Radome, also called electromagnetic, is known as the transparent electromagnetic window of radar antenna. Aircraft radome, as functional component, needs not only to protect radar antenna from adverse external environmental impact, but also to meet the requirements of electrical performance of radar antenna. In accordance with the aircraft generally-given requirements of radome shape, coat, installation of accessories, lightning protection, anti-environment, price, weight, reliability and maintainability, under the premises of not reducing structural strength, environmental and service life, and radome’s electrical performances should be optimized.
Structurally, radomes should meet the exteriority requirements of the aircraft, support aerodynamic loads, protect radar antenna from wind and rain, snow and ice, dust, solar radiation, static electricity, lightning and other environmental impacts, and ensure the stability and reliability of radar antenna.
Radome should meet the electromagnetic penetrating requirements in the electrical aspect. Electrical performance indicators of radome involves power transmission efficiency, aiming error, and pattern distortion (including beam width, near zone minor level, mirror image lobe level, far zone RMS minor level, zero level and cross polarization level).
According to the shape, working bands, electrical performance indexes and weight limits of aircraft radome, the following wall structural types are available for selection.
(1) Sandwich Structure
Thin-gauge skin is commonly used for laminated plate of wave penetrating composite materials, e.g. glass fiber / resin composite material; the sandwich is often of honeycomb or foam structure. Honeycomb, usually hexagonal, is made of glass cloth or Nomex paper impregnated resin. Foam core materials are in two types of polyurethane and artificial medium, of which, the dielectric constant of artificial medium is programmable. Sandwich structure covers:
A-type interlayer: it is composed of inner and outer skin and sandwich layer in the middle. The dielectric constant of the inner and outer skin materials is larger than that of core materials. Optium height of sandwich layer is approximately 1/4 of wavelength. This kind of radome is lighter in structural weight but larger in bandwidth, more sensitive to polarization and angle of incidence, and suitable for intermediate and low angle of incidence.
B-type interlayer: Its composition is similar as A-type interlayer, but the dielectric constant of the inner and outer skin materials is smaller than that of core materials. Due to poor structural reliability, it’s generally not used in aircraft radome.
C-type interlayer: of the sandwich structure of 5 layers. It can be regarded as a back-to-back combination of two A-type interlayers. This type of structure is great in strength and stiffness and its bandwidth is wide, suitable for high angle of incidence.
Composite multi-layer sandwich: refers to sandwich structure of over 7 layers. The composite multi-layer sandwich, with wider bandwidth, can progress in multi-waveband. The wall section structure of the radome in sandwich structure is usually symmetrical.
(2) Half-wave Wall
In the design of angle of incidence and frequency, with the solid wall structure, the wall thickness of the radome is half of the medium wavelength, so the half-wave wall is applicable to narrow bandwidth and high angle of incidence. Mostly it adopts the profiling fiber glass woven fabrics and resin transfer molding (RTM), and some use the filament winding and placement technique.
(3) Quasi-half-wave Wall
It is made up of two layers of thin-gauge skin and one layer of artificial medium sandwich. The artificial medium is filled with electric particles to match the dielectric constant of thin-gauge skin materials. The electric wall thickness and performance of this structure is basically the same as the half-wave wall, but its weight is reduced by more than 30%. It can be made in the way of paperhanging or winding and placing technique.