Differences in EM Performance Between Multi-Panel Faceted and Spherical Radomes

Differences in EM Performance Between Multi-Panel Faceted and Spherical Radomes Aleksey Solovey 1 1 Engineering Dept., L-3 ESSCO, Ayer, MA, USA, Aleksey.Solovey@L-3com.com Abstract Differences in the EM performance between radomes with multi-panel faceted and multi-panel spherical radome wall designs were investigated. Conditions when those differences may be practically important were derived. Index Terms antennas, multi-panel radomes, wave propagation in complex media. I. INTRODUCTION This paper investigates the differences in the EM performance of radomes with multi-panel faceted and multipanel spherical shapes of the radome wall. In faceted radomes the radome wall surface consists of flat, usually equal, polygonal panels. Assembled together they constitute a polyhedron inscribed into a perfect sphere. In spherical radomes each panel is curved in a way that when assembled together they create a perfect sphere. The faceted radome design is used by all MSF (Metal Space Frame), DSF (Dielectric Space Frame) and some sandwich radome manufacturers [1, 2]. The spherical radome design is used by L-3 ESSCO and some other radome manufacturers in its sandwich radome constructions. The differences in faceted and spherical radome wall EM performances are caused solely by a different distribution of incidence angles across the antenna illuminated spot on faceted and spherical radome surfaces. Finding conditions when those differences may or may not be practically important constitutes the main goal of this paper. Studied faceted and spherical radome EM performances were numerically simulated using highly specialized proprietary software package developed in-house by the L-3 ESSCO, the world leader in radome manufacturing. The core of the simulation engine contains the analytical solver for the EM wave propagation through the multi-layer dielectric media and the numerical solver for the EM wave scattering from the metal frame beams. Then, taking into account the distribution of the EM wave incident angles for the particular radome wall panelization pattern in front of the antenna aperture, all EM distortion parameters of interest for faceted and spherical radomes were calculated. II. INVESTIGATED ANTENNA AND RADOME DESIGNS The transmission through the radome wall depends on many factors: the radome wall lay-up, the ratio between the radome and the antenna aperture size(s), shape, illumination, steering range, placement within the radome and frequency band of operation. The number of combinations of those factors is virtually limitless. Thus, only two but in many sense most representative type of the radome wall designs were selected for investigation: typical sandwich or DSF radome wall lay-up tuned to the 5.625 GHz, and typical MSF radome panel membrane at 45 GHz. In addition, the wideband transmission loss characteristics at 1 45 GHz frequency range for both radome wall types were also studied. The most typical, circular dish aperture antenna whose size is times smaller than the radome diameter with the most common, -10 db aperture edge illumination taper placed in the center of the radome sphere was chosen for simulations. The 5 thick, 4 pcf density polyurethane foam core with two 33 mil fiberglass inner and outer skins for the sandwich or DSF, and the 2-ply ESSCOLAM 6 membrane for the MSF radomes were selected for the panel wall lay-ups. There are several reasons why those combinations of frequencies, antenna and radome wall designs were chosen as representative examples for which the differences between the faceted and the spherical sandwich and DSF radome wall EM performances ought to be studied. First, the :1 ratio of the radome vs. antenna sizes is common for most multi-panel radomes. Second, chosen combination of frequency and sandwich or DSF radome wall design is used in a large number of faceted and spherical radomes that cover the weather radars. Third, chosen radome wall design is one of the best from the minimum radome EM distortions standpoints. As such, chosen combinations of frequency, antenna and radome wall lay-up represent the most favourable cases for investigation whether or not sandwich or DSF faceted radome wall design is meaningfully worse than the spherical one. For those cases the restraints under which the faceted radome EM performance would not be noticeably worse than that for the spherical radome would be the weakest. For other multi-panel faceted radome wall design and frequency combinations those restriction will be even stronger. The 2-ply ESSCOLAM 6 membrane was chosen because it is used in a vast majority of the L-3 ESSCO MSF radomes. Except of few instances, the 45 GHz is the highest frequency that is currently used in most MSF radome applications. III. FACTORS THAT DIFFERENTIATE FACETED AND SPHERICAL RADOME WALL EM PERFORMANCES The single most important factor that differentiates scattering characteristics through the faceted and spherical

radome walls is the ratio between the size of antenna aperture and the size of faceted panel. With very few panels within the antenna view, the incidence angle distributions across the illuminated spot for faceted and spherical radome walls are considerably different. That is exactly what causes differences in faceted and spherical radome wall EM performances. Alternatively, with the increase of number of panels within the antenna view, the incidence angle distributions within the illuminated spot for the faceted and spherical radomes approach to each other. As a result, so do the faceted and the spherical wall EM performances. Second factor that differentiates scattering characteristics through the faceted and spherical radome walls is the antenna steering within the radome. At each particular antenna orientation in azimuth and/or elevation antenna looks through certain distinct combination of partially and entirely illuminated panels. Unlike for the spherical radome with the antenna at the center of the radome sphere, alteration of the panelization pattern within the antenna view varies the incidence angle distribution across the illuminated spot on the faceted surface. That causes the variation of faceted radome scattering characteristics with the antenna steering. In present study this effect is described through the panels shift parameter that is corresponded with variation of the radome wall panelization pattern within the antenna view during the antenna steering. The panels shift parameter varies from 0 to where, the value 0 corresponds with the case when antenna illuminates the exact whole number of radome panels. The value corresponds with the case when antenna illuminates two half-panels at both edges of the illuminated spot, while other panels are entirely illuminated. The intermediate values of panels shift parameter correspond with cases when some unequal portions of panels at the edge of illuminated spot are presented within the antenna view. IV. FACETED VS. SPHERICAL RADOME EM PERFORMANCE This section describes the differences in EM performances of faceted vs. spherical radome wall designs accompanied by several illustrative examples. A. Faceted vs. Spherical Sandwich or DSF Radome Wall Scattering Characteristics The wideband transmission loss of faceted vs. spherical sandwich or DSF radome wall lay-up described in previous section (the 5 thick, 4 pcf density polyurethane foam core with two 33 mil fiberglass inner and outer skins) is shown on the first plot of Fig. 1 for the case when the antenna illuminates the exact whole number of panels (1, 2, 3 or 4). As it can be seen from this plot, from the 1 45 GHz wideband transmission loss prospective the faceted character of the sandwich or DSF wall might be practically ignored above the C-band only if, depending on frequency, the size of antenna aperture is by 2 4 times bigger than the size of faceted radome panel. Variations in the transmission loss, beamwidth change, boresight errors, first sidelobe increase and null depth with the Solid Red Spherical Radome Dot-Dashed Light Blue Faceted Wall (3 facets) Double Dot-Dashed Yellow Faceted Wall (4 facets) 6 5 4 3 2 1 0 0.18 0.16 0.14 0.12 8 6 0.15 5 0-5 10 20 30 40 Frequency, GHz Boresight Error, miliradian Beamwidth Increase, percent - Fig. 1. Sandwich or DSF Radome Wall Wideband Transmission Loss and Transmission Loss, Sum Boresight Error and Beamwidth Change Variations with Antenna Steering @5.625 GHz

Solid Red Spherical Radome Dot-Dashed Light Blue Faceted Wall (3 facets) Double Dot-Dashed Yellow Faceted Wall (4 facets) 0.15 5 First Sidelobe Increase, db Difference Boresight Error, miliradian 0-40 -60-80 -100 Null Depth, db -120 Fig. 2. Sandwich or DSF Radome Wall First Sidelobe Increase, Difference Boresight Error and Null Depth Variations with Antenna Steering @5.625 GHz antenna steering within the multi-panel faceted and spherical sandwich or DSF radome walls were compared at 5.625 GHz. The results are shown in the last three plots of Fig. 1 and in Fig. 2 for radome panel sizes that are 2 4 times less than the size of antenna aperture. For the antenna positioned in the center of the radome sphere, the radome wall does not affect any EM distortion parameters except the transmission, reflection and absorption loss and noise temperature. For that reason, values of all spherical radome EM distortions except the transmission loss on plots in Fig. 1 and 2 correspond with its maximum values introduced by a typical well-tuned radome panel joints. As is seen from Fig. 1 and 2 (backed up by the broader studies), when the faceted radome panel size is comparable with the size of the antenna aperture, the faceted character of the radome wall becomes the main source of all radome EM distortions. To neglect the faceted nature of the radome wall design the size of the faceted radome panel should be at least 2, 3 or even 4 times less than the size of antenna aperture, depending on frequency and the EM distortion parameter of interest. For instance, the size of the faceted panel should be at least 2 times less than the size of antenna aperture for the transmission loss, the difference boresight error and the null depth increase, at least 3 times less for the sum boresight error and the first sidelobe increase and at least 4 times less for the beamwidth change radome EM distortion characteristics. It should be accentuated that this 2 4 times value of antenna aperture to panel size ratio criterion is based on one of the best, most tuned to the investigated 5.625 GHz frequency radome wall design. To neglect the faceted nature of the radome wall design in case of less tuned and/or working at higher than C frequency band of operation radome, the value of this ratio might be even bigger. B. Faceted vs. Spherical MSF Radome Wall Scattering Characteristics Although actually manufactured MSF radomes are made out of flat panels and thus, have the faceted wall surface, it s scattering through the panel membrane usually is simulated as a scattering through the spherically shaped membrane. Therefore, it is important to investigate the conditions when such theoretical approximation is valid. As is illustrated in Fig. 3, when the exact whole number of panels is within the antenna view, the faceted character of the radome wall makes no meaningful difference for the wideband transmission loss of the MSF radome wall. It is true even for highest, 45 GHz frequency and even when the size of the faceted panel is equal to the size of the antenna aperture. Solid Red Spherical Radome Dashed Dark Blue Faceted Radome (panel shift = 0) Dashed Green Faceted Radome (panel shift = 0.15) Dot-Dashed Light Blue Faceted Radome (panel shift = 0.30) Double Dot-Dashed Yellow Faceted Radome (panel shift = 0.45) 0.8 0.6 0.4 0.2 10 20 30 40 Frequency, GHz Fig. 3 Variation of Wideband Transmission Loss with Antenna Steering for MSF Membrane Wall when Size of Faceted Panel is equal to Size of Antenna Aperture

Solid Red Spherical Radome Dot-Dashed Light Blue Faceted Wall (3 facets within antenna view) 1.15 1.10 5 0 0.95 0.90 05 04 03 02 01 00 Boresight Error, miliradian -01 3.0 Beamwidth Increase, percent - - First Sidelobe Increase, db 35 30 25 20 15 10 05 Difference Boresight Error, miliradian 00-40 -50-60 -70-80 -90 Null Depth, db Fig. 4 Variation of Transmission Loss, Boresight Error, Beamwidth Change, First Sidelobe Increase and Null Depth with Antenna Steering for MSF Radome Membrane Wall @45 GHz However, above the C-band those wideband transmission losses have noticeable variation with the antenna steering and the faceted character of the MSF radome membrane wall can be ignored only if the faceted panel size is at least 2 times less than the size of the antenna aperture (as is seen from the first plot in Fig. 4). Comparison of the transmission loss, boresight error, beamwidth change, first sidelobe increase and null depth variations with the antenna steering within the faceted vs. spherical MSF radome walls at 45 GHz is shown in Fig. 4 for the faceted panel sizes that are 2, 3 or 4 times less than the size of the antenna aperture. Similarly to the sandwich radome wall case, for the MSF spherical radome wall values of all EM distortion parameters except the transmission loss on plots shown in Fig. 3 correspond with its maximum values introduced by the typical MSF radome metal beam frame. As is seen from Fig. 4 (backed up by broader studies), the faceted character of the MSF radome can be neglected only if, depending on frequency and the EM distortion parameter of interest, the size of the radome panel is 2 4 times less than the size of the antenna aperture. For instance, the size of the faceted panel should be at least 2 times less than the size of antenna aperture for the transmission loss, null depth increase and beamwidth change, at least 3 times less for the first sidelobe change and at least 4 times less for sum and difference boresight error radome EM distortion parameters.

V. CONCLUSIONS Results of present study that was done for many more number of the radome wall lay-ups and frequency combinations, than were mentioned in in this paper can be summarized as follows: 1. The faceted character of a sandwich, DSF and MSF multipanel radome wall design has negative impact on all EM distortion characteristics introduced by the radome wall. This impact might be slight, significant or even plays the major role in the overall level of the EM distortions caused by the radome. Its severity depends on frequency, antenna illumination and placement within the radome, radome wall lay-up, and the ratio between the size of antenna aperture and the size of radome faceted panel. The radome illumination influences the faceted radome EM performance through the effective size of the antenna. Thus, in many cases an approximation the faceted radome wall by a perfect sphere might be very misleading. 2. Unlike for the spherical wall with the antenna placed in the center of the radome sphere, where the radome wall affects only the radome transmission, reflection and absorption loss and the noise temperature, the faceted wall also affects all other radome EM performance distortion characteristics such as boresight error, beamwidth change, sidelobe increases, etc. Moreover, the value of those EM distortions depends on the antenna orientation within the radome making the faceted radome EM performance less uniform with the antenna steering in azimuth and/or elevation. This constitutes the main disadvantage of the faceted radome wall design. 3. The single most important parameter that defines the impact of the multi-panel faceted radome wall design on all radome EM performance characteristics is the ratio between the size of antenna aperture and the size of the radome panel. 4. Apart from very special circumstances, the faceted character of the MSF radome multi-panel membrane wall design makes no meaningful difference for the radome scattering characteristics at frequencies below the X-band. In order to neglect the faceted character of the MSF radome membrane wall at frequencies above the C-band, the ratio between size of the antenna aperture and size of the radome panel should be at least 2, 3 or even 4, depending on particular combination of the antenna characteristics, frequency and radome EM distortion parameters of interest. 5. Apart from very special circumstances, the faceted character of the sandwich or DSF radome multi-panel wall design makes no meaningful difference for the radome scattering characteristics at frequencies below the C-band. To neglect the faceted character of the sandwich or DSF radome wall at frequencies above the S-band, the ratio between size of antenna aperture and size of the radome panel should be at least 2, 3 or even 4, depending on particular combination of the antenna characteristics, frequency, and radome EM distortion parameters of interest. 6. The disadvantages of faceted radome wall design stated above most probably would be noticeable for the relatively small, less than 25 35 sandwich or DSF radomes at frequencies above the S-band. When at the same time the size of the antenna aperture is two or more times less than the radome diameter, these disadvantages become even more severe. Same is true for the MSF type of radomes although in lesser degree and at frequencies above the C- band. 7. At the contrary, for bigger radomes (more than 40-45 ) that house the antennas whose size is less than the radome diameter no more than two times, additional EM distortions associated with faceted radome wall design most probably might be neglected at all frequencies below 45 GHz. This information has been released into the public domain in accordance with International Traffic in Arms Regulations (ITAR) 22 CFR 120.11(a) (2). REFERENCES [1] L. N. Ridenour, Editor-in-Chief, Radar Scanners and Radomes, Part II, MIT Radiation Laboratory Series. McGraw-Hill, 1948. [2] Merrill I. Skolnik, Editor-in-Chief, Radar Handbook, Second Ed., McGraw-Hill, 1990, Chapter 6.9.