International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
[(]
Transmit and Receive Antenna Design for C-band Satellite Communication
Yi Yi Aye, Zaw Min Naing, Chaw Myat Nwe, Kyaw Soe Lwin, Zaw Myo Lwin
Abstract— This paper describes the dual reflector antenna design for C-band satellite communication. The selected frequencies of C-band are (5.9-6.4) GHz for up link and (3.7-4.2) GHz for down link. In this paper, dual reflector (Gregorian) antenna is designed for obtaining high performance and considering the effects of losses. This paper describes Gregorian antenna for both up link and down link model of C-band satellite communication. The parameters of Gregorian antenna are evaluated to achieve the optimum range of efficiency and gain. Simulation of the proposed antenna will be carried out using ICARA (Induced Current Analysis of Reflector Antennas) software.
Index Terms—C-band satellite, Gregorian antenna, ICARA, antenna efficiency.
I. INTRODUCTION
Gregorian antenna is a double reflector system with elliptical sub-reflector which has many interesting features such as high gain, high efficiency, more transmission rate and more compact structure. Gregorian antennas also have feed pattern reshaping, convenient feed location with shorter feed line, better illumination and little spill over losses. Especially, Gregorian antenna has large depth of focus [1]. Gregorian antennas are used in remote sensing, broadcasting and telecommunication satellites. Gregorian antenna is designed with minimum blockage condition in this paper. Reflector type antenna, phased array antenna and global horn antenna are used in space communication depending on types of satellite service. A phased array antenna has high aperture efficiency, no spill over and no aperture blockage but it has weight complexity and high losses in power distribution system. Horn antennas are used for full earth coverage in geostationary satellites [2].
Reflector type antennas are most widely used in communication satellite. A reflector antenna is the most desirable type for spacecraft antenna because of its lightweight, structural simplicity and design maturity.
Reflector type antennas consist of single reflector, offset reflector and dual reflector (Cassegrain & Gregorian). This paper designs dual reflector (Gregorian) antenna of C-band communication satellite for both downlink and uplink. Dual-reflector antennas have almost no spill over toward ground [3].
II. Design Consideration of antenna for C-band satellite transponder
The block diagram of transmit and receive antenna with C-band satellite transponder model is shown in Fig.1. The selected frequency range of C-band in this design is 3.9-4.6GHz (for down link) and 5.7-6.4GHz (for up link). The uplink frequencies are transmitted from the ground station to the satellite and need a lot of power to be generated to reach the satellite. The maximum frequency is selected to reduce the antenna size. The size of the antenna can be 4m (in maximum) on satellite and the size of antenna in consideration is larger than practical one [4]. Although the size of the dual reflector antenna is larger, it has more advantages than single reflector antenna.
Fig.1 Transmit and receive antenna for C-band satellite Transponder model
In dual reflector, the rays of reflection in Cassegrain antenna are more complex. Gregorian sub-reflector is beyond the prime focus, it is naturally farther from the feed and it can reduce the aperture blockage [5]. Gregorian antenna system consists of main reflector, sub-reflector and feed. The type of Gregorian antenna is used on SINOSAT satellite [6]. The feed is circular horn feed. The axis symmetric antenna can have blockage loss by the sub reflector. So, blockage efficiency is considered in this design. The losses depend on the effective focal length and the diameter of the sub reflector.
A. Design procedure of Gregorian antenna model
Firstly, C-band frequency range is selected. The diameters of main and sub-reflector are calculated. In this calculation, the diameter of main dish should be ten times larger than the diameter of sub-reflector. Then the focal length to diameter ratio is selected to reduce the losses. The subtended angles for main dish and sub-reflector are calculated.
Fig. 2 Block diagram of design procedure
Finally the gain and efficiency are calculated. This step is the main part of the design and losses must be considered to get the powerful efficiency and gain. The efficiency of proposed antenna design is typically up to 70% and the gain is 35-50dB. If the result is within optimum range, design will be completed or will be returned to the calculation of diameters and selection of f/D ratio. The design procedure is described in Fig. 2.
B. Calculation for the Geometrical Parameters
The geometry of Gregorian antenna is given in Fig.3, where and are the diameters of the main reflector and sub-reflector, respectively. The focal distance of the main reflector is F, and is the distances between the apex of the main reflector and the phase centre of the feed and is the distance between the sub-reflector and the phase centre of the feed (the secondary focus). The angle between the z axis and the edge ray on the sub-reflector is θ.
The parameter is the distance between the foci. The angle, ψ is the half of the angle of the main reflector of its () ratio.
Fig. 3 The Geometry of Gregorian antenna
The selected frequency range C-band is (3.9-4.6) GHz for downlink and (5.7-6.4)GHz for uplink. Frequency of design being taken is 6.4GHz (for receive antenna) and 4.6GHz (for transmit antenna) to reduce the antenna diameter. The sub-reflector diameter must be less than 20% of the dish diameter to reduce blockage by the sub-reflector. The following table shows the geometrical parameters of selected Gregorian antenna.
Table I
Geometrical parameters for selected Gregorian Antenna
Parameter / Up link value (receive) / Down link value(transmit)
Frequency / 6.4GHz / 4.6GHz
Wavelength / 0.046m / 0.0652m
Main dish diameter(50λ) / 2.3m / 3.2m
Sub reflector diameter(5λ) / 0.23m / 0.32m
The half subtended angle of the sub-reflector, θ, relates to
(1)
= focal-length to diameter ratio of sub-reflector (effective feed f/D)
=1.2
θ=23.54˚
The half angle of the main reflector ψ, will be chosen to minimize the spill over efficiency and it relates to f/D by
(2)
Where,
=the ratio of focal length to diameter of main reflector.
= 0.6
=45.239˚.
By using equation (1) and (2) the subtended angles can be calculated. The subtended angle of sub-reflector as a function of its focal-length-versus-diameter ratio is shown in Fig.4.This result illustrates that ratio increase and the subtended angle decrease. The selection of focal length to diameter ratio is the main part of the design calculation [8]. By adjusting, the efficient parameters are gained.
Fig.4 The subtended angle as a function of its focal-length-versus-diameter (f/D) ratio.
The distance between the two foci,
(3)
=0.224m.
The distance between the sub reflector and feed, is
(4)
=0.36m
(5)
=1.7m
The geometrical values have been evaluated by the equations (3), (4) and (5). The calculated parameters of selected Gregorian antenna are shown in Table 2. These values have been calculated for up link and down link C-band frequencies. The frequency is used in maximum value during the selected range. The diameter of main reflector is ten times bigger than sub-reflector diameter to reduce the losses by sub- reflector. The magnification, M is the ratio of effective feed f/D and main dish f/D. The value of magnification, M is 2. The amount of curvature is eccentricity,
(6) =0.33
Table 2
Calculated parameters for selected Gregorian Antenna
Parameter / Up link value 6.4GHz (receive) / Down link value 4.6gHz(transmit)
Main dish diameter / 2.3m / 3.2m
Sub reflector diameter / 0.23m / 0.32m
Main dish focal length / 1.32m / 1.92m
Sub dish focal length / 0.26m / 0.38m
Magnification / 2 / 2
f/D ratio for main dish / 0.6 / 0.6
f/D ratio for sub reflector / 1.2 / 1.2
III. Efficiency and Gain Calculation
The maximum gain of the antenna at a given frequency is for η=1. This efficiency includes aperture-illumination efficiency, spill over efficiency, edge taper and blockage efficiency.
In dual reflector, the blockage effect is the important factor. To reduce the blockage, the ratio of sub-reflector diameter to main reflector diameter should be about 0.1. The blockage efficiency can be calculated by using equation (7). The blockage efficiency as a function of the blockage ratio is shown in Fig.5. The more () ratio, the more the blockage losses. The blockage efficiency is
(7)
Where, =blocked area by sub-reflector
=total aperture area
=0.0302
Fig. 5 The blockage efficiency as a function of the blockage ratio ()
The spill over is the loss due to the power scattered by the sub-reflector outside the main reflector [9]. The relation of spill over efficiency and f/D ratio is shown in Fig.6. This graph shows that the more the f/D ratio is increase, the less the spill over efficiency. The spill over efficiency,
(8)
(9)
(10)
Fig. 6 Spill over efficiency as a function ratio (f/D)
Less energy is arriving at the edge of reflector than the centre, it is called edge taper [10]. Taper efficiency is increased when the f/D ratio is increased. This is shown in Fig.7. Taper efficiency,
(11)
Fig. 7 Taper efficiency as a function ratio (f/D)
Illumination loss is the missing energy at the edge and it is the product of spill over and edge taper [11].
Illumination efficiency,
(12)
The total aperture efficiency of an antenna consists of taper efficiency, spill over efficiency, blockage efficiency, illumination efficiency and phase efficiency [12]. In this design, edge taper, spill over and blockage efficiencies are considered. The total aperture efficiency is
(13)
=0.81
The gain of a Gregorian antenna is given by
(14)
The relation of gain and efficiency is shown in Fig.8. In this figure, the greater the efficiency is, the more the gain. The gain of this design is 42.81dB (for transmitting) and 43.12dB (for receiving) at efficiency 81%. This gain and efficiency is efficient performance.
Fig. 8 The relation between gain and efficiency
IV. simulation Results
The simulation results has been carried out using ICARA (Induced Current Analysis of Reflector Antenna) software by the antenna group using physical optics (PO) and physical theory of diffraction (PTD).
Fig.9 shows the geometric values of the transmitting antenna at frequency 4.72GHz. In this geometry, main dish diameter is 3.2m, sub-reflector diameter is 0.32m, the focal length of main reflector is 1.92m and the sub-reflector focal length is 0.38m. The magnification factor, M is 2 and the eccentricity is 0.333.
Fig. 9 Gregorian Antenna Geometry for transmitting antenna
Fig.10 shows the geometric values of the receiving antenna at 6.94GHz frequency. In this geometry, the main dish diameter is 2.2m, sub-reflector diameter is 0.22m, the focal length of main reflector is 1.32m and the sub-reflector focal length is 0.26m. The magnification factor, M is 2 and the eccentricity is 0.333.
Fig. 10 Gregorian Antenna Geometry for receiving antenna
Fig. 11 Far field Phi constant at 4.72GH (transmit antenna)
Fig. 12 Far field Phi constant at 6.94GHz (receive antenna)
In the first simulation, the desired parameters are placed and the feed is chosen as cos-q type. Fig. 11 and Fig.12 show the magnitude of Far-field Phi contact cuts for uplink and downlink frequencies. This results show the radiation of main lobe and side lobes at Phi constant and Theta set between -30˚ and 30˚. This result figures show the magnitude of co-polarization and cross-polarization. The co-polarization is over 43dBi and cross-polarization is -60dBi.
Fig. 13 and Fig.14 show Far-field u-v plot for uplink and downlink frequencies. The simulation results show the three dimension Far-field u-v plot magnitude by defining u =sin (θ) cos (Φ) and v = sin (θ) sin(Φ). The magnitude of u-v set between -0.5 and 0.5. The main lobe increase over 43dBi and side lobes drop near -20dBi.
Fig. 13 Far Field u-v plot at 4.72GHz (transmit antenna)
Fig. 14 Far Field u-v plot at 6.94GHz (receive antenna)
III. Conclusion
The proposed design of Gregorian antenna has high gain, high efficiency and fewer blockages. The typical efficiency of Gregorian antenna is up to 70%. The total aperture efficiency of this proposed Gregorian antenna design is 81%. The transmitting antenna gain of the proposed design is 42.812dBi and receiving antenna gain is 43.12dBi. The gain and efficiency of this proposed design is effective. The limitation ratio f/D is generally 0.4-0.8 or greater in order to avoid distribution losses and large blockings. If the main reflector diameter is 10λ-20λ, the sub-reflector diameter is to be 1λ or 2λ. This paper describes the results of an investigation on dual reflector (Gregorian) antenna configuration for receive and transmit antenna at C-band frequencies. The procedure based on the parabolic concept. In addition, shaping of reflector antenna must also be considered and further investigation is highly recommended.
Acknowledgment
I would like to express the deepest graduate to Dr. Zaw Min Naing, Technological University (Maupin), for his patient guidance, supervision, suggestions and encouragement during a long period of this study. I would like especially thank to Dr. Chaw Myat Nwe for her encouragement and suggestions. The author wishes to express her special thanks to Dr. Kyaw Soe Lwin for his kindness, helpful suggestions for this paper. I would like to thank all teachers in MTU. Especially, I would like to express my special thanks to my parents for their noble support and encouragement.
REFERENCES
[1] Paul Wade W1GHZ ©2004 , “Multiple Reflector Dish Antennas”.
[2] Samuel Silver.,” Microwave Antenna Theory and Design”.
[3] LECTURE 19: Reflector Antennas.
[4] Girish K.P., “Satellite Communication Systems Module 2” pp