Radome a bassa RCS

A radome is a cover placed over the antenna that protects the radiating element from its physical environment (e.g., wind, rain, ice, sand, and ultraviolet rays). It protects the antenna's exposed parts with a sturdy, weatherproof material, typically fiberglass, which keeps debris or ice away from the antenna to prevent any serious damage or malfunctioning. From an electromagnetic point of view, a radome should be transparent to radio frequencies so that it does not degrade the electrical performance of the enclosed antenna.
Under many circumstances, it is desirable to employ a frequency selective radome in order to prevent coupling from nearby transmitting antennas that may interfere with the electronic circuitry. Other than the control on out of band emissions, the role of the radome can be of crucial importance in reducing the antenna Radar Cross Section (RCS). To this purpose, a radome is usually designed as a frequency selective filter which allows the operating frequencies to pass with the least amount of insertion loss, whereas it reflects the out of band signals. Generally, in-band RCS is dominated by the antenna itself, not by the radome, whereas far out of band, the RCS will be dominated by the shape and by the reflection properties of the radome. Consequently, the frequency selectivity together with a proper shape of the radome tends to reduce the antenna RCS since impinging out of band power, representing a potential interrogation signal of an unknown radar, would be reflected towards a specular direction with a minimum monostatic reflection. This approach can be classified as a fictitious RCS reduction technique since it is effective for monostatic interrogations and if no additional scatterers are located in the close proximity of the antenna.
More in general, an ideal low-RCS radome should transmit the in-band power and absorb the incoming out of band power. This solution is usually not considered since it is a common belief that the employment of lossy material for manufacturing a radome causes a high insertion loss.
In our work [1] we proposed a transmissive/absorbing radome by means of opportunely designed resistive and metallic periodic surfaces in order to achieve high in-band transmission and, at the same time, a wideband absorption behavior. The main goals to achieve are summarized below:

• Low in-band insertion loss both for transmitted and received signals.
• Wideband absorption of out of band incoming signals.
• Periodicity of the frequency selective surface lower than a wavelength at the upper frequency limit of the absorption band for avoiding grating lobes.