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Radome Configurations

Several radome configurations are used to minimize RF reflections, including electrically thin, half-wave, A-sandwich, C-sandwich and others [3]. The best configuration for a particular application depends on the mechanical requirements and operating frequency.


Figure 3: In an electrically thin dielectric layer, reflections at the air/dielectric boundary are cancelled by the reflections on the other side of the laminate at the dielectric/air boundary, resulting in low-loss transmission of the incident wave.
A radome that is electrically thin (less than 0.1 wavelengths) [4], as shown in Figure 3, will generally deliver good RF performance. This is because signal reflections at the free-space/dielectric boundary are cancelled out by out-of-phase reflections from the dielectric/free space boundary on the other side of the dielectric material.

Figure 4 shows that signal losses are low and the net transmission from an electrically thin dielectric laminate is very high. Unfortunately, electrically thin radomes provide very little thermal insulation and are not suitable for locations with wide temperature extremes and a requirement for controlled temperatures.

 


Figure 4: The insertion loss through an electrically thin dielectric laminate at normal incidence is plotted here as a function of frequency. The laminate is 0.12 inches thick, has a dielectric constant of 4 and a loss tangent of 0.01. Loss increases with increasing frequency (and decreasing wavelength). When the thickness of the laminate is 0.1 wavelengths (at 4.9 GHz), the loss is 0.81 dB.

 


Figure 5: In a half-wavelength-thick radome, the round trip of signals through and reflected by the laminate introduces a 360° phase shift. The reflections at each interface cancel because they are out of phase, resulting in high net transmission of incident signals.
Another radome approach that works well is a configuration based on the half-wavelength-thick solid laminate shown in Figure 5. It is similar to the electrically thin configuration because the reflections cancel out. The wave travels 180° through the laminate, is reflected with a phase shift of -180°, and travels another 180° on the return trip to achieve the net 180° phase shift required for cancellation. Figure 6 shows the performance of the same laminate described in Figure 4 at higher frequencies (through 35 GHz) where it is 0.5 wavelengths thick.

 


Figure 6: The insertion loss of the laminate analyzed in Figure 4 is plotted here at higher frequencies. When the laminate material is 0.5 wavelengths thick, the reflections cancel and the net signal transmission is high.

 


Figure 7: An A-sandwich radome construction consists of a foam core with thin laminates on either side.
An A-sandwich radome configuration consists of a low dielectric foam or honeycomb core sandwiched between two thin laminates as shown in Figure 7. Its operation is similar to the half-wavelength-thick solid laminate. However, it is 0.25 wavelengths thick because the reflection coefficients from the skins have the same amplitude and phase. The round trip for the reflection from the second skin is 0.5 wavelengths. The reflections, which are 180° out of phase, cancel (Figure 7).

 


Figure 8: Reflections of an A-sandwich radome are plotted versus frequency. The foam core is designed to be 0.25 wavelengths at 5 GHz, which provides maximum performance at <7 GHz (and 15 GHz where the phase shift is an odd multiple of 180°).

 


Figure 9: C-sandwich configuration. Two layers of core material provide additional parameters to tune for multiband operation.
A C-sandwich radome consists of three skin layers and two foam layers as shown in Figure 9. The thickness of each foam layer, and possibly the skins, can be tuned for optimal RF performance in the bands of interest. This can lead to many potential construction combinations that can provide good RF performance and high mechanical strength. C-Sandwich constructions provide better performance than A-sandwich radomes; however, the added complexity increases material and labor costs.

Radomes Homepage
Fundamentals of Radomes
Radome Basic Functionality
Understanding RF Reflections
Radome Configurations
Radome Support Structures
Incident Angle Impact
Performance Variables
Radome Terms and Definitions
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