Design and Realization of an X Band Monopulse Feed Antenna for Low Earth Orbit (LEO) Satellite Ground Station

. In this work, design of a fve-element compact dual-polarization X band monopulse feed antenna using a 7.3m Cassegrain-type refector for Low Earth Orbit (LEO) satellite ground station is presented. Te designed antenna has reached 32dB/K G/Tvalue, the side lobe level of the designed antenna is at the level of − 24dB, the crosspolarization value is − 35 dB, and the port isolation value is below − 29dB in the 8–8.5GHz frequency range. Te antenna efciency of the designed antenna is measured as 79.93%.


Introduction
Low Earth Orbit (LEO) or remote sensing satellites move faster relative to ground stations.Terefore, tracking fastmoving earth observation LEO satellites precisely is essential to get image data without interruption.In such cases, the efciency of the tracking system highly depends on the feed of the antenna system; hence, the feed architecture is important for an efective tracking system [1].
In the literature, sequential lobing, conical scan, and monopulse tracking are well-known target tracking techniques [2].Among these techniques, monopulse tracking has shown to have a high signal-to-noise ratio (SNR), is mechanically vibration-free [3], and provides high-precision tracking [4].For example, the trajectory of an LEO satellite has been estimated precisely using orbital Keplerian characteristics in the program tracking approach in [5].
Monopulse tracking systems use multiple antennas to gather angular data in the azimuth and elevation axes with diferent confgurations; e.g., four-elements [6,7], fveelements [8,9], and multimode feed [10] have been used depending on the application, system specifcation, and costefectiveness.Monopulse feed is the most commonly used and efective tracking technique [4].Te feed design provides an accurate tracking system with the optimum sum and diference patterns for high gain and low beam width refectors [11].Various research studies for the single-band operation have been made to fnd the best feed for monopulse tracking [12].Monopulse feeds have been shown to demonstrate a dual-polarized tracking element for a fveelement feed [13].Te total signal is extracted from a conical or corrugated horn antenna with a larger aperture in a setup with fve horn antennas.Te error signal in the azimuth and elevation axes is retrieved from a smaller waveguide with a circular aperture.Modern satellites use polarization diversity to concurrently transmit information in two channels of the same frequency to support a higher data rate within the constrained bandwidth available.Right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) signals are concurrently delivered by LEO satellites operating in circular polarization under frequency reuse systems.Terefore, tracking the LEO satellites using both polarization techniques to collect error-free satellite data is crucial [14].
Tis study focuses on designing and implementing a combined single-band with dual-polarization monopulse feed at 8-8.5 GHz with a high efciency of 79.93% and a high gain to construct a high-precision tracking system.Te proposed feed is ideal for X single-band operation, which is small, straightforward, and simple.Te proposed X band feed antenna consists of a septum polarizer, a corrugated horn antenna, and a monopulse unit.Te proposed X band feed antenna is constructed using a 7.3 m Cassegrain-type refector.Te CST Studio Suite [15] software is used to design, model, and analyze the proposed antenna.Te performance outputs of the designed antenna match the design criteria and are compared with the similar designs in the literature.
In the proposed design, the crosspolarization isolation value is above 35 dB, the side lobe level exceeds 24 dB, and the antenna directivity is 54.97 dBi resulting in superior performance.Tis directly infuences the system G/T value.Te monopulse tracking block diagram in this study difers from conventional monopulse tracking systems [7].In the presented structure, the TE11 mode from both azimuth and elevation is initially combined and then fed into a 180 °hybrid coupler and subsequently into a 90 °hybrid coupler.Tis confguration allows the system to track in both RHCP and LHCP, creating a versatile tracking system.
Te rest of the paper is designed as follows: in Section 2, the design of each component of the X band feed antenna, i.e., septum polarizer, corrugated horn antenna, and monopulse unit, and their assemblies are presented.In Section 3, the assembly of the refector antenna and the designed feed antenna is explained.In Section 4, measurement results are presented, and conclusions are drawn in Section 5.

Design of X Band Feed Antenna
Te X band feed antenna system comprises a septum polarizer, a corrugated horn antenna, and a monopulse feed unit as shown in Figure 1.Tese components must be designed for the feed antenna to successfully receive the communication and tracking signal and to maintain the system's continuity.Each component must provide the required RF characteristics individually, and when all components are assembled, the RF characteristics of the system must also be maintained.Te targeted system performance specifcations of the proposed X band feed antenna system for the LEO ground station are given in Table 1.While designing components in this section, the wavelength λ at the 8.25 GHz is used.In this study, an X band septum polarizer is selected for the polarization conversion of the signal received by the feed antenna, the X band feed antenna is selected as a corrugated horn antenna, and a monopulse unit is used to calculate the diference between the satellite and the ground station boresight from the signal received by the antenna during satellite communication.
Te X band feed system is evaluated on the 7.3 m Cassegrain refector.In this study, a Cassegrain antenna structure is used in the X band feeding system because a Cassegrain antenna has less obstruction in front of the refector when using two refectors than a single refector.In the Cassegrain structure, the X band feed antenna is placed in the virtual focus between the primary and second refectors.Te feed antenna provides the receiving part of the communication signal in the X band feed antenna.
Next, the design of each component and their assembly are explained in detail.
2.1.Septum Polarizer Design.Te primary purpose of the design is to linearly transmit half of the power of the RHCP or LHCP wave from the square waveguide to one of the two rectangular waveguides depending on the polarization characteristics.While achieving this goal, it is necessary to keep refection values at the inputs and outputs of the structure and the isolation value between the rectangular waveguides low.
Te design of a septum polarizer consists of three parts: the square waveguide, the septum step structure, i.e., a segment consisting of several steps, and two rectangular waveguides, i.e., the parts at the end of the septum where linear signals are received.Te square waveguide dimensions are calculated depending on the frequency band.For a rectangular waveguide, the sizes are calculated as follows: where f c m,n represents the cutof frequency of the (m, n) mode and (a, b) indicates the dimensions of the waveguide.Te square waveguide size for 8-8.5 GHz is calculated as a � b � 24 mm, and the size of the waveguide structure is shown in Figure 2(a).
In the septum polarizer design, a 4-step septum structure is used.Additionally, a conductive tooth is designed near the square waveguide for impedance matching.Te dimensions of the septum steps are designed depending on the wavelength.Te lengths of the septum step structure are shown in letters in Figure 2(b), and the dimensions of the steps are shown in millimeters in Figure 2(c).
Finally, the parts where linear signals are received at the end of the septum polarizers are designed.Tese parts can be designed with a waveguide or a pin structure.In this study, WR112 rectangular waveguides are used for the design.At the end of the septum, waveguides are rotated 90 °and designed as two waveguides on the right and left sides.Te appearance of the structure is given in Figure 2(d).
Aluminum is used as the material for the design and fabrication of the septum polarizer.Te common port is called port 3, and it is expected as the input port for a circularly polarized wave; then, the LHCP signal is received from port 1 (left port in Figure 2(d)), and the RHCP signal is received from port 2 (right port in Figure 2(d)).
If a common port is used as input, then two signals are generated: parallel and perpendicular to the septum plane and summed.Tus, the phase diference between these signals is 90 °.
Teoretically, half of the signal sent from the common port is expected to reach one of the linear ports, corresponding to −3 dB and a 90 °phase diference between them.

Monopulse Unit Design.
Te tracking system is realized by the monopulse unit, which consists of four circular waveguides placed two by two in azimuth and elevation of the feed antenna as shown in Figure 4. Te number, position, and dimensions of the waveguides used in the monopulse unit are calculated precisely for the design.Initially, the signal modes are analyzed to calculate the size of the waveguides [16].Te input of the feed antenna is designed as a circular waveguide since the signal received by the antennas is a circularly polarized wave.Te feed antenna must receive the communication mode of the signal.Te communication mode corresponds to TE11 among the circular waveguide modes.Te X band feed antenna is required to operate in the 8-8.5 GHz frequency band.Te cutof frequency must be lower than the minimum frequency value so that the structure can operate efciently after 8 GHz.Te cutof frequency chosen for this design is 7 GHz.Te circular waveguide's input radius is 12.56 mm.As a result of the simulations, the optimum radius value is 14.25 mm.
Designing an accurate tracking system with optimum sum and diference patterns for a high gain narrow beamwidth refector is mainly based on the feed design.Te monopulse section is added to the structure after the feed antenna design.Te monopulse section samples the signal from the feed antenna.Tus, the dimensions of the circular waveguides in the monopulse unit are designed as 14.25 mm.Data in elevation and azimuth are required to calculate the error signal by sampling the signal to the antenna.Terefore, four monopulse couplers are designed to receive data from the circularly polarized signal in each axis.Tese couplers are placed two each in azimuth and elevation of the feed antenna.Te fveelement structure consisting of a corrugated antenna and four circular waveguides is shown in Figure 4.A block diagram of the monopulse tracking system is given in Figure 5. Te system can perform monopulse tracking for both RHCP and LHCP without the knowledge of the polarization type of the satellite to be tracked.A quadrature tracking system separates the feed antenna for elevation (E) and azimuth (A) axes.
In this system, septum polarizers are installed at the rear part of the waveguide structure to generate RHCP and LHCP polarizations in the tracking error signal.Te output signals of E1 and E2 waveguides along the elevation axis are frst combined using a 2-way power combiner.Similarly, the output signals of A1 and A2 waveguides along the azimuth axis are combined using a 2-way power combiner.As a result, the signals are connected to a 180-degree hybrid coupler, creating an elevation error signal with RHCP polarization for tracking.Both elevation and azimuth error signals, being in phase, are combined using a power combiner to create the tracking error signal.Tis signal passes through a low-noise amplifer and is then sent to an X band RF scan plate.Te sum signal at the septum polarizer output has the characteristics of RHCP and LHCP circular polarizations, and a low-noise amplifer amplifes each polarization line and then switches.Switching is done based on the tracked satellite's polarization, and the sum signal is sent to the X band RF scan plate.

SUM sw
Te RF scanning plate consists of an isolator, power combiner, variable attenuator, phase shifter, and frequency down-converter.Te azimuth and elevation information generated from the diference and sum signals of the antenna is sent to the automatic tracking receiver to enable the antenna to track the satellite signal.

Assembly of Septum Polarizer, Corrugated Horn Antenna, and Monopulse.
Te septum polarizer, corrugated horn antenna, and monopulse unit are combined and simulated.Te design is shown in Figure 6.
Ten ports are defned on the structure.Two of these ports are the ports that will receive the circularly polarized communication signal.Four ports are the ports of the circular waveguides on the elevation axis for the tracking signal; the other four ports are the ports of the circular waveguides on the azimuth axis for the tracking signal.
After the design of the feed antenna, four circular waveguides, called monopulse units, are added in azimuth and elevation.Te cross-sectional view and lengths of the design are given in Figure 7.
Te feed antenna, circular waveguides, and septum polarizers are made of aluminum.Te elements are connected with bolts, and sealing elements are used between them.Te three-dimensional radiation pattern of the X band feed antenna and its assemblies at 8.25 GHz is shown in Figure 8. Te gain of the antenna system is 19.4 dBi.

Assembly of Reflector Antenna with the X Band Feed Antenna
Te ground stations require high-gain antennas to achieve high-gain levels for tracking LEO satellites.Tus, the developed X band feed antenna is integrated with a Cassegraintype refector antenna to attain high gain levels.Cassegrain refector antennas are referred to as dual refector antennas.Te Cassegrain-type refector structure is shown in Figure 9. Te point indicated by f 2 in Figure 9 shows the focal points of the main parabolic refector and the hyperbolic subrefector.Te focal points of both structures are placed so that they overlap.Te point indicated by f 2 is the virtual focal point of the hyperbolic subrefector.Te waves coming from the virtual focal point (f 2 ) onto the hyperbolic subrefector travel in the direction of the real focal point of the hyperbolic refector (shown as dashed lines in Figure 9) reach the main parabolic refector.Te focal point f 2 is also the focal point of the main parabolic refector, and for the main parabolic refector, all waves coming from the focal point travel in a straight line.Terefore, the waves from f 2 are refected once from both International Journal of Antennas and Propagation refectors and propagate as plane waves.Te developed X band feed antenna is placed at the focal point f 2 in the Cassegrain structure [17].
Te parameters of sigma and focal point are considered when integrating the X band feed antenna onto the Cassegrain refector.Tese parameters play a crucial role in the design and 8 International Journal of Antennas and Propagation production of refector antennas, as they afect performance characteristics such as focusing capability and gain.A higher focal point and lower sigma value result in better performance and sensitivity.After the feed antenna system is placed on the focus of the 7.3 m Cassegrain refector antenna, the diference model is calculated for the RHCP ports; the LHCP ports are the same as the RHCP ports.Figure 10 shows the feed antenna placed on the Cassegrain refector antenna.International Journal of Antennas and Propagation Te system's gain is 55.3 dBi when the feed antenna system is placed on the focus of a 7.3 m Cassegrain refector antenna.To maximize the secondary gain, the secondary sum and diference models are optimized for the given refector and subrefector profle.Te simulated secondary sum and diference pattern are shown in Figure 11.
When the antenna diameter is determined based on the required data rate for communication, the link budget is calculated as the G/T value.Te progression of this design is illustrated in the fowchart shown in Figure 12.
Te sum pattern fulflls the gain requirement, while the diference pattern provides the required depth and slope.Te diference pattern has no distortion (Figure 11).Although the diference pattern provides a broader tracking pointing accuracy, reliable tracking is achieved by operating the system in the main lobe of the sum beam.Tis is achieved by providing appropriate threshold-level adjustments for tracking errors at the tracking receiver.

Experimental Setup
Te real-time implementation of the X band feed antenna is shown in Figure 6.Te fabricated version of the feed antenna is given in Figure 13.
RF measurements of the X band feed antenna were performed in an anechoic chamber.Te measurements were tested at 8 GHz, 8.25 GHz, and 8.5 GHz frequencies.Te X band feed antenna was placed on a motion pedestal in the anechoic chamber test.On the opposite side of the pedestal, a reference antenna with a known gain, frequency range, cross-polarization value, and input refection value was placed in a line of sight (Figure 14).It was noted that the International Journal of Antennas and Propagation reference antenna had the same polarization as the proposed antenna.
Te X band feed antenna was moved in azimuth and elevated on the movement pedestal.Patterns were realized with the spectrum analyzer (Figure 15).Te radiation patterns of the X band feed antenna are given in Figures 16(a Te measurement results of the diference pattern shown in Figure 16(c) for the X band feed antenna also matched the simulation results.Te null level at 0 degrees and the consistent progression of the pattern along the theta angle was critical.Te measurement results supported these observations.
Te desired VSWR value for the X band feed antenna was below 1.2.Te VSWR value was below 1.06.A VSWR value close to 1 improved the matching performance,  transmission efciency, communication quality, and overall system performance (Figure 16(d)).
Te crosspolarization level for the X band feed antenna was above 30 dB, as observed in both the simulation and measurement results.A high crosspolarization antenna could efectively handle both horizontal and vertical polarizations.Te measurement results confrmed that this requirement was met (Figure 17).
Te port isolation value was above 28 dB in Figure 16(e).Te measurement and simulation values were similar.
High port isolation prevented signals from one port from interfering with signals from other ports.Tis isolation provides advantages such as interference reduction, signal clarity, accurate measurements, and system reliability.Tese features contributed to improved antenna system performance and stable communication.

Results
Te X band feed antenna was placed on the refector antenna with a diameter of 7.3 m, as shown in Figure 18.
Te gain of the sum signal was 54.97 dBi, the communication signal (Figure 19).Te antenna efciency was approximately 79.93%.Additionally, the null level of the tracking signal had a smooth pattern at 0 (Figure 19).It is desirable to have these patterns to be smooth for good satellite tracking.
G/T was measured from the terrestrial sources from Sun Methods [18], 32 dB/K.A low-noise amplifer (43 K noise temperature) was integrated with the feed to minimize noise.Satellite autotracking was established by realizing a single-band multielement monopulse tracking.

Conclusion
An X band feed antenna is designed for the LEO satellite ground station.X band feed antenna system ofers a high G/T ratio (32 dB/K for the X band) and a low crosspolarization response and low side lobe level.
A ground station design was implemented by integrating an X band feed antenna with a 7.3 m Cassegrain-type refector.According to the simulation results, the gain values at 8 GHz, 8.25 GHz, and 8.5 GHz frequencies were measured at above 55.0 dBi, the VSWR value was 1.1, the side lobe level was below −24 dB, the crosspolarization value was below −35 dB, and the port isolation value was below −29 dB.Te measurement results showed a gain value of 54.97 dB at 8.2 GHz frequency, VSWR value of 1.15, side lobe level above −24 dB, crosspolarization value above −30 dB, and port isolation value above −28 dB.G/T is a satellite system's fgure of merit (FOM).G is the received antenna gain.T is the system noise temperature [3].Te fgure of merit, the G/T value for the ground station system, was measured above 32 dB/K.A high G/T value is an essential parameter for ground stations because it provides signifcant advantages such as extending the transmission distance, achieving sensitivity even at low power levels, reducing noise, and improving data quality to have reliable and highperformance communication systems.
Considering our target requirements, the results indicate that all performance outputs match our target ones.Te efectiveness of the ground station antenna was found to be 79.93%.Compared to the 7.3 m refector antenna diameter, such a high efciency was attained.Te suggested feed antenna is highly efcient, small, cost-efective, adheres to the overall design style, and is ideal for use as a ground station refector feeder antenna that fulfls strict industry standards.
In comparison to [7] in this study, our antenna demonstrates better performance in terms of efciency, especially when considering the antenna diameter.Additionally, in the presented design, the crosspolarization isolation value is above 35 dB in Figure 17, and the side lobe level exceeds 24 dB in Figure 19, resulting in superior overall performance.Tis directly infuences the system G/T value.Te monopulse tracking block diagram provided in Figure 9 difers from [7], showcasing a distinct tracking architecture.In the presented structure, the TE11 mode from both azimuth and elevation is initially combined and then fed into a 180 °hybrid coupler and subsequently into a 90 °hybrid coupler.Tis confguration allows the system to track in both RHCP and LHCP, creating a versatile tracking system.
Considering our target requirements (Table 1), the gain of the proposed study is better than other comparable works' gain, except for [19], which has lower efciency and crosspolarization than the proposed study.Compared to International Journal of Antennas and Propagation similar designs [20,21], the proposed design improves the gain, efciency, and crosspolarization.Te comparison of feed antennas with their features which are single/dual, gain, efciency, polarization, and crosspolarization is presented in Table 2. Various research studies for the single-band operation have previously been made to fnd the best feed for monopulse tracking [13] Out of available monopulse feed confgurations, four elements [19] and multimode feed [20,21] have been used depending on the application, system specifcation, and costefectiveness.

2
International Journal of Antennas and Propagation 2.2.Corrugated Horn Antenna Design.Te feed antenna is the interstage system that receives the electromagnetic wave from the air and transfers it to the transmission line.In this study, the X band feed antenna is chosen as a corrugated horn antenna as shown in Figure3.Tree critical are taken into consideration in the design of the feed antenna: antenna beamwidth, illumination angle phase variation, and feed antenna-air matching.Corrugations are added after the input radius of the structure is

Figure 2 :Figure 1 :
Figure 2: Te septum polarizer: (a) the front view, (b) the septum step structure and length parameters, (c) the septum step dimensions, and (d) the position of the waveguide in the septum polarizer.

Table 1 :
Target specifcations of the X band feed antenna with 7.3 m Cassegrain-type refector.Parameters Specifcations Gain 54.80 dBi @8250 MHz Voltage standing wave ratio (VSWR) 1 : 1.2 Crosspolarization −30 dB Null depth 30 dB Port isolation −28 dB Side lobe level (SLL) −18 dB G/T @8250 MHz 32 dB/K International Journal of Antennas and Propagation calculated.Te depth of the frst corrugation placed at the entrance of the antenna is λ/2, and the depth of the subsequent corrugations is gradually reduced from λ/2 to λ/4 with a total of 10 corrugations.Tis section consisting of 10 corrugations with decreasing dimensions is called the mode converter.Te depth of the rest of the corrugations in the body of the antenna is equal to λ/4, and the number of corrugations used in the body is given parametrically with the width and tooth thickness of the corrugations in the CST Studio Suite [15] software, which optimizes the structure by changing the parameter values.Te resulting schematic and dimensions of the designed corrugated system after the optimization are shown in Figure 3(a).Te designed corrugated structure shown in Figure 3(a) is rotated 360 °in the waveguide axis, and the mechanical structure is obtained.A cross-sectional view of the mechanical structure of the feed antenna is given in Figure 3(b).

Figure 4 :
Figure 4: Tree-dimensional view and front view of the feed and tracking antenna.

Figure 7 :
Figure 7: Dimensions of feed antenna and its assemblies.

Figure 8 :
Figure 8: Te radiation pattern of the X band feed antenna system at 8.25 GHz.

Figure 9 :Figure 10 :
Figure 9: Te structure of the Cassegrain antenna structure.

Figure 11 :
Figure 11: Secondary sum and diference pattern plot for 7.3 m Cassegrain antenna.

Figure 13 :
Figure 13: Te measurement setup of the feed antenna.

Figure 17 :
Figure 17: Measurement results of the crosspolarization of the feed antenna, (a) 8 GHz, (b) at 8.25 GHz, and (c) at 8.5 GHz.
) and16(b).Te simulation and measurement results were observed to match, and the gain patterns of the X band feed antenna were measured as 19.4 dBi.

Figure 19 :
Figure 19: Measurement and simulation results of the pattern of the refector antenna at 8 GHz, 8.25 GHz, and 8.5 GHz.