This repository contains the assignments for the Academic Course “Special Antennas-Antenna Synthesis" taught in the Spring of 2018-2019 in Aristotle University of Thessaloniki - Electrical & Computer Engineering. The goal of the Project is the analysis and optimization of three antennas with different parameters. For the simulations 4NEC2 software was used, after the construction of the geometry of the antennas using MATLAB.
The antennas which were analysed are:
-
Discone Antnenna
-
Helical antenna
- Television Antenna with 4 sets of crossed dipoles and a reflector
The behavior of each antenna was studied and explained through simulations (such as Input impedance of the antenna, 2D or 3D radiation patterns, Reflection Coefficient and SWR) that were considered for different wavelengths and frequency bands in order to draw conlcusions regarding broadband operations or resonances.
The respective folders contain the Matlab files that were used for the design of the geometry, as well as .txt files that were generated by the scripts and were used in 4NEC2. The geometry of the 2nd antenna was created exclusively through 4nec2 software.
The design will be for the central frequency fo=3Ghz. Wavelength: λ=0.1m.
The dimensions of the antenna are r = 0.3λ for the radius of the disc, l = 0.5λ for the length of the wires (of the cone) and 2*θ = 60 for total angle of the cone (θ= 30 from the axis). We assume wire diameter λ / 100. In the design of the NEC file, we cosider that the cone wires are connected at a common point a short distance (let d = λ / 20) down from the point where the disc wires connect (see photo, below).
A Matlab script was written that takes r, l, θ and d as variables and constructs the geometry of the antenna. Specifically, it will build GW lines, as accepted by the NEC file (.nec).
For the frequency band [0.5fo - 4fo] in the below plot, we can see the change of real and imaginary part of the input impedance Zin, as well as the reflection coefficient of the antenna.
We observe that the reflection coefficient is desirable (<-10db) only in one small frequency band close to 6.5 Ghz. To achieve good operation throug al the above zone, looking at the magnitude of the Zin of the antenna (green in the picture) we see that its 'average value' is close to 110 Ω.
Therefore, we set the characteristic impedance equal to Zo = 110, and run the simulations again for the SWR and the Reflection coefficient in the same frequency band.
We see significant changes both in SWR which is desirable (<2) for all frequencies (except 1.5 to 2.5 Ghz) in the following plot and in Reflection Coefficient which is below -10 dB.
Below we can see the Radiation Pattern @f=fo=3Ghz at the Verical and Horizontal plane as well as 3D.
3D Radiation Pattern:
We continue with the diagrams of the reflection coefficient, changing the angle 2θ = 10, and 150 degrees while keeping all other parameters unchanged, in order to see the behavior of the antenna.
The discone antenna tends to behave as a λ/2 dipole in the central frequency of 3 Ghz. As we can see in the vertical plane radiation patter, a difference is that the maximum gain has been shifted towards θ=115 degrees (instead of 90 in λ/2 dipole). This happens because of the circular disc. In higher frequencies the discone creates 2 z-axis symmetrical lobes, ranging between +-130 and +-160 degrees. Regarding the angle of the cone, we conclude that the more the cone opens, the fewer the zones in this frequency band in which satisfactory operation is achieved. In fact, for larger angles the reflection coefficient changes abruptly in some frequencies and more specifically there is a reasonance frequency at 6.4Ghz.
We would like to design a helical antenna with 20 turns, C=λ for central frequency fo=100Mhz. Ground will be defined as a disc of r=λ/2, using 12 radial and 5 circular wires. The excitation will be fed in a segment between ground and the edge of the helix. Using the geometry builder we design the ground and that the helix. Below we can see all the geometrical characteristics choices in 4NEC2 for the desired frequency and N=20 turns. The helix was designed λ/60=0.05m above ground, for the excitation segment.
For the frequency band [0.3fo - 2fo] in the below plot, we can see the change the absolute value and phase of the input impedance Zin.
We observe that the absolute value of Zin is quite steady through the whole frequency band, excepting the initial frequencies. This is very desirable because, we can achieve a very good impedance matching for a big freq. band. Based on the above plot, we select 65 Ohm characteristic impedance and plot the Reflection Coef.
Indeed, as we can see, starting from 85Mhz, the Reflection coef. is already below -10dB and the antenna can work properly in a broad band with quite unimportant Return Loss.
In the below photo, we can see the radiation pattern of the antenna in different frequencies.
f=0.7*fo= 70 Mhz
f=fo=100 Mhz
3D Radiation pattern at cental frequency fo
f=1.3*fo=130 Mhz
We can conclude that, as the frequency increases, the antenna radiates in the desired axial mode. (At low frequencies we have a normal mode behaviour). The main lobe is desirably at 0 degrees and max gain is 18dBi @fo. Furthermore, HPBW decreases, when the frequency ranges around the cenrtal fo. Also, the back lobe is insignificant. One disadvantage is that there are a lot of side lobes, which are not minor in higher frequencies. The number of Side lobes, as well as their gain increase while the frequency increases respectively.
The antenna in the photo below is a well known TV receiving antenna.
It consists of four elements (dipoles), in X shape (for more broadband behavior, as simplified form of a discone antenna), which are fed from a central power point (the black box in the image). This point (which is the antenna input), branches in two transmission lines with characteristic impedance Z1 that supply power to the intermediate elements. Then these elements they are also fed to the ends, via TL of the same characteristic impedance Z1 but with reverse wiring as shown in the image. There is a reflector behind the dipoles, at a distance of λ/4 (in the central operating frequency). The dipoles length is λ/2 (at the central operating frequency). We also consider that the distance between the elements is also λ/2 (although in the image it seems not to be uniform). Let, the angle from x axis for each dipole be 25 degrees.
We would like to design the above antenna with central frequency of fo=60MHz. Choosing Z1=100Ohm, we can see the change of the absolute value of the input impedance Zin for the frequency band [0.3fo - 2fo] in the below plot.
We choose Z1 = 90 Ohm. It is not the mean Zin in all the above freq. band, but we are more interested in frequencies around 60 Mhz as well as good operation in a wide band. Below, we can see the Reflection Coefficient after this change:
We obsreve that our antenna is quite broadband now. For frequnecies around [52-66] Mhz the Refl. Coef. is below -10 dB and we have only minor reflections. The fractional bandwidth is almost 1.27.
In the below photos, we can see the radiation pattern of the antenna in different frequencies in vertical and horizontal plane.
f=0.5*fo=30 Mhz
Vertical plane:
Horizontal plane:
f=fo=60 Mhz
Vertical plane:
Horizontal plane:
3D diargam @cenreal freq:
f=1.5*fo= 90 Mhz
Vertical plane:
Horizontal plane:
The HPBW is quite large at 30Mhz. Furthermore. the side lobes and the back lobe (which are only 2) are unimportant @30Mhz and @60Mhz. Max gain @60Mhz is 14dBi at the vertical plane and almost 11.5 @30 and@ 90 Mhz. While the frequency increases, @90Mhz, we observe that a lot of side lobes have been generated and now the main lobe cannot be distinguished. On the other hand, for all these frequencies the maximum gain is at the desired direction.