A Low-frequency Compact Lightweight Dual-polarization CATR Feed Antenna

To address the issue of a large axial distance for low-frequency feed antennas, this paper presents a low-frequency compact lightweight dual-polarization CATR feed antenna. This feed antenna consists of a near-rhombus-shaped crossed dipole antenna with chamfers, a third-order horn enclosure, and a novel feed network based on 180° hybrid couplers. Through design, the axial length of this feed source is approximately 290mm. The VSWR is less than 1.5, the HPBW is greater than 44.5° from 300MHz to 500MHz. The phase centre varies less than 0.3λ and the cross-polarization isolation is below in the whole working frequency band.


Introduction
With the rapid development of satellite communication and radar communication, antennas are indispensable components in various communication systems.Compact Antenna Test Ranges (CATR), as a method for testing and evaluating antennas, are relatively mature within the frequency range of hundreds of MHz to hundreds of GHz [1].However, the effectiveness of CATR testing relies heavily on high-performance compact range feed antennas.At present, it is common to find phase-stable and well-behaved beamforming compact range feed sources operating above 1 GHz.Designing compact and lightweight compact range feed antennas in the low-frequency and ultra-low-frequency range remains a challenging task due to limitations imposed by site size and antenna dimensions.
In CATR feed antennas, dual-polarization horn antennas [2] are the most common type.In [3], a method combining open boundaries and closed angles was proposed to create a low-sidelobe dualpolarization antenna for the 120MHz-300MHz range, but the horn antenna had a length of 1.71m.In [4], a horn antenna design extended the open boundary quadridge horn (OBQH) to lower frequencies, but it still had an axial length of 1.52m.The large axial distance of low-frequency feed antennas is not conducive to practical assembly and usage, posing significant challenges for indoor testing.Therefore, reducing the electrical size of the feed source using a special antenna structure while maintaining antenna performance is a key focus of this research [5][6][7].In [8], an array of dual-polarization Vivaldi antenna elements was used with multi-feed excitation to compress the axial length to 0.3m.However, its VSWR was not ideal.[9] proposed that microstrip patch antennas could achieve similar performance to traditional horn feed sources.Cross-dipole antennas offer wideband performance and have a uniform radiation pattern.[10,11] introduced triangular and circular ring dipole structures that help reduce the axial length of low-frequency feed sources.However, these antennas have low radiation efficiency, and their antenna gain is not ideal.Therefore, combining microstrip patch antennas with horn antennas seems to be a research direction worth exploring.
In this paper, we used a crossed-dipole antenna as the radiating element and nested it within a designed circular stepped horn antenna enclosure to create a low-frequency, compact, and lightweight dual-polarized CATR feed antenna.This antenna operates in the frequency range of 300MHz to 500MHz and exhibits excellent directional characteristics with appropriate structural dimensions, meeting the requirements of compact range testing for low-frequency feed sources.Additionally, it offers a conceptual approach for designing compact feed sources in the low-frequency range.

The structure of the antenna
The antenna, as shown in Figure 1, is primarily divided into three parts: the crossed-dipole antenna, the stepped horn enclosure, and the feed network.
The dimensions of the crossed-dipole antenna with chamfers can be found in Figure 2(a).This part is fabricated on an FR-4 TG170 board with a dielectric constant of 4.4.Unlike traditional microstrip antennas, the ground plane is not connected to the other side of the FR-4 board.Instead, each of the four dipole arms is fed by a 50Ω coaxial transmission line.The inner conductor of the coaxial line is connected to the dipole arms, while the outer conductor is connected to a ground metal very close to the board, providing a common ground connection.This part exhibits a symmetric radiation pattern and a relatively wide beamwidth and serves as the main feeding section.
The dimensions of the circular stepped horn enclosure, as shown in Figure 2(b), are similar to a three-stage stepped horn.This design reduces the axial length of the antenna and primarily aims to correct the radiation pattern.It also contributes to reducing cross-polarization.
The feed network, as depicted in Figure 3, is constructed using three 180° hybrid couplers.It allows for the measurement of radiation performance in both horizontal and vertical polarizations without changing the orientation of the feed source.
The data for the first two sections can be found in Table 1.

The design principles of the antenna
This section will introduce the design principles of the three parts separately.

Cross dipole
The crossed dipole antenna consists of two pairs of orthogonally placed dipoles.In this paper, a neardiamond-shaped structure with chamfers is used as the unit of the dipoles.It gradually increases the gap between adjacent dipoles from L1 to L2, which helps enhance the isolation level and reduce crosspolarization.The fundamental resonance point shifts towards lower frequencies as the radiating element size increases.The second harmonic resonance point is generated by the coupling between dipoles, and when one pair of dipoles is excited, the other pair of dipoles also oscillates.Reducing the coupling distance shifts the second harmonic resonance point towards higher frequencies, but this reduction in coupling distance also leads to a decrease in the size of the radiating element, causing the fundamental resonance point to shift towards higher frequencies.Therefore, the adjustment of both parameters is crucial for widening the bandwidth.Therefore, we added chamfers at the corners of the dipoles to mitigate the variation of the fundamental resonance point.L0 represents the perpendicular line cut from the chamfered triangle, and Figure 4 clearly shows the relationship between L0 and the frequencies of the two resonances.As the chamfer size increases, the fundamental resonance point remains stable at around 300MHz.Meanwhile, the second harmonic resonance point shifts towards higher frequencies, achieving the goal of bandwidth expansion.However, as L0 continues to increase, the matching at mid-frequencies appears to deteriorate.When L0 reaches 26.46mm, the antenna's S11 remains below -14dB within the 300MHz-500MHz range.We also observed changes in antenna gain with varying L0, as shown in Figure 5, and found that the gain curve is more favourable when L0 is set to 26.46mm.Therefore, this structure significantly aids in the antenna's broadband performance while significantly reducing its axial length.

Stepped horn enclosure
The cross-dipole antenna solves the issue of excessive axial length in traditional feed sources.However, simulations revealed that this antenna had high side lobes and insufficient gain.To address this, a horn structure was added externally to the dipole structure.
Figure 6 and Figure 7 compare the simulation results between antennas with and without the horn enclosure.It can be observed from the simulations that the antenna's matching and gain were not ideal when there was no horn enclosure.Figure 6 shows that the matching performance was better with a three-stage horn enclosure compared to the other two cases.Figure 7 demonstrates that the three-stage horn enclosure exhibited the strongest linear relationship between antenna gain and frequency, indicating better performance.The three-stage horn enclosure provided a stable phase centre, effectively correcting the antenna's radiation pattern and matching.The axial length of this antenna is approximately 290mm.

Feeding network
The four feeding coaxial cables are divided into two groups, as shown in Figure 3.Each group of coaxial cables is connected by a 180° hybrid coupler, with a 180° phase difference between the two cables.They are ultimately connected to a common 180° hybrid coupler, leading to two different polarization ports.Through the use of 180° hybrid couplers, the phase relationships of the four cables can be adjusted to be (0°, 180°, 0°, 180°) and (0°, 180°, 180°, 0°), respectively, when fed through ports 1 to 4. This allows for the realization of both vertical and horizontal polarization, effectively suppressing crosspolarization.
This novel feeding method based on 180° hybrid couplers allows for different polarization directions without the need to adjust the antenna's orientation.This feeding method provides an excellent solution for testing larger low-frequency feed sources without requiring changes to the antenna's orientation.

Measured and simulation results
We conducted electromagnetic simulations of the above structures, and the results are as follows.
The VSWR for both polarizations is shown in Figure 8. From the graph, it can be observed that the VSWR for both polarizations remains below 1.5 in the frequency range of 300MHz to 500MHz.The antenna has a bandwidth of approximately 1.67.

Conclusion
This paper presents a low-frequency, compact, lightweight, dual-polarized CATR feed antenna.The crossed dipole antenna increases bandwidth while reducing the axial distance of the feed antenna.The stepped horn enclosure ensures a stable phase centre and plays a role in correcting the antenna's radiation pattern and impedance matching.The feeding network provides a solution for dualpolarization testing.Operating within the frequency range of 300MHz to 500MHz, this antenna exhibits excellent radiation patterns, a wide beamwidth, and a stable phase centre.The novel feeding method offers a new solution for low-frequency antenna testing.This antenna can be utilized for compressed-field antenna testing and RCS measurements.

Figure 2 .
The dimensions of the antenna.

Figure 1 .
The configuration of the antenna.(a) The crossed-dipole antenna with chamfers, (b) The circular stepped horn shell.

Figure 3 .
Figure 3. Feeding network of the antenna.

Figure 8 .
Figure 8. VSWR for vertical polarization and horizontal polarization.

Figure 9 toFigure 9 .
Figure9to Figure13depict the simulated results for various performance metrics of the antenna.Within the operating frequency range of 300MHz to 500MHz, the antenna exhibits the following characteristics: Half Power Beamwidth (HPBW) is greater than or equal to 44.5°, indicating a relatively wide beam.The variation in phase centre with respect to antenna height is minimal and stable.Examination of the directional patterns at three different frequency points within this frequency band reveals that the antenna possesses favourable radiation patterns, characterized by a high main lobe and low horizontal sidelobes.Cross-polarization between the horizontal and vertical directions exceeds 44dB, demonstrating strong isolation performance.

Figure 10 .Figure 11 .
Figure 10.HPBW for vertical polarization Figure 11.Realized Gain for vertical polarization and horizontal polarization.and horizontal polarization.

Figure 12 .
Figure 12.Phase centre for vertical Figure 13.Xpol for vertical polarization polarization and horizontal polarization.and horizontal polarization.

Table 1 .
Detailed dimensions of the proposed antenna.