EE 525 Antenna Engineering - Questions
Using the transmission line model, draw the equivalent transmission line network of a
rectangular patch type resonant microstrip antenna, symmetrically fed by a microstrip
line on one edge, and loaded by a rectangular stub at the opposite edge. Name and
define the significant parameters.( Microstrip patch and the stub have dimensions of
aXb and cXd respectively).
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Derive the expression for the transformation of polarisation of the E-field of an
incoming wave in undergoing reflection from a perfectly conducting reflector in using
the aperture field integration technique. Comment on the underlying assumptions.
( given : aXbXc = b(a.c)-c(a.b) , where a,b,c are vectors )
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Consider an X–band linear phased array antenna operating at 10 Ghz and employing
rectangular waveguide type of radiating elements. The array is capable of performing
beam scanning in azimuth over  500.
 What should be the minimum spacing between the centers of the elements? Why?
 If the interelement mutual coupling cannot be ignored, then the active element
pattern must be used to determine the overall (composite) array pattern. Give the
definition for the active element pattern.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Consider a sectoral H-plane horn antenna with dimensions a X b where, a = 80 [Hplane; (yz-plane)] dimension, b = 0.50 [E-plane; (xz-plane)] dimension. The aperture
electric field distribution of the horn is approximated by a uniform (both in amplitude
and phase) one. A centrally placed metallic iris of dimensions a’ = 10 (in the HPlane) and b’= 0.50 (in the E-plane) causes aperture blocking. Determine the
normalized pattern of the antenna in the H – Plane and comment on the effect of
blocking on the boresight gain and the 1st sidelobe level.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------A plane wave is incident on the the concave side of a reflector antenna consisting of
a half sphere of radius R located in the negative hemisphere. The center of the half
sphere coincides with the origin of the coordinate system and the aperture lies on the z
= 0 plane.
The E-field of the incident wave given by
 
E  a x E0 e+jk0z
; (with ejwt type of time dependence)
 Determine the expression for the surface current density Js induced on the reflector
surface using the “Physical Optics” (PO) approach.
 Briefly discuss the underlying assumptions of the PO approach.
 In calculating the far-out sidelobes of reflector antennas, which technique yields
more accurate results; “aperture integration” or “physical optics”? Discuss briefly.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------A rectangular patch type of microstrip antenna is excited using a symmetrically
placed coaxial probe.
 Using the cavity model draw the direction and distribution of the equivalent
magnetic surface currents at the edges when the excited mode is TM11.
(No formulation needed)
 Discuss the expected shape of the E and H plane patterns (providing rough plots)
giving emphasis to the directivity in the broadside (perpendicular to patch surface)
direction.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Consider a log-periodic antenna.
 Provide a rough sketch of this antenna and explain why such an antenna is a limited
gain device; i.e, why doesn’t the gain of the array increase by increasing the number
of elements?
 If two log-periodic antennas are to be used to realize an array with increased gain,
then how should the log periodic antennas be arranged in order to maintain the
broadband characteristics.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Explain the principle of operation of a horn antenna employing the multi-mode
concept for aperture field shaping (consideration of two modes is sufficient). Provide
the sketch showing the cross-section of this antenna and explain the functions of the
different sections of this design.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------A paraboloidal reflector antenna of radius “a” is symmetrically fed by means of a
small conical feed horn placed at the focus. The aperture distribution of the feed horn
is such that it results in a linearly polarized (in the x direction; z axis being
perpendicular to the aperture), constant amplitude ( E 0 ) and uniform phase projected
aperture electric field distribution . The far-zone E - field expression due to this
distribution is given by
e  jk0r 2 J 1 k 0 a sin  


E  a  cos   a cos  sin   jk0 E0a 2
2r k 0 a sin  
The radius of the aperture is equal to 5  0 where  0 is the free-space wavelength
( 0  2 / k 0 ).
The feed horn having an aperture radius of b =  0 “shadows” the radiation from the
reflector.
Obtain the expression for the far field radiation pattern of the antenna, including the
effect of aperture blocking and give an estimate about the degradation in the gain of
the antenna system in the boresight direction due to the blocking effect.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------A feed-reflector combination is to be used in a frequency re-use system, (two different
communication channels using the same frequency but having two orthogonal
polarizations). It is desired to use a reflector system that can overcome aperture
blocking problems. Suggest such a system (giving the name and a rough sketch). For
this particular application, would you prefer a system possessing a large or a small
f/D (focal length to diameter ratio)? Why? Explain by providing sketches of the
paraboloid cross-sections for different f/D ratios and associated polarizations of
projected aperture E-fields.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------A Gregorian reflector system is to be used as the receiving antenna in a
radioastronomical system. An important antenna parameter related to such an
application is called the ‘figure of merit’. Give the definition for ‘figure of merit’ and
provide explanation about the factors contributing to ‘figure of merit’. Also provide a
sketch of the Gregorian system showing the related ray trajectories.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------A rectangular patch type of microstrip antenna is excited using a symmetrically
placed coaxial probe.
 Using the cavity model draw the direction and distribution of the equivalent
magnetic surface currents at the edges when the excited mode is TM11.
(No formulation needed)
 Discuss the expected shape of the E and H plane patterns (providing rough plots)
giving emphasis to the directivity in the broadside (perpendicular to patch surface)
direction.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Consider a log-periodic antenna.
 Provide a rough sketch of this antenna and explain why such an antenna is a limited
gain device; i.e, why doesn’t the gain of the array increase by increasing the number
of elements?
 If two log-periodic antennas are to be used to realize an array with increased gain,
then how should the log periodic antennas be arranged in order to maintain the
broadband characteristics.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Explain the principle of operation of a horn antenna employing the multi-mode
concept for aperture field shaping (consideration of two modes is sufficient). Provide
the sketch showing the cross-section of this antenna and explain the functions of the
different sections of this design.
----------------------------------------------------------------------------------------------------------A paraboloidal reflector antenna of radius “a” is symmetrically fed by means of a
small conical feed horn placed at the focus. The aperture distribution of the feed horn
is such that it results in a linearly polarized (in the x direction; z axis being
perpendicular to the aperture), constant amplitude ( E 0 ) and uniform phase projected
aperture electric field distribution . The far-zone E - field expression due to this
distribution is given by
 jk0 r
2 J 1 k 0 a sin  


2 e
E  a  cos   a cos  sin   jk0 E0a
2r k 0 a sin  
The radius of the aperture is equal to 5  0 where  0 is the free-space wavelength
( 0  2 / k 0 ).
The feed horn having an aperture radius of b =  0 “shadows” the radiation from the
reflector.
Obtain the expression for the far field radiation pattern of the antenna, including the
effect of aperture blocking and give an estimate about the degradation in the gain of
the antenna system in the boresight direction due to the blocking effect.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Explain the principle of operation of a horn antenna employing the multi-mode
concept for aperture field shaping (consideration of two modes is sufficient). Provide
the sketch showing the cross-section of this antenna and explain the functions of the
different sections of this design.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Drawing the related diagrams, explain the principle of operation of a compact range
system using (i) a lens antenna (ii) an off-set parabolic antenna both fed by means of a
horn. What is meant by ‘the quiet zone’?
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Consider the cavity model for the rectangular patch type microstrip antennas.
 Assuming that Hz = 0 and the fields to have no z-variation, show that { Ex = 0 , Ey =
0 } are valid solutions for the TM – modes.
 Taking only he dielectric losses into account, derive the expression for the complex
resonant frequency for the TMmn mode. (Hint: It is known that the mode excitation
coefficient Cmn assumes very large values when the propagation constant is equal to
the wave number of the (m,n)th mode).
-----------------------------------------------------------------------------------------------------------------------------------------------------------------The polar form of the equation for a broadband plane spiral antenna is given by r = r0
ea(-0). Briefly discuss the manner in which the parameters r0, a and 0 determine the
electrical performance of the antenna.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Draw the simplified diagram of a corporate feed system , including the related phase
shifter and the radiating elements.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Explain the principle of operation of a horn antenna employing the multi-mode
concept for aperture field shaping (consideration of two modes is sufficient). Provide
the sketch showing the cross-section of this antenna and explain the functions of the
different sections of this design.
------------------------------------------------------------------------------------------------------------------------------------------------------------------
Consider a log-periodic antenna:
 Give the rough sketch of this antenna and explain why, such an antenna is a limited
gain device; i.e, why does the gain of the array does not increase by increasing the
number of elements?
 If an array of log-periodic antennas is to be used to realize an incresed gain, then, in
order to maintain the broadband characterisrics, in what manner should be the
arraying done? Explain briefly
------------------------------------------------------------------------------------------------------------------------------------------------------------------Drawing the related diagrams, explain the principle of operation of a compact range
system using (i) a lens antenna (ii) an off-set parabolic antenna both fed by means of a
horn. What is meant by ‘the quiet zone’
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Illustrate the difference between a switched beam and an adaptive smart antenna
system by providing rough plots of the associated radiation patterns. Using those
radiation patterns, briefly discuss which system is superior when it is required to
receive signals from a source located in a specific direction while trying to avoid
reception of interfering signals from mobile jammers.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Give the block diagram of a general form of an optical beam forming feed network
and provide a brief explanation regarding the principle of operation of this BFN
system.
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