# Multifunction RF Systems for Naval Platforms
## 3 MFRF System Design Considerations
### Practical Limitations and Trade-Offs
#### 3.2.2 Wideband vs. Multiband Operation  
In the ideal MFRF system architecture, all of the hardware components are sufficiently wideband to support the full range of operating frequencies for the RF functions of interest. This implies component bandwidths of 0.5-40 GHz, based on the RF function requirements of Table 1. However, such bandwidths are not available with current state-of-the-art technology. Referring to Figure 2, limitations begin with the radiating element itself. A number of different types of antenna elements have been designed for use in dual-polarized wideband AESAs, but the element design that provides the largest bandwidth along with relatively good cross-polarization and reflection coefficient properties remains the well-known flared notch, often referred to as a Vivaldi antenna [26; 30]. Figure 3 depicts a single AESA element comprising two flared notches positioned orthogonally to provide dual linear polarization operation. The metal flared notches can simply be printed on dielectric substrates, which facilitates cost-effective fabrication and assembly of a large array of such elements, with small element separation if necessary. The bandwidth is largely determined by the element aspect ratio \(h/d\), while the element separation on the array corresponds to the dimension \(d\). The maximum instantaneous bandwidth achievable with a flared notch element occurs with an aspect ratio of \(h/d\approx 5\), which yields a bandwidth of about 10:1, or a decade of bandwidth. However, the overall required 0.5-40 GHz range of operating frequencies for the naval RF functions represents almost two decades of bandwidth. Consequently, the limitation on achievable AESA element bandwidth necessitates the use of multiple AESAs operating in different bands that collectively cover the entire operating band of interest. In considering various multiband MFRF system designs, one design approach is to populate every AESA with elements that are as wideband as possible, with the aim of accommodating all RF functions that operate within the large bandwidth of each array. An alternative scheme could include some AESAs that are designed to be utilized only by RF functions that have relatively small operating frequency ranges, albeit in different bands. This potentially allows narrowband elements to be used for those arrays. For example, a dual-band dual-polarization AESA design has been reported that supports operation at both S-band and X-band [31]. The geometry of the array is shown in Figure 4. The spacing of the S-band and X-band elements is chosen to minimize grating lobes in their respective bands. The radiating elements themselves consist of metal patches printed on different sides of four stacked dielectric substrates. The separation distance between the stacked substrates is adjusted to optimize the bandwidth of the elements. Each X-band element comprises two stacked diamond-shaped patches-one patch is active and the other is parasitic. There are two feed ports to the active patch on adjacent corners to realize dual orthogonal polarizations. Each S-band element consists of stacked patches that include two modified coupling feed patches, an active perforated patch and a parasitic perforated patch. The purpose of the perforations is to expose four X-band elements that underlie each S-band element, so that the presence of the S-band element does not affect the performance of the X-band elements. The measured bandwidths of the S-band and X-band elements are 0.6 GHz and 2.7 GHz respectively, corresponding to operating frequency ranges of 2.8-3.4 GHz and 9.0-11.7 GHz. The element reflection coefficient is less than \(-10\) dB over these frequency ranges. This type of AESA would appear to be a good candidate for shared usage by the S-band and X-band radar subfunctions listed in Table 1. The array design provides the flexibility of allowing any part of the AESA to be accessed simultaneously by radar functions in both bands.