CONICAL HORN ANTENNA WITH PARABOLIC REFLECTOR DESIGN

December 24

2012COURSEWORK C

RF DESIGN ECM617

NAME: NORAZLIN BINTI MOHAMAD RAZALI

STUDENT ID: 2009297332

LECTURER: DR. MOHD. KHAIRUL BIN MOHD. SALLEH

1

In this third project assignment, we are required to design a parabola reflector antenna

using Computer Simulation Technology (CST) Studio Suite. CST has number of solvers in it both

frequency and time domain. However in this project only transient solver is used which is time

domain solver. CST is based on finite domain time difference method (FDTD). The antenna is

front-fed by a circular horn waveguide antenna with rectangular waveguide feed of a given

standard S as prescribed in the table below. The aperture angle of the conical horn is 60◦. The

antenna is working at 8.2 GHz.

Table 1: Frequency bands & interior dimensions of waveguide antenna.

Waveguide Standard, S

Frequency Band Freq. Limits (GHz) Inside Dimensions ( mm)

WR-112 H band 7.05 – 10.00 28.4988 12.6238

Figure 1: Conical Horn Waveguide Antenna with parabolic reflector specifications

The above antenna design is simulated in CST Design Suite using the following

parameters in Table 2. The model of the antenna in CST is designed with Perfect Conductivity

Conductor (PEC) as the material. The horn antenna is the combination of a cone, followed by a

cylinder and then being connected to a rectangular waveguide.

Radius of parabolic (mm)

Diameter of cone, d (mm)

Length of cone, l (mm)

New frequency limit (GHz)

Distance, a (mm)

Angle of cone, 2

1000 108 d/(2*tan(pi/6)) 7.05 – 9.35 700 60 Table 2: Conical waveguide antenna with parabolic reflector dimension specifications.

𝜽

𝒅

𝟐

𝒅

𝒍

𝒍

𝟑

𝒙

𝒚

𝒂

2

The horn antenna is placed adjacent to parabolic reflector as such that the wave is to be

transmitted parallel to conical horn aperture in order to be shifted 180 in phase and being

reflected back parallel to the main axis. The final antenna designed in CST is shown in the

following figures.

Figure 2: Conical horn antenna fed by rectangular waveguide.

Figure 3: Back side of the horn antenna.

Figure 4: Conical horn antenna with parabolic reflector from side view.

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The simulation takes up about 5 hours to complete with default mesh properties. Such

long period is taken because the CST uses time domain transient solver instead of frequency

domain solver in other software for example High Frequency Structure Simulator (HFSS) that

based on finite element method (FEM). Screenshots of simulation result was obtained and

shown below.

RESULTS

Figure 5: S-parameter S1,1 magnitude vs. frequency

Figure 5 shows S-parameter 1D plot marked at frequency of 8.1968 GHz as the nearest

frequency to the operating frequency of this antenna which is 8.2 GHz. The graph shows that the

magnitude in dB of its return loss is -12.17. Generally, the preferred value is in the range of -10

to -20 dB. However, the value less than -10 dB proved that the antenna is transferring the

maximum power and thus almost no power is reflected back. Further adjustments can be made

to achieve its desired performance by varying the distance of the horn antenna to the reflector,

a, size of the antenna, and others.

Figure 6: Port signal plot

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Figure 7: Energy plot.

Figure 8: Far-field radiation pattern in polar plot.

Based on figure 8 shown above, the plot clearly indicates that the main lobe which

resembles correct signal radiation is much bigger than the side lobe level. This fact strongly

suggest this is a good result of directivity because the signal radiates straight at the centre and

less signals radiates on its side avoiding from signal loss. This is why horn must be designed so

in such a way that waves direction from antenna is perpendicular to horn aperture, as shown in

Figure 4. These causes outgoing waves resemble TEM waves. Therefore the gain increases

purity of waves modes increase and finally side lobe level decreases.

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In addition, the direction of the main lobe is at 180.0 degree which is true as the horn

antenna needs to radiate the signal straight to the parabolic reflector since it is being placed

perpendicularly to the axis. The angular width at 3 dB is 2.3 degree which is narrow enough as

the directivity of this antenna is quite high and hence the flare angle is small. Therefore, the gain

of the antenna should also be high. Having a high directivity is directly related with the fact of

having a big aperture where the fields could be generated properly.

Figure 9: 3D far-field radiation pattern.

Figure 10: 3D far-field radiation pattern from top view.

The simulation makes the radiated fields generated by the electric charges and currents

could be determined as shown in figure 9 and 10. We can see that the radiation aperture is

created inside the waveguide. From the figure also, it is important in a parabolic reflector that

the position of the feed phase centre exactly at the focus of the reflector. There are important

losses because of axial defocusing. Hence, the best feed-horns must present the same phase

centre position for E and H planes and as stable as possible in its usable band.

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Figure 11: Radiation pattern of the antenna.

From the screenshots earlier, a very narrow beam is obtained with side lobes created

outside the waveguide horn. The narrow beam formed is as expected since it is the

characteristic of a horn antenna with reflector. The side lobes can be treated as a loss if its size

is dominating the radiation pattern. In figure 9, 10, and 11, we can observe the value of

directivity of the antenna is 36.64 dBi. Since the value is greater than 30 dBi, we can say the

directivity is very good and fulfilling the requirement of the antenna.

On the other hand, we have the value of its gain which is 36.62 dB as stated in figure 12

below. It is also a desired gain since the best value of gain falls in between the range of 30 to 40

dB. Theoretically, the value of the directivity and gain of the antenna is supposed to be the same

value and in comparison, we have the values differ in a very small value. Hence, the overall

performance of the antenna is very good and closer to what being expected theoretically.

Figure 12: 3D radiation pattern.

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Figure 13: Power delivered plot.

Other information obtained is the power delivered plot as in figure 13 where at the

frequency of 8.2 GHz, the total power delivered is 0.9344 Watt. In addition, the total radiated

power of the antenna is 26.68 dBmW which is high enough as required.

Figure 14: E-field of the antenna.

Figure 15: H-field of the antenna.

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Based on figure 14 and 15, at zero degree, the E-field of magnetic charges is parallel to y-

axis while H-field of electrical charges is parallel to x-axis. Hence, at the same time, a

perpendicular waves that resemble TEM waves formed at the rectangular feed of the horn that

necessary to generates radiated fields in a stable state.

SIMULATION OF ANTENNA WITH a = 1000 mm.

The same antenna is simulated at the same frequency with the other value of a which is

1000 mm where a is the value of distance between the centre of the parabolic reflector to the

aperture of horn antenna. The previous value being used is 700 mm and now we are comparing

the results obtained and summarized in the table as below.

Table 3: Comparison of performances of antenna with different value of a.

Characteristics being measured

Horn antenna with a = 700 mm

Horn antenna with a = 1000 mm

Directivity 36.64 dBi 39.61 dBi Gain 36.62 dB 39.56 dB Return loss at S-Parameter -12.17 dB -25.7922 dB Total radiated power 26.68 dBm Watt 26.93 dBm Watt Power delivered at 8.2 GHz 0.9344 Watt 0.9974 Watt

The following figures show the result obtained after the simulation. From the

comparison, it is clearly shows that the performance of the antenna is the best at the distance, a

of 1000 mm. The farther distance of the horn being placed from the parabolic reflector ensures

the radiated signal being reflected by the reflector more efficiently since the side lobes formed

can still be reflected instead of losing the signal. The radiated power is at the maximum at

frequency of 8.2 GHz causing the antenna is much better than the previous antenna with smaller

distance of a.

Figure 16: S-Parameter of antenna with a = 1000 mm.

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Figure 17: Radiation pattern of the antenna with a = 1000 mm.

Figure 18: Power delivered of the antenna with a = 1000 mm.

Figure 19: Polar plot of the antenna with a = 1000 mm.

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CONCLUSION

The design of a conical horn antenna fed by rectangular waveguide with parabolic

reflector is very easy to be design using CST. However, the time domain transient solver used by

the software cause the simulation to take so much time to complete the simulation. HFSS

software is recommended to simulate such a complex design because it can simulate by

frequency domain solver in a sweep of time. The antenna is working at the given frequency of

8.2 GHz with necessary dimensions. The analysis of the overall results of the antenna strongly

suggests that the antenna has achieves its desired performance in terms of directivity and gain

with 36.64 dBi and 36.62 dB respectively. The radiation fields obtained was a narrow beam that

also resembled a characteristic of a horn antenna with parabolic reflector. Return loss on the S-

Parameter plot of less than -10 dB also proved that a maximum power transfer occurred and

thus ensures the best performance of the antenna. In addition, polar plot formed shows that the

antenna has small side lobes compared to its main lobe. This is a desired performance since the

outgoing waves from the horn successfully propagate in the behaviour of TEM waves toward

the reflector. The comparison between two antennas with different distance from its reflector

shows that the farther distance performed the best achievement with maximum power transfer

at the required frequency of 8.2 GHz.