# Multifunction RF Systems for Naval Platforms
## 4 MFRF System Prototype Development Programs
### Amfrc
#### 4.1.2 Technical Description  
The system was designed to operate over 6-18 GHz. The original AMRFC design was based on a lower band, with an emphasis on the radar function. However, the US Navy decided early in the development to prioritize demonstration of modern EW and communication capabilities, which was better accommodated by the high band design. The AMRFC testbed design represents the trade-offs of Section 3 that were made in this case, based on the state-of-the art in RF and digitizer technologies at that time [44]. Other design decisions were driven by the goal to demonstrate real-time operation, given the limitations in processing power in the late 1990s compared to today. Key features of the AMRFC testbed design are described below. * The separate Rx and Tx arrays were each approximately 32 cm square, and populated with wideband dual-polarized radiating elements based on orthogonal flared notches. The element spacing was set on the Tx array to ensure grating lobe-free operation up to 18 GHz over a scan volume of \(\pm 50^{\circ}\) in azimuth/elevation. The arrays had a centre-to-centre separation in the same plane of about 3.7 m to ensure sufficient EM isolation when transmitting and receiving simultaneously. * The Tx array had 1024 elements, segmented into four quadrants of 256 elements each, with each quadrant further subdivided into four subarrays to yield a total of 16 subarrays. There was a RF Tx module behind each subarray, comprising a HPA and a pair of RF channels (one per polarization) feeding the subarray elements, with full amplitude and phase control provided in each polarization path. The HPA was a GaAs device, capable of generating several watts across the operating band in either linear or saturated modes. Note that with this relatively low HPA power, combined with the small Tx array size, the EIRP was only high enough to allow demonstration of a radar function equivalent to a short-range navigation radar, rather than the naval radar functions of Table 1; this was a reflection of program priorities, as well as limitations in cost and HPA technology at that point in time. There was a separate DWG allocated to each array quadrant, where each DWG included a DRFM component for coherent EA, and was capable of generating waveforms of up to 1 GHz bandwidth. By using photonic switches, each DWG could be routed to any or all of the four array quadrants for maximum flexibility. This configuration allowed the formation of Tx beams using any combination of quarter, half or full array, up to a maximum of four independent simultaneous beams (one per quadrant). * The Rx array had 1152 elements in total, grouped into nine 128-element subarrays, with an Rx module behind each element. Each Rx module had four independent RF receive channels: three linearly polarized channels, which were each fed by one of the two orthogonal flared notches of the dual-polarized element, and one polarization agile channel, which carried the sum of the two polarization signals from each element. It appears that there were no beamforming elements in the Rx channels. The Rx channel data was utilized as follows. * The RF signals from nine elements arranged in an interferometer configuration on the Rx array were downconverted and routed to a remote ES processor for precision direction-finding of narrowband strong emitters. This involved processing the phases between the nine inputs using interferometric algorithms to compute azimuth and elevation. The interferometric approach, while only feasible for strong emitter signals, has the advantages of covering a wide range of operating frequencies and wide field-of-view. The RF signals from two other elements were provided directly to auxiliary receivers for potential use by the DWG DRFMs in support of coherent EA techniques. * The RF signals from all the Rx modules in each subarray were combined downstream on a channel basis, that is, all of the Channel 1 signals from the Rx modules in a subarray were combined, all of the Channel 2 signals were combined, etc.