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Unformatted text preview: YEDİTEPE UNIVERSITY
DEPARTMENT OF ELECTRICAL and ELECTRONICS ENGINEERING
ANTENNAS and MICROWAVE LABORATORY
EE 421 – ANTENNAS AND PROPAGATION EXPERIMENT 3: Array Antennas and Beam Scanning Application
Measuring fundamental antenna parameters of slotted waveguide and microstrip
array antennas, and observing effects of number of array elements on antenna
parameters, and also observing frequency-dependent beam rotation of slotted
waveguide antenna array.
737 405 Gunn Oscillator
Set of Microwave Absorbers
Rotating Antenna Platform with:
Stand Rod (345 mm.)
BNC cable with length = 2/(3.5) m.
Plug in power supply
RS 232 cable
31177 Steel tape measure
Stand Base MF
PC with Windows 95/98/NT or higher version 737424
737399 Slot Antenna
Large Horn Antenna
BNC cable with length = 1 m.
Support for Waveguide Components
Set 10 Thumb Screws
Aluminum Foil or Aluminum Adhesive Tape
10 / 1 1
1 General Information: 1. ARRAY ANTENNAS
The radiation characteristic can be considerably improved when an array is
used instead of a single radiator. In contrast to the parasitic elements of the
Yagi-Uda antenna, the radiators of an array are combined together by a
feeder system. 1.1 Slotted Waveguide Array Antenna
In practice the slot antenna is very frequently used as a linear array. Due to
its fan-formed beam characteristic and high side-lobe attenuation it is
preferred in surveillance radar equipment for ship navigation. Slot antennas
consist of several coupled single slots. When several slots are arranged in a
waveguide, a linear array antenna is born. The slots are fed by a guided wave
and radiate a portion of the power supplied into free space. The arrangement,
size and alignment of the slots determine the directional characteristic of the
complete antenna. Necessary for radiating power out of a slot is that the slot
interrupts the surface currents of the wave traveling along the waveguide. If
the waveguide is fed with the fundamental mode according to Figure. 1.1.1,
maximum radiation results for slots (2) and (3), while the longitudinal slot (1)
running symmetrically and located on the broad side of the waveguide
(theoretically) will not radiate. Thus slots in this position (2) or (3) are
suitable for the assembly of slotted lines, as they are commonly used in
waveguide measurement techniques. Figure. 1.1.1
The construction of the slot antenna for a rectangular waveguide carrying the
fundamental mode is depicted in Figure. 1.1.2.
10 / 2 Figure. 1.1.2
The slots are alternately arranged around the broad side of the waveguide,
showing a slot offset x from the symmetry axis. The slot distance d0 amounts
to approx. λG/2. The alternation from slot to slot around the symmetry axis
compensates for the polarity reversal of the wall currents. If the slot length
amounts to somewhat less than half a free-space wavelength λ0/2 at
operating frequency, then the slot is in resonance. The antenna is fed at one
end (e.g. by a waveguide). The opposite end is terminated by a short-circuit
plate located at a distance d1 of λG/4 from the last slot. Thus, the waveguide
transforms the short into an open circuit at the location of the last slot. The
parallel connection of no-load with the slot impedance leads to no change in
impedance. Because the slot spacing d0 amounts to λG/2, the waveguide
functions like a λ/2 transformer and reproduces the slot impedances
unchanged at the location of the adjacent slot. Thus, the sum total of all N
slots has the same effect at the feeding point of the antenna as the parallel
connection of N impedances of equal magnitude! The construction of a slot
antenna is carried out in the following steps:
• The operating frequency f0 and guided wavelength λG determine the length
and spacing of the radiating slots = λ0/2, d0 = λG/2.
• The number of slots N is a consequence of the desired directivity or the
mechanical construction requirements.
• The matching requirements at the feeding point give the necessary
transverse impedance ZS for each slot.
• From this the required slot offset x can be determined.
The feeding of a resonant operating slot antenna is carried out with standing
waves resulting in all slots being fed with waves of equal phase. However, if
the "output" of the slot antenna is terminated reflection-free with a
waveguide termination, then excitation of the slots is carried out with
propagating waves. A change in the frequency leads to excitation of the slots
with waves of different phase due to a change in the wavelength λG of the
guided wave. In the directional diagram of the entire antenna array this
leads to a squinting effect of the major lobe. If the frequency of the exciting
wave is varied periodically, then the direction of the major lobe varies
10 / 3 periodically too and beam scanning results. Phase-controlled antennas permit
the electronic control of the beam direction and are thus superior to the
naturally sluggish mechanical systems in detecting and tracing fast radar
targets. 1.2 Microstrip Array Antenna 1.2.1 Microstrip Antennas: Microstrip antennas received considerable attention starting in the 1970s,
although the idea of a microstrip antenna can be traced to 1953 and a patent
in 1955. Microstrip antennas, consist of a very thin (t << λ0 where λ0 is the
free–space wavelength) metallic strip (patch) placed a small fraction of a
wavelength (h << λ, usually 0.003 λ ≤ h ≤ 0,005 λ) above a ground plane. The
microstrip patch is designed so its pattern maximum is normal to the patch
(broadside radiator). This is accomplished by properly choosing the mode
(field configuration) of excitation beneath the patch. End–fire radiation can
also be accomplished by judicious mode selection. For a rectangular patch,
the length L of the element is usually λ0/3 < L < λ0/2. The strip (patch) and
the ground plane are separated by a dielectric sheet (referred to as
substrate). (Figure 126.96.36.199) Figure 188.8.131.52 Microstrip antenna with rectangular patch. 10 / 4 In high–performance aircraft, satellite and missile and applications, where
size weight, cost, performance, ease of installation, and aerodynamic profile
are constraints, low profile antennas may be required.
Presently there are many other government and commercial applications,
such as mobile radio and wireless communications that have similar
specifications. To meet these requirements, microstrip antennas can be used.
These antennas are low–profile, conformable to planar and non–planar
surfaces, simple and inexpensive to manufacture using modern printedcircuit technology mechanically robust when mounted on rigid surface,
compatible with MMIC designs, and when the particular patch shape and
mode are selected they are very versatile in terms of resonant frequency,
polarization, pattern and impedance. In addition, by adding loads between
the ground plane and the patch, such as pins and varactor diodes, adaptive
elements with variable resonant frequency, impedance, polarization and
pattern can be designed.
Major operational disadvantages of microstrip antennas are their low
efficiency, low power, high Q (sometimes in excess of 100), poor polarization
purity, poor scan performance, spurious feed radiation and very narrow
frequency bandwidth, which is typically only a fraction of a percent or at
most a few percent. In some applications, such as in government security
systems, narrow bandwidth is desirable. However, there are methods, such
as by increasing the height of the substrate, that can be used to extend the
efficiency (as large as 90 percent if surface waves are not included) and
bandwidth (up to about 35 percent). However, as the height increases,
surface waves are introduced which usually are not desirable because they
extract power from the total available for direct radiation (space waves).
The surface waves travel within the substrate and they are scattered at
bends and surface discontinuities, such as the truncation of the dielectric and
ground plane, and degrade the antenna pattern and polarization
characteristics. Surface waves can be eliminated, while maintaining large
bandwidths, by using cavities. Stacking, as well as other methods, of
microstrip elements can also be used to increase the bandwidth.
In addition, microstrip antennas also exhibit large electromagnetic signature
at certain frequencies outside the operating band, are rather large physically
at VHF and possibly UHF frequencies, and in large arrays there is a tradeoff
between bandwidth and scan volume.
There are numerous substrates that can be used for the design of microstrip antennas, and their dielectric constants are usually in the range of 2.2 ≤ ε r ≤
12. The ones most desirable for antenna performance are thick substrates
whose dielectric constant is in the lower end of the range. Because they
10 / 5 provide better efficiency, larger bandwidth, loosely bound fields for radiation
into space, but at the expense of larger element size. Thin substrates with
higher dielectric constants are desirable for microwave circuitry because they
require tightly bound fields to minimize undesired radiation and coupling,
and lead to smaller element sizes; however, because of their greater losses,
they are less efficient and have relatively smaller bandwidths. Since
microstrip antennas are often integrated with other microwave circuitry, a
compromise has to be reached between good antenna performance and circuit
Often microstrip antennas are also referred to as patch antennas. The
radiating elements and the feed lines are usually photoetched on the
dielectric substrate. The radiating patch may be square, rectangular, thin
strip (dipole), circular elliptical, triangular or any other configuration.
Square, rectangular, dipole (strip), and circular are the most common because
of ease of analysis and fabrication, and their attractive radiation
characteristics, especially low cross-polarization radiation. Microstrip dipoles
are attractive because they inherently possess a large bandwidth and occupy
less space, which makes them attractive for arrays. Linear and circular
polarizations can be achieved with either single elements or arrays of
microstrip antennas. Arrays of microstrip elements with single or multiple
feeds may also be used to introduce scanning capabilities and achieve greater
1.2.2 Microstrip Antenna Arrays: One of the best features of microstrip antennas is the ease with which they
can be formed into arrays. Arrays are very versatile and are used, among
other things, to synthesize a required pattern that cannot be achieved with a
single element. They are used to scan the beam of an antenna system,
increase directivity, and perform various other functions which would be
difficult with any one single element. The array element can be fed as a
series-feed network. Both the series-feed network and the radiating elements
can be made photolithographically, without any need for soldering to the
elements. Because of the fact that a change in the excitation of one element
affects the excitations of all other elements in a series-fed array, it is very
helpful to have an accurate CAD capability for the design of such arrays. 10 / 6 Figure 184.108.40.206 Microstrip Antenna Array Configurations with different feeding
Corporate-fed arrays are general and versatile. With this method the
designer has more control of the feed of each element (amplitude and phase)
and it is ideal for scanning phased arrays, multibeam arrays, or shaped-beam
arrays. The phase of each element can be controlled using phase shifters
while the amplitude can be adjusted using either amplifiers or attenuators.
Radiation from the feed line, using either a series or corporate feed network,
is a serious problem that limits the cross-polarization and side lobe level of
the arrays. Both cross-polarization and side lobe levels can be improved by
isolating the feed network from the radiating face of the array. This can be
accomplished using either probe feeds or aperture coupling. Arrays using
proximity coupled elements have the advantages of improved bandwidth and
reduced spurious radiation over microstrip line-fed elements, but feed
radiation is still high enough so that cross-polarization or side lobe levels
better than about 20-25 dB are unlikely to be attained. This is not a problem
with aperture coupled patch arrays, since the feed lines are shielded by the
ground plane , .
A planar antenna array is build up, when several linear arrays are combined
in a parallel configuration. Planar arrays in microstrip line technology are of
particular interest. Here it is possible to integrate microwave components
directly into the antenna structure (PCB-technology). Thus a more
economical production of compact systems can be realized.
Unfortunately the radiating patches of the array often interact with each
other in an uncontrollable manner. The patch radiator of a microstrip
antenna can be considered as a stripline resonator operating at no-load at
both ends. Frequently, rectangular or disk shaped elements are used as
10 / 7 radiators. Figure 220.127.116.11 and 18.104.22.168 show the cross section of a rectangular
resonator and its related field distribution. Figure 22.214.171.124 Microstrip line and its electric field lines, and effective dielectric
constant . Figure 126.96.36.199 Physical and effective lengths of rectangular microstrip patch .
The radiation from a single rectangular-shaped patch has almost dipole
character. The field distribution at the edges of the resonator demonstrates a
"spilling out" of electrical field lines from the resonator. Thus, the electrical
length is greater than the mechanical length. The dimensions of the patch
along its length has to be extended on each end by a distance ΔL . Here, the
following holds true: Leff = L + 2 ΔL 10 / 8 ⎛W
⎛ ε reff + 0.3 ⎞ ⎜ h + 0.264 ⎟
ΔL = 0.412 h ⎜
⎜ ε − 0.258 ⎟ ⎜ W
+ 0.8 ⎟
⎠ ε reff = ε r +1 ε r −1 ⎡
2 + h⎤
⎢1 + 12 W ⎥
⎦ −1 2 Preliminary Study:
1. Draw the radiation diagrams in E–Plane ( φ =0) of the following antenna array
expression “ Earray ” which simulates the radiation field expression of the array
antenna configuration with reflector plate as shown below in the figure.
⎛ ψx ⎞
⎛ ψz ⎞
sin ⎜ N z
⎟ sin ⎜ N x
Earray = f (θ ) ⋅
⎛ψ z ⎞
⎛ψ x ⎞
sin ⎜ ⎟
sin ⎜ ⎟
HWDA Array factor where ψ x = α x + β d x sin (θ ) cos (φ )
ψ z = α z + β d z cos (θ )
N x , N z correspond to numbers of array elements in x and z directions, respectively α x , α z correspond to phase differences between array elements in x and z directions, respectively
d x , d z correspond to distances between array elements in x and z directions, respectively
cos (θ ) ⎟ − cos ⎜
⎟ cos ⎜ cos (θ ) ⎟
⎝ 2 ⎠=
⎠ for = λ / 2
f (θ ) =
sin (θ )
sin (θ ) β= 2π λ0 , λ0 ≅ 3 ⋅108
f0 a) Operating frequency f 0 : 9.40 GHz
αx = π , αz = 0 N x = 2, N z = 7 or 5 or 3
d x = 2.10 cm., d z = 2.25 cm. , d x 2 is reflector − antenna distance .
Trials for N z = 7 ,5,3 , compare the results.
Is HPBW affected by the number of array elements? 10 / 9 b) Operating frequency f 0 : 9.40 GHz α x = π , α z = − β d z cos (θ desired )
N x = 2, N z = 7 d x = 2.10 cm., d z = 2.25 cm. , d x 2 is reflector − antenna distance . Trials for θ desired = 80o , 65o , 45o , 20o ; compare the results.
Do the lobes of different phase differences ( α z ) lead to beam rotation at the desired θ desired angles? Array elements are half wave dipole antennas ( = λ / 2 ) aligned along the z axis, and
with a reflector plate. 2. Draw the radiation diagrams in E–Plane (θ=π/2) and in H-Plane ( φ =0) (in 3-D if
possible) of the following antenna array expression “ Earray ” which simulates the
microstrip array antenna radiation field expression, to be experimented in the
laboratory, as shown below.
⎡ sin ( X ) sin ( Z ) ⎤
⎛ β Leff
j β h W E0 e− j β R
sin (θ ) ⎢
sin (θ ) sin (φ ) ⎟
⎥ cos ⎜
Eφ = X= βh sin (θ ) cos (φ ) , Z = βW 2
and also array factor can be defined as; cos (θ ) , β = 10 / 10 2π λ0 , λ0 ≅ 3 ⋅108
f0 ⎛ ψ⎞
sin ⎜ N y y ⎟ sin ⎛ N z ψ z ⎞
⎛ψ y ⎞
⎛ψ z ⎞
sin ⎜ ⎟
sin ⎜ ⎟
where ψ y = α y + β d y sin (θ ) sin (φ )
ψ z = α z + β d z cos (θ )
N y , N z correspond to numbers of array elements in y and z directions, respectively α y , α z correspond to phase differences between array elements in y and z directions, respectively
d y , d z correspond to distances between array elements in y and z directions, respectively Earray = Eφ ⋅ Karray
Eφ corresponds to radiation field expression of only one rectangular patch array element. Karray is the array factor which simulates the 4x4 planar array
configuration. (Thus in fact, this multiplication Earray , is the total field contribution
16 of all 16 array elements→ Eφ ⋅ Karray ≡ ∑ Eφi )
i =1 Note: You can take E0 as an arbitraray value such as 1, and the radial distance away
from the antenna R λ0 should be selected at far field region!
Operating frequency f 0 : 9.40 GHz
Substrate data: RT duroid, with εr= 2.2 and h= 1.57 mm.
Feed network: Corporated/Series.
Reflector plate: 148 mm. x 130 mm.
Array Configuration: Array elements are rectangular patches, 4 × 4 uniform planar array. 10 / 11 Experiment Set-Up Views:
Set-up of Horizontal Diagram Measurement of Slotted Waveguide Array Antenna
Test Antenna (Receiver) Set-up of Vertical Diagram Measurement of Slotted Waveguide Array Antenna
Test Antenna (Receiver) 10 / 12 BEAM SCANNING BY MEANS OF FREQUENCY CONTROL:
Set-up of Horizontal Diagram Measurement of Slotted Waveguide Array Antenna at Different
Test Antenna (Receiver)
Source Antenna (Transmitter) Slotted Waveguide Array Antenna with Matched Load
(Waveguide) Termination Vertically Polarized at different frequencies
(Moveable Short Position: 21.70 mm – 8.9 GHz)
(Moveable Short Position: 16.30 mm – 9.9 GHz)
(Moveable Short Position: 12.50 mm – 10.9 GHz) Set-up of Vertical Diagram Measurement of Microstrip Array Antenna
Test Antenna (Receiver)
Source Antenna (Transmitter) 10 / 13 Set-up of Horizontal Diagram Measurement of Microstrip Array Antenna
Test Antenna (Receiver)
Source Antenna (Transmitter) Procedure:
Notes: If free rotation of the rotary plate is hindered during the measurement, this
leads to faulty position entries in the measuring table. In this case, repeat the
measurement or enable the menu item go to reference point.
Please, place the stand rods into the rotating antenna platform, SLOWLY!
Sometimes, on the radiation pattern graph, one can observe that the 360o
cycle is not completed, therefore resulting an incomplete radiation pattern
graph. You can follow one of the ways:
– Click “Approach Reference Point” on the CASSY–LAB software, and then
restart the measurement again by pressing F9.
– Turn the main–beam direction point of the receiving antenna on the rotating
antenna platform into the 180° direction (opposite of the 0° direction). An then
measure again. Since it starts and finishes at the back lobe position (if there exists),
one can not easily become aware of the incomplete radiation diagram. You can only
apply this procedure for directional antennas with small back lobe!
1. Horizontal Diagrams of the slotted waveguide array antenna with number of
array elements N=7, 5 and 3: Assemble the experiment set-up as shown in
Experiment Set-Up Views, above. Set up the slotted waveguide array antenna so
that the slots are located on the side facing away from the source antenna. Thus
you manage to get the major lobe to appear on the cartesian plot at the center!
The source antenna radiates vertically polarized waves.
10 / 14 i) Record the horizontal radiation diagram of the slotted waveguide array
antenna with N=7. Save the measurement and record on the result sheet.
(Click on the “Append New Measurement” on CASSY-LAB software) ii) Cover the external slots (left and right) with aluminum foil or aluminum
adhesive tape. Record the horizontal radiation diagram of the slotted
waveguide array antenna with N=5. iii) Cover each of the outer 2 slots with aluminum foil or aluminum adhesive
tape. Record the horizontal radiation diagram of the slotted waveguide
array antenna with N=3. Compare the results. One can cover the outer slots with aluminum foils or aluminum adhesive tapes.
2. Vertical Diagrams of the slotted waveguide array antenna with number of
array elements N=3 and 7: Assemble the experiment set-up as shown in
Experiment Set-Up Views, above. The source antenna radiates horizontally
polarized waves. Replace the short plate by the short with the holder, in order to
set up a vertical assembly. Insert the slot antenna into the central bore for stand
rods in the rotating base.
i) Firstly, each of the 2 outer slots of the antenna remain covered with
aluminum foil. Record the vertical radiation diagram of the slotted
waveguide array antenna with N=3. Save the measurement and record on
the result sheet. (Click on the “Append New Measurement” on CASSYLAB software) ii) Remove the aluminum foils or aluminum adhesive tapes from the slots. Be
careful not to accidentally rotate the antenna. Record the vertical radiation
diagram of the slotted waveguide array antenna with N=7. Compare the results. 10 / 15 3. Beam Scanning by Means of Frequency Control: Assemble the experiment setup as shown in Experiment Set-Up Views, for beam scanning by frequency
change. Remove the coupling diaphragm between the Gunn oscillator and the
isolator. Replace the simple short plate with the moveable short. The oscillator
frequency can be determined using the frequency meter (if possible). The setting
of the frequency can be carried out approximately using the following table:
Moveable Short Position
12.50 Gunn Oscillator Frequency
10.9 4. Vertical and Horizontal Diagrams of the microstrip array antenna: Reassemble the Gunn oscillator back into its basic form with a fixed frequency of f
= 9.40 GHz (removing the moveable short configuration).
i) Vertical Diagram: Assemble the experiment set-up as shown in
Experiment Set-Up Views, above. Excitation is carried out with a
vertically polarized E-field of the source antenna. Save the measurement
and record on the result sheet. (Click on the “Append New Measurement”
on CASSY-LAB software) ii) Horizontal Diagram: Excitation is carried out with a horizontally polarized
E-field of the source antenna. Save the measurement and record on the
result sheet. Compare the results. 10 / 16 Reference List:
1. Pozar, D. M., “Microstrip Antennas”, Proc. of the IEEE, Vol. 80, pp. 79-91, 1992.
2. Balanis, C. A., Antenna Theory, Analysis and Design, John Wiley & Sons Ltd., New
3. Canbay, C., Anten ve Propagasyon I, Yeditepe University Press, Istanbul, 1997.
4. T 7.6 Antenna Technology Book, Third Ed., LD Didactic Company.
5. Kraus, J. D. and Marhefka, R. J., Antennas for All Applications, New York: McGrawHill, 2002. 10 / 17 ...
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