# Multifunction RF Systems for Naval Platforms
## 3 MFRF System Design Considerations  
### Ideal MFRF System Architecture  
To provide context for the discussion in this section, it is useful to consider the key features of an ideal MFRF system architecture that would allow all AESA elements and Tx/Rx channel hardware to be utilized by any of the RF functions of interest. Such a configuration would maximize the potential benefits discussed in Section 1. The principal hardware components of a Tx and Rx channel in an ideal MFRF system architecture are depicted in the simplified block diagram of Figure 2 for a single AESA element. If the radiating elements are dual-polarized, each element polarization has a similar Rx and Tx channel associated with it. The above figure conveys the notion that each radiating element is shared by a Tx channel and a Rx channel, where generally each of the two channels can be used by a different RF function at the same time. The circulator is the key element that enables this configuration. A circulator is a nonreciprocal three-port device that utilizes the properties of certain magnetic materials like ferrite to allow an RF signal to pass between ports in ideally only one direction, while preventing the signal from proceeding in the reverse direction around the circulator. So referring to Figure 2, the transmit signal entering the circulator is routed to the radiating element, and blocked in the reverse direction so that it does not emerge from the circulator into the Rx channel. Thus, the circulator serves to isolate the sensitive LNA in the Rx channel from the high power output of the HPA during transmission. This is important because while the limiter in the Rx channel is designed to attenuate excessively large RF power inputs that could cause LNA damage, signals at the input of the LNA may still be large enough to saturate the LNA, resulting in nonlinear amplification and resulting distortion of any input signals. Signal distortion could lead to adverse effects such as increased bit error rates for SATCOM signals or degradation of ES function capability to characterize modulation attributes of a threat radar signal. Use of the circulator also ensures that a RF signal entering the circulator from the radiating element is sent to the receive channel only, thereby providing isolation between the radiating element and HPA under these circumstances. Otherwise, RF energy may potentially be fed back into the HPA due to external signals picked up by the radiating element, or reflections from the radiating element as a result of element impedance mismatches. If such signals are large enough, the HPA may be damaged and/or suffer performance degradation [20]. Figure 2: Block diagram of a single Tx and Rx channel in an idealized MFRF system. Typically, the circulator, limiter, HPA and LNA are packaged together as part of a transmit/receive (T/R) module that is located close to the radiating element to mitigate system losses. In an ideal system, these T/R module components, along with the associated radiating elements, would have a sufficiently large bandwidth to accommodate the operating frequencies of all RF functions which may use the Tx and Rx channels. Based on the information in Table 1, this implies that operating frequencies of the radiating elements and T/R module components must extend from 0.5 GHz to 40 GHz for a naval MFRF system. Each radiating element of an array used in an ideal MFRF system is also dual orthogonally polarized so as to support all polarization states. This is necessary to meet all of the polarization requirements listed in Table 1, particularly those for the EA and ES functions. To prevent grating lobes in the array gain pattern for typical maximum array scan angles of \(\pm 60^{\circ}\), the radiating elements are spaced at a distance of \(0.54\lambda_{g}\), where \(\lambda_{g}\) is the wavelength value at the highest frequency of operation. From Table 1, the highest frequency is 40 GHz, implying an element spacing of 0.4 cm.