Phase Compensation of Composite Material Radomes Based on the Radiation Pattern
- Peng LI^{1, 2},
- Na LI^{1}Email author,
- Wanye XU^{1} and
- Liwei SONG^{1}
https://doi.org/10.1007/s10033-017-0133-1
© The Author(s) 2017
Received: 6 May 2016
Accepted: 2 April 2017
Published: 12 April 2017
Abstract
Some compensation methods have been proposed to mitigate the degradation of radiation characteristics caused by composite material radomes, however most of them are complex and not applicable for large radomes, for example, the modification of geometric shape by grinding process. A novel and simple compensation strategy based on phase modification is proposed for large reflector antenna-radome systems. Through moving the feed or sub-reflector along axial direction opportunely, the modification of phase distribution in the original aperture of an enclosed reflector antenna can be used to reduce the phase shift caused by composite material radomes. The distortion of far-field pattern can be minimized. The modification formulas are proposed, and the limitation of their application is also discussed. Numerical simulations for a one-piece composite materials sandwich radome and a 40 m multipartite composite materials sandwich radome verify that the novel compensation strategy achieves satisfactory compensated results, and improves the distortion of the far-field pattern for the composite material radomes. For one-piece dielectric radome, more than 60% phase difference caused by radome is reduced. For multipartite radome, the sidelobe level improves about 1.2 dB, the nulling depth improves about 3 dB. The improvement of far-field pattern could be obtained effectively and simply by moving the feed or sub-reflector according to phase shift of the radome.
Keywords
1 Introduction
Radomes not only protect enclosed antennas against wind, rain, ice, snow and solar radiation, but also reduce the manufacturing cost and extend the service time of enclosed antennas [1, 2]. However, from the viewpoint of electromagnetics, the radomes also lead to the degradation of radiation characteristics of the enclosed antennas, such as gain loss, boresight error and the rise in side-lobe level [3, 4], which can be represented by the distortion of radiation pattern. Therefore, minimizing the degradation of radiation characteristics and maximizing structural stiffness are the prior purposes in the design of modern high-performance antenna-radome systems [5, 6].
One of the main reasons of radiation pattern distortion is the change of phase difference caused by radome. In order to mitigate the phase difference, some novel approaches and materials have been proposed. Virone et al. installed a special metal periodic structure in the joints to reduce the phase difference between the joints and planes [7, 8]. A good agreement between the experimental and simulated results is reported [9], and the results indicate that their compensation strategy is valid and effective for mitigating the degradation caused by radomes. The absorbing materials is used to reduce the effect of scattered field caused by induced current of the metal frame by enwrapping the metal bars [10].
A shaped reflector antenna is designed to change the original antenna phase distribution and compensate the phase distortion caused by dielectric radomes [11]. A shaped radome (nonuniform thickness) can also get a minimal phase difference and reduce the pattern distortion [12]. The degradation caused by glass fiber material error can be compensated [13] by grinding the geometrical thickness based on phase equivalent [14].
The metamaterial is applied to design a radome. By optimizing the structure parameters of the radome [15], the transmission coefficient of the plane radome is close to 1, and the degradation is almost invisible. Another metamaterial radome which has a planar centrosymmetric honeycomb-shaped structure can obtain a higher gain about 2.5 dB before the use of this metamaterial radome [16].
However, all these methods above are not easy to apply in practice. The methods in [7–14] need large modifications of the radomes or antennas and lead to high cost. The metamaterials radome is not suitable for large rotatable antenna-radome system.
Phase compensation is a common method and could be applied to many electronic devices to improve their electromagnetic performance, such as microstrip crossover structure [17], microwave patch antenna [18], large reflector antenna [11], and radome [12].
In order to mitigate the distortion of radiation pattern caused by composite materials radome, a novel compensation strategy based on phase compensation is proposed. Some simulation examples are presented to exhibit the validity of the novel strategy for multi-band system, multipartite dielectric sandwich radomes.
2 Analysis Methods of Dielectric Sandwich Radomes
T _{H} or T _{V} is the amplitude of transmission coefficient of the wave, η is the inserted phase delay (IPD) according to the incident angles of different points, the subscripts H and V indicate the horizontal and vertical polarizations of the wave respectively, and β is the polarization angle.
Transmission coefficient and IPD are the functions of relative permittivity, loss tangent, thickness of radome materials d and wavelength λ [1].
3 Compensation Strategy
To reduce the degradation of radiation characteristics, both transmission coefficient of dielectrics and inducted current of metal should be cut down. In this study, we mainly paid attention to the effects of transmission coefficient, and tried to reduce the phase difference in transmission aperture.
The phase of transmission coefficient T is the key factor that causes the distortion of radiation pattern. In the original aperture, the phase of the wave is a constant, namely the phase difference is 0, and the radiation pattern has no distortion without the radome. In contrast, with the radome, the phase of T in the transmission aperture is not a constant, but varies with the incidence angle γ. A different γ leads to a different IPD, thus induces a phase difference of T in the transmission aperture, and finally causes the distortion of radiation pattern.
In antenna-radome systems, radome is an indispensible part and we cannot remove it, but we could change the phase distribution in the original aperture. If phase difference in the original aperture has an inverse trend to that in the transmission aperture. The wave propagates through the radome. Thus, the phase change will turn out to be a constant in the transmission aperture. Finally, an ideal radiation pattern can be obtained.
3.1 Shaping the Main Reflector
However, it is not practical to shape the main reflector for an installed reflector antenna.
3.2 Moving the Feed
3.3 Moving the Sub-Reflector
At the same point of the sub-reflector, \( \xi^{\prime}_{\hbox{max} } \) is less than ξ, and their relationship is \( M = \tan \left( {\xi_{\hbox{max} } /2} \right)/\tan \left( {\xi^{\prime}_{\hbox{max} } /2} \right) \), where M is the amplification factor of the double reflector antenna, tan (ξ/2) = D/4f, D is the diameter of the main reflector, and f is the focal length.
In the aperture S, from the center to the edge, the two flare angles increase from 0 to \( \xi^{\prime}_{\hbox{max} } \) and ξ _{max}. If d _{fz} is negative, namely the feed moves to the −Z direction, the distribution of phase difference in the original aperture is bigger in the center and smaller at the edge. If d _{sz} is positive, namely the sub-reflector moves to the +Z direction, the distribution of the phase difference is similar to that with a negative d _{fz}.
The distribution of \( \Delta \eta_{\text{T}} \) is smaller in the center and bigger at the edge, which is just inverse to the distribution of phase difference caused by negative d _{fz} or positive d _{sz}. Hence, the two-phase difference can offset each other and realize the compensation of the distortion of radiation pattern. This means \( \Delta \eta_{\text{T}} = \Delta \eta_{\text{f}} \) or \( \Delta \eta_{\text{T}} = \Delta \eta_{\text{s}} \).
From the above analysis, it is known that the offset focus or T leads to a phase difference in the transmission aperture and result in the distortion of radiation pattern as well as the degradation of radiation characteristics. However, if they work together, the degradation will be mitigated significantly.
Flare angle ξ′ is less than ξ at the same point in the sub-reflector. To compensate the same Δη _{T}, the value of d _{fz} is larger than that of d _{sz}. Thus, the compensation by moving the sub-reflector is more appropriate than by moving the feed in practice. For most reflector antenna in engineering, both the sub-reflector and feed are fixed by bolts, if we adjust the bolts, the position of the sub-reflector or feed could be changed in a small range.
By numerical integration, the calculation of Eq. (12) is implemented [22].
4 Simulation Examples
Two antenna-radome systems are used for simulation: (1) a 5.2 m reflector antenna with a 9.14 m one-piece dielectric sandwich radome (Fig. 1(a)); (2) a 26 m antenna with a 40 m multipartite dielectric sandwich radome (Fig. 1(b)). Two examples are used here for different purposes. The first purpose is to verify the effectiveness of the compensation strategy and its feasibility for the multi-band antenna-radome system. The second is to test the validity of the strategy for multipartite sandwich radomes.
Information of simulation examples
Antenna/radome’s diameter D/m | Frame | Panel | Frequency f/GHz |
---|---|---|---|
5.2/9.14 | – | A | 2.3/5.3 |
26/40 | Glass fiber | C | 5.7 |
Material parameters of the radome
Component | Material | Thickness d/mm | Permittivity ε/(F·m^{−1}) | Loss tangent tanδ/(W·m^{−3}) |
---|---|---|---|---|
Core | Foam | 50 | 1.15 | 0.009 8 |
Skin | Glass fiber | 0.5 | 4.20 | 0.026 |
4.1 5.2 m reflector antenna with a 9.14 m one-piece radome
The one-piece radome means that there is no connected joint (ignored) between two panels. Hence, the jointed panels is considered to be a smooth hemisphere. The enclosed antenna is a Cassegrain antenna, and the focus diameter ratio is 0.4, with \( \xi^{\prime}_{\hbox{max} } \) = 31° and ξ _{max} = 80°.
Main parameters of 9.14 m radome under 4 situations
Parameters | Max field value E _{max}/dB | Sidelobe-level SLL/dB | Nulling depth ND/dB | Phase difference PD/rad | Max field value E _{max}/dB | Sidelobe-level SLL/dB | Nulling depth ND/dB | Phase difference PD/rad |
---|---|---|---|---|---|---|---|---|
Frequency f/GHz | 2.3 | 5.3 | ||||||
No radome No offset focus | 62.47 | −17.58 | −43.13 | 0 | 69.73 | −17.58 | −43.26 | 0 |
Radome No offset focus | 62.27 | −17.34 | −27.18 | 0.401 | 69.06 | −16.31 | −19.94 | 0.922 |
No radome Offset focus | 62.42 | −17.25 | −33.24 | 0.402 | 69.60 | −16.84 | −24.28 | 0.609 |
Radome Offset focus | 62.33 | −17.58 | −41.25 | 0.055 | 69.32 | −17.49 | −38.06 | 0.321 |
After compensation, the power loss is only 0.14 dB, which is less than the uncompensated value. The sidelobe-level (SLL) is improved greatly and approaches to the value of no radome. The nulling depth (ND) increases about 14 dB from −27.18 dB to −41.25 dB. The phase difference (PD) in aperture is only 0.055, which is much less than the uncompensated value 0.4.
At 5.3 GHz, the results are similar to those at 2.3 GHz, but the degradation is relatively obvious. Before compensation, the power loss is 0.67 dB, the sidelobe-level degradation is 1.26 dB, the nulling depth degradation is 23 dB, and the phase difference of aperture is 0.92. Hence, the d _{sz} will be more than 0.1λ, which leads to the degradation of the enclosed reflector antenna. Previous studies show that if the value of the offset focus is less than 0.1λ, the degradation is so tiny that can be ignored. In this viewpoint, there is a limitation for the compensation strategy, that is, the maximum value of moving the sub-reflector is 0.1λ. This means that if the phase difference of T is too large, it cannot be compensated completely. In spite of this, a partial compensation of 0.1λ is still available.
Here, after partial compensation, the sidelobe-level degradation reduces from 1.26 dB to 0.1 dB, the nulling depth degradation reduces from 23 dB to 5.2 dB, the power loss reduces from 0.67 dB to 0.4 dB, the phase difference reduces from 0.92 to 0.32.
For comparison, the results from antenna with offset focus are also involved in Table 3. They are worse than the results from antenna without offset focus and those from antenna under radome with offset focus.
The above simulation results illuminate that, the radome leads to the degradations of radiation characteristics, and so does the offset focus. However, if the radome and the offset focus exist at the same time with appropriate values, the respective degradation of radome or offset focus can be significantly reduced. That means that the offset focus can compensate the degradation caused by radome, especially for sidelobe-level and nulling depth.
The distribution of phase difference of T is small in center and large at edge, while that of offset focus is large in center and small at edge. This result is just consistent with previous expositions. After compensation, the maximum phase difference appears in the center and at the edge, and the minimum is a ring in the aperture. Overall, the whole phase difference is decreased greatly.
Notably, it is impossible to reduce the phase difference to zero by the compensation of offset focus, since the curvatures of the radome and the reflector are generally different.
4.2 26 m antenna with a 40 m multipartite dielectric radome
In practice, one-piece radomes are extremely rare, and most radomes are multipartite dielectric ones, especially for large radomes. In this simulation, both the panels and joints (composed of glass fiber) are included. The metal bolts are ignored. The effect of metal bolts is extremely small, since the area blocked by the bolts is so small as a literature indicated [26]. The thickness of joints is 30 mm. The joints are considered as thick dielectrics with an analysis strategy of ray tracing. The diameter of the enclosed antenna is 26 m, M is 3, and focus diameter ratio is 0.3.
Main parameters of the 40 m radome before and after compensation (5.7 GHz)
Parameters | Max field value E _{max}/dB | Left sidelobe-level L-SLL/dB | Right sidelobe-level L-SLL/dB | Nulling depth ND/dB | Beam width BW/(°) | Phase difference PD/rad |
---|---|---|---|---|---|---|
Ideal antenna | 107.22 | −14.53 | −14.53 | −25.36 | 0.204 | 0 |
Before compensation | 103.85 | −12.45 | −12.48 | −13.51 | 0.221 | 2.65 |
After compensation | 104.56 | −13.65 | −13.68 | −16.47 | 0.212 | 2.26 |
After compensation, the phase difference in the transmission aperture reduces from 2.65 to 2.26, the side-lobe level varies reduces from −12.45 dB to −13.65 dB and the degradation reduces from 2.1 dB to 0.9 dB. The SLL can satisfy the design requirements. Meanwhile, the nulling depth is improved by about 3 dB from −13.51 dB to −16.47 dB. Owing to the asymmetry of radome joints, the right and left SLLs are different.
The simulation results indicate that the offset focus can compensate the degradation of dielectric sandwich radomes and have a good potential application for metal-space frame radomes, especially the distortion of radiation patterns.
5 Conclusions
- (1)
For the one-piece dielectric radome (9.14 m radome), more than 60% phase difference caused by radome is reduced, and more than 80% of the radiation characteristics degradation is compensated by the proposed compensation method, since the main reason of the degradation is the phase difference of the radome.
- (2)
For the multipartite radome (40 m radome), although there are more reasons lead to the degradation (such as joints, frame and large phase difference of a practical radome), a satisfied results can still be achieved by the compensation strategy, about 50% of the degradation is reduced, especially for the sidelobe level and nulling depth.
- (3)
For the case of the large phase difference, the degradation is reduced partly by the proposed method. Because there is a limitation of the adjustable range of the sub-reflector or feed.
- (4)
The method is only useful for a reflector antenna with a standard parabolic dish. For a modified parabolic reflector antenna, reflector shaping is a appropriate method.
Notes
Declarations
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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