennas & Propagation
Summer Examinations 2018
Section A
1. Consider an electromagnetic plane-wave propagating from air into dry soil (permittivity εr = 7, conductivity σ = 10-3 S/m and permeability μr = 1) as sketched below. At z = 0 the plane-wave is E1 = 10 V/m. The frequency of the plane-wave is f = 2.6 MHz.
air
εr=1
pr=1
Find the total electromagnetic field (amplitude and phase of the total electric and magnetic fields) at z = -50 m (air). Take the origin of spatial phase at z = 0.
Find the total electromagnetic field (amplitude and phase of the total electric and magnetic fields) at z = 50 m (dry soil). Take the origin of spatial phase at z = 0.
Define penetration/skin depth. What is the value of the penetration/skin
depth in dry soil? (Notice that dry soil is not a good conductor, and
therefore the approximated solution 6 = O山(2)o is not valid).
Find the average power dissipated in the volume of dry soil 1 m × 1 m × 50 m outlined in the sketch.
You may require the following:
= 土 - j r =
2. Consider two infinite perfectly conducting parallel plates separated by a distance a as shown in the figure:
x
source free
|
(a) Find kx and kz, as well as Hy, Ex and Ez of the electromagnetic field that results [15]
from the superposition of 2 uniform plane waves travelling along the waveguide with an angle +θ and -θ with respect to the z-axis. The frequency of both plane waves is ω . The electromagnetic wave that propagates with an angle +θ with
respect to the z-axis has the following expression Hy = H+ e一jk .r ejOt , with
k = ksin9+ kcos9 .
(b) Demonstrate that the superposition of these two plane waves satisfies
Helmholtz equation 2Hy + k2Hy = 0 and it is a particular solution of it with
k =入(2几) .
Section B
3.
Briefly list the advantages and disadvantages of microstrip patch antennas.
Draw a diagram depicting the electric field distribution for a resonant microstrip patch antenna, clearly labelling the fields corresponding to radiating slots (apertures).
The design equations for a microstrip patch antenna of width, , length, , on a substrate of thickness, , and relative dielectric constant, , are:
where is the effective length extension due to the fringing fields at a radiating edge of the antenna.
Assuming that the dominant mode is excited, design a rectangular
microstrip patch antenna using RT/Duroid 5880 ( , )
to resonate at 2.45 GHz, giving the patch dimensions to the nearest .
Given that the half power beamwidths of the antenna designed in (c) in the E and H planes can be approximated by,
respectively (all remaining symbols have their usual meaning), estimate its
directivity, in dBi, using Krauss’ approximation .
Consider the perfectly conducting cylindrical dipole shown in Figure 4.1 below for the case when a << H, 入, i.e. assuming that a ) 0 . The boundary conditions pertinent to this dipole are:
1) I(z = +H) = I(z = -H) = 0 , and
2) Ez (z, p= a) = 0
Figure 4.1: A thin cylindrical perfectly conducting dipole .
Derive an ordinary differential equation for the non-zero magnetic vector potential on the surface of the dipole, starting from the solution which expresses the electric field vector in terms of the electric scalar and magnetic vector potentials, and the Lorenz gauge condition,
1 ?V
c2 ?t
State the general solution to the ordinary differential equation for the non-zero magnetic vector potential in (a) above for the case of time-harmonic fields and currents.
Explaining carefully all approximations involved, show that the z-directed component of the magnetic vector potential is related to the current flowing in the dipole by, Az (z)如 I(z)Lav , and derive an expression for the dipole’s average inductance per unit length, Lav , in integral form.
If the dipole is centre-fed using a balanced transmission line and the maximum current is I0 , show that the current distribution is given by, Iz = I0 sin kH ± z ] for the upper and lower arms of the dipole, respectively.
2023-01-05