Antenna Simulation and Design Exercise
Student: Cagin Sari
ID:10206074
Course: MEng Electrical and Electronic Engineering
Lab Supervisor: Dr Laith Danoon
Date: 18/11/2020
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Table of Contents
Introduction ............................................................................................................................ 2
Background Theory ............................................................................................................... 2
Procedures .............................................................................................................................. 4
Simulation result, discussion and analysis ............................................................................. 4
Conclusion ........................................................................................................................... 10
References ............................................................................................................................ 10
Introduction
The report aims to discover how the impedance bandwidth, S-parameters, and input
impedance of the dipole antenna changes with changing frequency and by using the
background theory it also aims to understand the meaning of these changes for the antenna
being modelled.
To achieve these aims a simulation has been done by using CST Microwave Studio which is
a specialist tool for simulating 3D Electromagnetic Radiation of high frequency components.
Using the software package mentioned a model of dipole antenna has been built and
simulation has been run across 1 GHz to 2 GHz. Field monitor is used to plot the electric,
magnetic and far-field radiation pattern and the plots are analysed. S-parameters, input
impedance parameters are also obtained from the simulation and used for analysis.
Background Theory
For a well matched resonant antenna where R in = Z0 the bandwidth of the antenna at any
level of VSWR is given by the following equation:
BW =
VSWR-1
(1)
√VSWRQ0
Impedance bandwidth is used to check the range of frequencies where the antenna has a good
impedance matching
Q0 is given by;
𝜔
Q0 = 2Z0
0
(2)
Where 𝜔0 is the angular resonant frequency and Q0 is the unloaded Q-factor of the antenna,
VSWR is the voltage standing wave ratio, it occurs when antenna is mismatched with the
generator or the transmission line the smaller the VSWR the better since it is the measure of
the mismatch. The mismatch causes the wave to reflect from the input of the antenna and the
measure of the reflection of wave is provided by the reflection coefficient 𝛤 or S11 .
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Bandwidth requirement of antennas is also given in terms of VSWR this can also be seen
from the equation 1 and the bandwidth is also measured at VSWR=2 which corresponds to
-10 dB.
The VSWR and S11 is given by the following two equations:
z −z
S11 = zin +z0
(3)
0
in
|
1+|S
VSWR = 1−|S11|
11
(4)
Other important equations involve Directivity and Gain. Directivity measures the power
density at a particular direction and compares it to the isotrope radiation at a distant r at the
far-field region [1]by this way one can calculate how concentrated antenna’s radiation pattern
is in a particular direction. Directivity is given by the following equation:
D=
Sav,max
Prad
4𝜋𝑟2
(5)
Where Sav,max is the maximum instantaneous power density at the point and Prad is the
maximum radiated power.
Gain of an antenna heavily depends on how efficient the antenna is and it is defined with
respect to the input power provided to the antenna[1].
G = 𝜂D
G=
𝜂=
Sav,max
Pin
4𝜋𝑟2
Prad
Pin
(6)
(7)
(8)
Where 𝜂 is the radiation efficiency of the antenna and Pin is the input power to the antenna[1].
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Procedures
Figure 1 shows the modelled dipole antenna structure with dimensions
The modelled antenna is shown in the figure 1 above the materials used for dipole is perfect
electric conductor (PEC)[2]. PEC is selected because the Antenna is assumed to be lossless.
For signal generation discrete port is used and frequency range is defined to be from 1 GHz
to 2GHz. The length of the antenna wasn’t changed during the simulations, the only changing
parameter in the structure of the antenna was radius. After the simulation is finished the Sparameters plot is analysed and resonant frequency is recorded by taking the minimum
frequency of the curve and Z matrix is analysed to measure the impedance bandwidth of the
antenna. Impedance bandwidth of the antenna is recorded by recording the frequency range at
-10 dB than at – 6 dB. From the 1D plots section on Navigation Tree. By using the 2D/3D
section of the navigation tree E-field, H-fields are observed and analysed and by using the
far-field section far-field radiation patterns are observed and corresponding gain and
directivity results are recorded. Further simulations are made by incrementing radius from 1
mm to 5 mm than to 10 mm to observe the changes it causes on the mentioned parameters
that are being analysed.
Simulation result, discussion and analysis
Figure 2 shown above displays the S-parameters for different radius values ranging from
1mm to 10 mm over the frequency range from 1 GHz to 2 GHz, fresonant or fmin is marked
on the figure.
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The marked points given on the figure 2 shows the resonant frequencies and changes in the
resonant frequency with the changing radius of the dipole. The simulation with 1mm radius
reached S11=-15.803 dB and 1.317 GHz which means that most of the power is delivered and
very small is reflected from the input of the antenna. Changing the radius to 5 mm caused a
very little change in the results S11 =-15.963 dB and 1.243 GHz however increasing the
radius to 10 mm changed the results tremendously S11=-9.0554 dB and fmin = 1.224 GHz,
S11 being that low means that the antenna is mismatched and important proportion of the
power is reflecting from the antenna input the reason for this might be the reactive part of the
impedance. Since the capacitive reactance does not cancel out the inductive reactance the
antenna does not reach the resonance so the maximum power transfer does not occur as can
be seen from the graph results in a very large reflection coefficient[3].
Figure 3 shown above displays the fR given at R= 50 Ω
Figure 3 above shows the real part of the input impedance against frequency and results with
different dipole radius values. Results show that dipole with radius of 1 mm reaches 50 Ω at
1.211 GHz and dipole with radius 5 mm reaches 50 Ω at 1.2009 GHz the difference between
these two frequencies is very small so changing the radius from 1 mm to 5 mm didn’t change
the real part of the input impedance until 1.6 GHz. However for radius = 10 mm it never
reached resistance of 50 Ω this is also a cause for the large reflection coefficient shown on the
Figure 2
Figure 3 shown above displays the fX given at X= 0 Ω
The figure 3 shows the change in the imaginary part of input impedance over frequency for
different radius values. The one with radius= 1 mm reaches the 0 reactance at f= 1.3326 GHz
this is the point where capacitive reactance and inductive reactance cancel each other out.
The one with radius of 5 mm does not reach the 0 Ω but it gets very close to it however 10
5
mm radius never gets close to the 0 Ω thus this also proves the prediction made in the Sparameter analysis using figure 2; it was predicted that capacitive part of the reactance does
not cancel the reactive part and as a result resonance can’t be reached so that large proportion
of the power gets reflected from the input port of the antenna.
Magnitude of S11
Radius of the dipole (mm)
Bandwidth (GHz)
Difference
-6dB
1
-10dB
5
1.19851.0671.4781
1.5726
0.2796
0.5056
10
1
5
10
1.058- 1.251.13991.4684 1.3944
1.3825
none
0.4104
0.1444
0.2426 none
Table 1 shows the bandwidths of the dipole antennas for different S11 magnitudes and
different dipole radius measured from the values presented in the figure 2
As can be seen from the table 1 above the bandwidths of dipoles with radius 5 mm and
radius of 10 mm at -6dB has broader bandwidth when compared to the dipole with radius 1
mm this might be useful when -6dB is enough for the reflection coefficient. When lower
reflection coefficient is required the dipole antenna with 10 mm radius fails to reach -10 dB
reflection coefficient for the the ones with 1 mm radius and 5 mm radius successfully operate
at -10 dB line. The 5 mm radius dipole antenna has broader bandwith than 1 mm one so it
might be useful for cases where broad bandwith is required.
Radius(mm)
1
5
10
far-field radiation results
directivity
gain(IEEE)
realised gain
dBi
dBi
dBi
2.224
2.223
0.7427
2.151
2.15
2.039
2.138
2.138
1.562
Table 2 shows the change in the gain and realised gain when the radius is changed
As can be observed from the table above for all of the radiusses gain and directivity is very
similar so according to equation 6 efficiency is very high close to 100%, this is expected
since the perfect electric conductor is used as the dipole antenna material. The gains and
directivities of the antennas for all radiusses are very similar. It can be observed from the
table 2 that for both 1 mm and 10 mm radius dipoles the realised gain is much different from
the gain the reason for this is for realised gain the losses are also taken into account [4]. So
this means that at radius 1 mm and 10 mm has lots of losses.
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a)
b)
Figure 4 above shows the e-field of dipole with radius of 1mm at fmin at a) YoZ and b) XoY
orientations
The figure 4 shows shows how the electric field is formed by the 1mm radius dipole at Yoz
and XoY frames
a)
b)
Figure 5 above shows the h-field of dipole with radius of 1 mm at fmin at a) YoZ and b) XoY
orientations
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Again the figure 5 shows how the h-field is formed from two different frames for radius of
1mm.
a)
b)
Figure 6 above shows the e-field of dipole with radius of 5 mm at fmin at a) YoZ and b) XoY
orientations
The figure 6 shows the e-field pattern variation at radius of 5mm when compared to the
figure 4 it can be seen that at 5 mm radius the field is much more intense and this might be
the result for significantly high realised gain which was indicated during the discussion of
table 2.
a)
b)
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Figure 7 above shows the h-field of dipole with radius of 5 mm at fmin at a) YoZ and b) XoY
orientations
The figure shown aove displays the images captured frm the h-field monitor from different
orientation to show how the h-field is forming for radius of 5mm.
a)
b)
Figure 8 above shows the e-field of dipole with radius of 10 mm at fmin at a) YoZ and b)
XoY orientations
The figure above showsthe e-field for the dipole with radius of 10 mm again when compared
to the figure 5 this field looks much weaker.
a)
b)
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Figure 9 above shows the h-field of dipole with radius of 10 mm at fmin at a) YoZ and b)
XoY orientations
Figure 9 above can be used to observe the changes in the h-field for the dipole with radius of
10 mm. H-field of the dipole with 10 mm radius looks the weakest compared to the other
figures.
Conclusion
This report has helped to gain first hand experience about the practical modelling and
analysis of dipole antennas by using CST microwave studio which is a very powerful tool for
modelling electromagnetic radiation. The report also helped to understand the design
requirements for a dipole antenna which is widely used in the industry, this report helped to
combine the theoretical part of the lecture to practical modelling. In the discussion part,
analysis of the figures and some research is done to reveal how the change in radius affects
the bandwidth, and reflection coefficient directivity and gain. The report and the simulations
have satisfied all of the aims and objectives and proved to be very valuable.
References
[1] L. Danoon, Antennas and Propagation 2020/2021. University of Manchester, 2020.
[2] ‘CST MICROWAVE STUDIO - Workflow and Solver Overview.pdf’. Accessed: Nov.
19, 2020. [Online]. Available: https://online.manchester.ac.uk/bbcswebdav/pid12062319-dt-content-rid-52144752_1/courses/I3132-EEEN-60121-1201-1SE022594/CST%20MICROWAVE%20STUDIO%20%20Workflow%20and%20Solver%20Overview.pdf.
[3] S. Rob, Transmission Lines. Manchester: University of Manchester, 2020.
[4] Z. Lodro, ‘RE:Why there is a big difference in between realize gain and gain in my
design?’, ResearchGate. https://www.researchgate.net/post/Why-there-is-a-bigdifference-in-between-realize-gain-and-gain-in-my-design (accessed Nov. 18, 2020).
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