design, implementation and comparative study … design, implementation and comparative study...
TRANSCRIPT
1
Design, Implementation and Comparative Study Slotted
Waveguide Antennas
João Carlos Ferreira Monteiro
Instituto Superior Técnico
Avenida Rovisco Pais, 1 — 1049-001 Lisboa
Abstract: Nowadays, wireless networks appear as a tool capable of providing the most varied services
(Television, Internet, Phone, etc). Such networks are available to all users, whether in study places
(Universities, etc), in recreational sites (bars, shopping malls, etc) or even created by the users themselves
for their own entertainment. The growing demands for such networks, has led to the development of
projects in this area, as well as optimization of existing recourses in order to make this service more
competitive. In this paper a comparative study of two antennas made of rectangular waveguides with slots
in two orthogonal Planes, operating in 2,45GHz band, is performed. The antennas have the same number
of slots, but the position of the slots differs; one contains the slots in the Plane zx, and the other in the
Plane zy. They also differ in the offset parameters; the antenna with slots in Plane zx requires the
dimensioning of the offset, while the antenna with slots in zy Plane presents no offset. The antennas
analysis will be based on characteristic parameters such as -3dB bandwidth, the gain in different
polarization Planes and the SWR. The project has included a simulation study using the CST-MWS
software and measurements in an anechoic chamber.
1. INTRODUCTION
Currently, wireless communication allows you
to establish communication between two
devices through the propagation of
electromagnetic waves (for example radio
waves, infrared light, laser, etc.). In the
telecommunications industry, its applicability is
remarkable in the transmitters and radio
receivers, remote controls, computer networks,
among others. In this context, the antennas
made of slots assume greater importance in the
wireless communication system (Wireless). A
slot in a waveguide is a metal radiating element.
Their behavior is identical to the functioning of
an electric dipole. By analogy, an aggregate of
slots has the identical behavior of an
aggregation of dipoles. It is in this context that
emerges the primary objective of this thesis: the
analysis of two antennas composed by
waveguides with different positioning of the
slots in the guide, so that it can be done a
comparison between antennas.
The antennas consist of a rectangular waveguide
with 4 slots made in the zy plane (antenna 1)
and in the zx plane (antenna 2). In this chapter it
is possible to understand its mode of
functioning.
In order to compare two types of antennas, it
will be presented their mode of operation,
dimensioning, positioning, width and length of
the slots to insert in the guide.
Waveguide
The wave guide used as a basic element of these
antennas was produced from an anodized
aluminum profile, which results in a low cost
for these antennas and also in the ease of
manufacture, two important characteristics
especially in military applications. These guides
were designed so to operate in fundamental
mode TE10.
Figure 1 – Rectangular waveguide
The measurements of the guides used are shown
in Table 1.
Table 1 – Measurements of the used waveguide
Dimensions (mm)
Width (a) 37
Height (b) 97
Thickness 1,5
Propagation
direction
2
Fundamental Mode
The fundamental mode of propagation, in the
guides with the dimensions mentioned above, is
the TE10 mode. This mode has its cutoff
frequency 1.55 GHz, thus allowing operation at
central frequency of work, 2.45 GHz.
In table 2 we can observe how to calculate the
characteristics of the mode of propagation in the
guide.
Table 2 –Waveguide characteristic parameters
Generic
writing
Rectangular waveguide
[rad ]
[GHz]
[mm]
[rad ]
[mm]
Type and positioning of the slots
The sizing of slots, by other words, the width
and length are the same for both antennas. The
criterion for the design corresponds to ensure
maximum radiation for each slot.
A half-wave resonant dipole or a resonant slot
has a length of 0.475 (1). Elliott and Kurtz
concluded that the length of the slot is given by
0.483 (2).
The same authors, using the curves of Stegen,
determined that the slot width is given by:
However, for the antenna with slots in the plane
zy simulations were performed to optimize the
width of the slot. In the following table you can
view the measurements of the slots
Table 3 – Slots Characteristics
Antenna ZY Plane XZ Plane
L (lenght) 59 59
W (width) 7 4
ZY Plane Slots
Figure 2 –Electric field lines, magnetic field and
electric current distribution (3)
Based on this structure of current lines (Figure
2), the slots were scaled so that they could have
a maximum radiation. The current lines have a
distribution along the guide according to figure
2. This distribution can be expressed by the
following expressions:
These expressions, as well as the distribution of
current lines in Figure 2, are essential to
understand the positioning of the antenna with
slots in the plane zy. The slots are placed in the
zy plane which corresponds to the points of
convergence and divergence of the electric
power lines (zx plane). The arrangement of the
slots along the guide can be seen in Figure 3.
Figure 3 – ZY Plane slots
3
ZX Plane Slots
Figure 4 – ZX Plane slots
Figure 4 shows that the slots are displaced from
the longitudinal axis of the face of the antenna,
this deviation is called offset. The next step is to
calculate this factor. Starting from the initial
formula of Stevenson (2):
where represents the conductance of the
slot and the represents the
conductance of the guide. Elliot, with the help
of curves Stegen, made some adjustments, so
the previous equation has acquired the
following form:
Using the above equation it is possible to
determine the value of the
disregarding the value of
. Then this
value is used, replacing it in the equation, along
with the and other parameters, and so we
can determine the offset that is given by d.
2. SIMULATION AND EXPERIMENTAL
MEASUREMENTS
ZY Plane Slots Antenna
Initially it was used in the simulations, an non
adapted antenna. The results are presented in
Table 4.
Tabela 4 – Simulação da antena sem stub
3D H Plane E Plane
S11 (dB) -6,25
VSWR 2.9
Gain (dBi) 10,01 10,0 10,0
SLL (dB) - 4,25 6,28
-3dB
bandwidth
- 10,3 147,7
As you can see in the table, the values of S11 (-
6.26 dB) and VSWR (2.9) reflect the misfit of
the antenna.
Later the antenna was adapted using a stub, first
it was scaled using the Smith Chart and then
optimized by successive simulations. For this
antenna the stub is at 40mm from the top of the
guide and has a depth of 13mm.
We then performed the analysis of the results
obtained by simulation of the antenna already
adapted
Figure 5 – S11 (Simulation)
Through the analysis of figure 5 it can be seen
that the antenna has a value of S11 of -18.25
dB, which results in a VSWR of 1.279. These
values can state that the antenna is adapted.
Figure 6 – Radiation diagram (H Plane)
Figure 7 – Radiation diagram (E Plane)
4
Analyzing figure 6, we can observe that for the
H plane the antenna has a main lobe with
approximately 11dB of gain and a level of
secondary lobes (NLS) of -3.7 dB. So is it
possible to analyze the figure 7, where the gain
has a value of 10dB and an NLS of -3.7.
Experimental measures
Figure 8 – S11 (Experimental)
In figure 8 we can observe that the experimental
S11 value is very close to-15dB. This value is
very close to the minimum acceptable value
which is-15dB. Despite it is slightly above the
desired value is considered acceptable.
Figure 9 – Polarization (E Plane)
Figure 10 – Polarization (E Plane)
Through the analysis of the previous figures we
can conclude that, in both planes of polarization,
the cross-polarization always has a value far
below to the normal polarization. Therefore the
antenna rejects the cross-polarization.
Figure 11 – Gain (E Plane)
Figure 12 – Gain (H Plane)
In the figures 11 and 12, in blue is the
distribution of gain along the azimuth, in red is
the maximum value obtained and in green the
level of secondary lobes. It is worth noting the
high amplitude of secondary lobes, which can
cause interference in the radiation. The
summary of the results obtained in the two
previous figures as well as the other
experimental measurements is present in Table
5.
Table 5 –Experimental measures
H Plane E Plane
S11 (dB) -13,8
VSWR 1.52
Gain (dBi) 10,75 10,61
SLL (dB) 5,18 6,48
-3dB bandwidth 9 146
Simulation and Experimental results
comparison
Figure 13 – Comparison S11
-15
-10
-5
0
2,4 2,45 2,5
S 11 [
dB
]
Frequency [GHz]
S11
S11
-80
-60
-40
-20
0
-180 -120 -60 0 60 120 180
Gai
n [
dB
]
Azimuth [º]
Polarization and Cross polarization
E
Ex
-70
-55
-40
-25
-10
-60 -40 -20 0 20 40 60
Ga
in [
dB
]
Azimuth [º]
Polarization and Cross polarization
H
Hx
-20
-10
0
10
20
-180 -120 -60 0 60 120 180
Ga
in [
dB
]
Azimuth [º]
E Plane Gain
E
Max
SLL
-40 -30 -20 -10
0 10 20
-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90
Ga
in [
dB
]
Azimuth [º]
H Plane Gain
H Max SLL
-25
-20
-15
-10
-5
0
5
2,4 2,45 2,5
S 11 [
dB
]
Frequency [GHz]
Comparison S11
NA
CST
5
The values of S11, experimental and simulated,
don't have the same value, showing a difference
of about 3dB. This may be due to the manual
adjustment of the stub, and to the difficulty in
placing the same to a precise depth of
penetration in the guide.
Figure 14 – Comparison (H Plane Gain)
Figure 15 – Comparison (E Plane)
The figures above show that the distribution of
the gain in H plan or in the E plan is coincident
almost always throughout the graph. It can be
concluded that the antenna, for these two
parameters, presents itself as an ideal antenna.
For a better understanding of the comparison
between simulations and experimental results is
presented in Table 6.
Table 6 – Comparison (Resume)
Simulation results Experimental results
H Plane E Plane H Plane E Plane
S11(dB) -18.25 -13,8
VSWR 1,28 1,52
Gain(dBi) 10,8 10,8 10,75 10,61
SLL (dB) 5,1 7,1 5,18 6,48
-3dB Bandwidth 10,3 147,9 9 146
ZX Plane slots Antenna
Simulations
Like the previous study, for this antenna it was
also made simulations for an antenna without
the stub, in other words, a non adapted antenna.
The results can be viewed in the following table.
Table 7 –Antenna simulations without stub
3D H
Plane
E
Plane
S11 (dB) -6,59
VSWR 2,76
Directivity
(dBi)
12,6
1 12,60 12,60
Gain(dBi) 11,4
9 11,50 11,50
SLL (dB) - -13,80 -18,70
-3 dB
bandwidth - 19,90º 74,10º
As we can see, the parameter values of the S11
and of the VSWR are distant from the intended,
respectively-15dB and 1.5, therefore, the
antenna is not adapted. This fact forced the
design of a stub (dimensioned similarly to the
dimension to the antenna above). The stub
appears as a characteristic length of 31mm to
40mm and is placed at the top of the guide.
After the adjustment of the antenna, new
simulations were conducted. The results of these
simulations are presented below.
Figure 16 – S11 (Simulation)
The figure 16 shows the distribution of the S11
over the frequency, the value of this for the
working frequency (2.45 GHz) is -19.34 dB,
which implies a VSWR of 1.24. The values
obtained confirm the good adaptation of the
antenna.
Figure 17 – Radiation diagram (H Plane)
Figure 18 – Radiation diagram (E Plane)
-60
-40
-20
0
20
-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90
Ga
in [
dB
]
Azimuth [º]
Gain comparison (H plan)
CST
CA
-20
-10
0
10
20
-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180
Ga
in [
dB
]
Azimuth [º]
Gain comparison (E plan)
CST
CA
6
In Figure 17 it is possible to identify a main
lobe with a range of 12.5 dB, there are also
visible two side lobes prominent in relation to
the others, this results in an S of -13.8 dB.
The SLL has a main lobe with -3dB bandwidth
of 19.9. On the E plane (Figure 18) the antenna
has a maximum gain of 12.5 dB and an SLL of -
18.7 dB. In this plane, the main lobe with -3dB
bandwidth of 74.3.
Experimental measures
Figure 19 – S11 (Experimental)
Looking at Figure 19, it appears that the S11 has
a value of around 20dB. This value is below-
15dB, which fact attests to the good design of
the stub, and the consequent good antenna
adaptation.
Figure 20 – Polarization (E Plane)
Figure 21 – Polarization (H Plane)
According to the figures analysis, the values of
the cross-polarization component distribution
are always lower than the normal polarization.
In H plan, for some azimuths the cross-
component is higher than the normal
polarization. However it may be said that this
antenna rejects the cross-polarization.
Figure 22 – Gain (H Plane)
Figure 23 – Ganho (Planeo E)
The gains of the different planes of radiation are
shown in Figures 22 and 23. At first, we can see
that there is a main lobe, the maximum
amplitude of it is 12.41dB, flanked by two side
lobes. The green line represents the SLL, which
has a value of -14.12dB. In the second Figure
SLL is much smaller than the previous one,
which means that, in this Plan, the secondary
lobes will cause less interference than H Plan
ones.
In the following table, we can see the analyzed
results so far and the remaining parameters in
the analysis.
Table 8 – Experimental results (Resume)
H Plane E Plane
S11 (dB) -19,74
VSWR 1,23
Gain (dBi) 12,41 12,35
SLL (dB) -14,12 -30,20
-3 dB
Bandwidth
20 73
-25
-20
-15
-10 -5
0
2,4 2,45 2,5
S11
[dB
]
Frequency [GHz]
S11 (4 Fendas)
S_11
-70 -60 -50 -40 -30 -20 -10
-180 -120 -60 0 60 120 180
Ga
in [
dB
]
Azimuth [º]
Polarization and Cross polarization
Ex
E
-80 -70 -60 -50 -40 -30 -20 -10
-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90
Ga
in [
dB
]
Azimuth [º]
Polarization and Cross polarization
Hx
H
-40 -30 -20 -10
0 10 20
-90 -60 -30 0 30 60 90
Ga
in [
dB
]
Azimut [º]
H Plane Gain (4 Slots) H
Máximo
NLS
-40 -30 -20 -10
0 10 20
-180 -120 -60 0 60 120 180
Ga
in [
dB
]
Azimuth [º]
E Plane Gain (4 Slots) E
Máximo
NLS
E…..
Max
SLL
E…..
Max
SLL
7
Simulation and experimental measures
comparison
Figure 24 – Comparison S11
In Figure 24 we can see the correlation between
experimental value and the value obtained by
simulation. For this parameter the objectives
have been met.
Figure 25 – Comparison (H Plane)
Figure 26 – Comparison (E Plane)
For the H plane (Figure 25) experimental and
simulated values are nearly coincident.
In the E plane (Figure 26) there are small
variations, particularly in the area of the side
lobes, however, the remaining values of the
graph are almost coincident.
The following table presents a summary of
comparisons made between experimentally
values and values obtained by the simulations
done.
Table 9 – Comparison (Resume)
Simulation Real
H Plane E Plane H
Plane
E
Plane
S11 (dB) -19,34 -19,74
VSWR 1,24 1,23
Gain (dBi) 12,51 12,51 12,41 12,35
SLL (dB) -13,80
-18,70
-14,12 -30,20
-3 dB
Bandwidth
19,90º 74,10º 20º 73º
3. ANTENNAS COMPARISON
Presented the two antennas, which are an
integral part of this study, it is time to compare
them taking into account the parameters used to
analyze them individually.
Results obtained by simulations
The first parameter to be analyzed will be the
standing wave ratio (S11).
Figure 27 – Antennas Comparison (Simulation -
S11)
From the viewpoint of results obtained using
simulations, the antennas have very close values
of S11, this values are approximately equal to -
20dB. However, the zx plane slots antenna has a
better adaptation, due to theirlower S11.
Figure 28 – Antennas Comparison (H Plane
gain-simulation)
-25
-20
-15
-10
-5
0
2,4 2,45 2,5
S11
[d
B]
Frequency [GHz]
Comparison S11 (4 Slots)
S_11 (NA)
S_11 (CST)
-50 -40 -30 -20 -10
0 10 20
-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90
Ga
in [
dB
]
Azimuth [º]
CST e CA gain comparison Ganho CA
Ganho CST
-40
-30
-20
-10
0
10
20
-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180
Ga
in [
dB
]
Azimuth[º]
CST e CA gain comparison Ganho CA
Ganho CST
-30
-20
-10
0
2,4 2,45 2,5
S 11 [
dB
]
Frequency [GHz]
Antennas S11 (simulation)
-40
-20
0
20
-90 -60 -30 0 30 60 90
Ga
in [
dB
]
Azimut [º]
Antennas Gain (H plane)
8
Figure 29 – Antennas E Plane Gain (Simulation)
In Figure 28 we can observe two main lobes,
one in the red curve and another in the blue
curve, but the lobe that belongs to the red curve
has a greater bandwidth than the blue curve.
This characterizes the antenna with slots in the
plane zy as more directive. However the
antenna with slots in zx plane has a higher gain,
of approximately 2dB.
In the plane E, Figure 29, the roles are inverted,
the antenna with slots in the plane zx is more
directive in the result of a narrower main lobe,
the antenna with slots in zy plane is nearly
isotropic, due to a larger width of the lobe main.
In the following table can have a perception of
all parameters examined in this comparison.
Table 10 – Comparison (Simulations)
ZX Plane slots
antenna
ZY Plane slots
antenna
Planeo
H
Planeo
E
Planeo
H
Planeo
E
S11(dB) -19,34 -18,25
VSWR 1,24 1,28
Gain (dBi) 12,51 12,51 10,8 10,8
SLL (dB) -13,80 -18,70 -5,1 -7,1
-3dB
Bandwith 19,90 74,10 10,3 147,9
Experimental measures
Similar to the comparison between values
obtained using simulations, a comparison was
made based on experimental results obtained for
both antennas
Figure 30 – Antennas Comparison
(Experimental – S11)
Contrary to what happened with the
measurements obtained by simulation, the
experimental measurements of the two antennas
have different values. This difference has a
value of about 5 dB. This is due to the fact that
adapting the antenna with slots in zy plane was
not expected. The manual adjustment of the
stub, and the difficulty in setting it may be the
reason for this bad adaptation.
Figure 31 – H Plane Experimental Gain
comparison
Figure 32 – E Plane experimental Gain
As experimental and simulated values in
relation to gain, practically match for both
antennas, the analysis to be made to the figures
31 and 32 matches with the analysis already
made to the figures 28 and 29. The main
conclusions of this analysis are the higher
directivity, in the H plane, of the antenna with
slots in the plane zy. However the antenna with
slots in zx plane is more directive in the plane E,
-20
0
20
-180 -120 -60 0 60 120 180
Ga
in [
dB
]
Azimut [º]
E Plane Gain(Simulation)
-20
-15
-10
-5
0
2,4 2,45 2,5
S 11 [
dB
]
Frequency [GHz]
Antennas S11(experimental)
-30
-20
-10
0
10
20
-90 -60 -30 0 30 60 90
Ga
in [
dB
]
Azimut [º]
H Plane Experimental Gain
-40
-30
-20
-10
0
10
20
-180 -90 0 90 180
Ga
in [
dB
]
Azimut [º]
E plane experimental gain
9
where its competitor plan is practically
isotropic. Table 11 present all values obtained in
the experimental values comparisons.
Table 11 – Comparison (Experimental
measures)
ZX Plane slots
antenna
ZY Plane slots
antenna
H Plane E
Plane H Plane E Plane
S11(dB) -19,74 -13,8
VSWR 1,23 1,52
Gain (dBi) 12,41 12,35 10,75 10,65
SLL (dB) -14,12 -30,20 -5,18 -6,48
-3dB
Bandwidth 20 73 9 146
4. CONCLUSIONS
The development of this work had as the initial
objective, the design, construction and analysis
of two individual antennas. This analysis was
divided into two main parts, results obtained by
simulation comparisons and results obtained
experimentally comparisons.
Subsequently, the objective was focused on a
comparison between antennas, while keeping in
mind all the results obtained in the individual
analysis.
Then, it is possible to see the 3D radiation
diagrams the antennas under study.
Figure 33 – 3D radiation diagram (zx Plane
antenna - E Plane)
Figure 34 –3D radiation diagram (zy Plane
antenna - E Plane)
Through observation of the previous graphics,
Figure 33 and Figure 34, there are differences in
the main lobe, the Figure 6.1 show a higher
bandwidth main lobe than the Figure 6.2.
This allows us to say that the zy plane slots
antenna is more directive, in the plane H, than
the zx plane slots antenna It is still possible to
see that the level of side lobes is greater in the
zy plane slots antenna.
Figure 35 – 3D radiation diagram (zx Plane
antenna - H Plane)
Figure 36 – 3D radiation diagram (zy Plane
antenna - H Plane)
10
Analyzing the figures 35 and 36, corresponding
to the 3D radiation diagrams of the antennas
under study, according to a different
perspective, we can conclude that the antennas
have different behaviors. Comparing the main
lobes, like the study for the figures 6.1 and 6.2,
is possible to verify that the antenna with slots
in the plane zx has an inferior bandwidth when
compared with the bandwidth of the antenna
with slots in the plane zy. For the antenna with
slots in zy plane, we can say that it is almost
isotropic in this plane of radiation. This
situation can be solved by placing a metal net in
the rear of the antenna.
Generally, it was found that the antenna with
slots in the zx plane is presented, accounting for
all parameters in the analysis, as the most viable
option. The parameters which are highlighted
and that gave advantage to the antenna with
slots in the plane zx were the gain, the level of
side lobes and the coefficient of stationary
wave.
Although both antennas are easy to build, the
antenna with slots in the plane zy presents
greater ease of construction because they do not
need an offset dimension. However, there are
features common to both antennas, such as low
cost and robustness. This last feature makes
them an option to be taken into account to
perform in military communications.
REFERENCES 1. Kraus, John D. Antennas. s.l. : McGraw-Hill
Book Company, 1950.
2. Wade, Paul. Microwave Antenna Book.
2003.
3. Faro, M. de Abreu. Propagação e Radiação
de ondas Electromagnéticas. s.l. : Técnica
AIST, 1984.