# Multifunction RF Systems for Naval Platforms
## 4 MFRF System Prototype Development Programs
### M-Aesa
#### 4.3.2 Technical Description  
The M-AESA system provided Tx functionality from 4.5-18 GHz and Rx functionality from 2-18 GHz. The extended frequency coverage at the low end of the spectrum for Rx, as compared to Tx, was to accommodate certain communication services. Two main antenna configurations were considered for the M-AESA program for the notional ship-mounted case [52]. The first configuration involves a wideband combined Rx/Tx AESA for each quadrant that is shared by radar, EA, ES and communication functions. It is representative of the ideal MFRF system architecture presented in Section 3.1, with all of its potential benefits. The second antenna layout comprises: a multiband or at least more narrowband combined Rx/Tx array that is utilized by radar functions and some in-band communication services; a smaller wideband Tx array for use by the EA function and the Tx portion of communication links; and a linear wideband Rx array to support ES and the Rx side of communication links. Note that beams formed with the linear array are narrow in azimuth but wide in elevation, so that AoA determination in the ES function is restricted to the azimuth dimension. Referring to the trade-off discussions in Section 3.2, the second configuration ensures sufficient isolation between Rx and Tx channels during periods of simultaneous reception and transmission. It also allows components of narrower bandwidth to be used at least for the radar function, with attendant advantages such as the availability of higher power HPAs and faster ADCs with higher dynamic range. It appears that this second configuration was ultimately selected, based on the developed RF components described below. The primary RF building blocks developed under the M-AESA program were wideband antenna arrays [28] and wideband T/R modules incorporating analog TTD beamforming elements [53; 54]. The wideband arrays were based on the type of flared notch element depicted in Figure 3. A test array was fabricated during Phase 2 of the M-AESA program [28]. It consisted of 25 \(\times\) 25 dual-polarized elements spaced about 1.5 cm apart, which provided grating lobe-free beamforming up to 10.5 GHz for scan angles within \(\pm\)60\({}^{\circ}\). The reflection coefficient for the centre element of the array was measured to be less than \(-\)10 dB over 2-18 GHz and over all scan angles. The simplified block diagram of the T/R module is presented in Figure 7. The wideband _ampl_ifiers (AMP), switches, attenuator, and TTD beamforming element were packaged together as a GaAs MMIC core chip. The entire T/R module had dimensions of 1.4 cm wide \(\times\) 5 cm long \(\times\) 0.4 cm thick. The 1.4 cm width requirement was a challenging one that was set by the specification for 1.5 cm element spacing in the array. The switches served to select between Rx and Tx paths, with isolation of greater than 40 dB provided over 2-18 GHz. Note that this design precluded the possibility of simultaneous reception and transmission within the same T/R module. The depicted arrangement of the two switches allowed the same TTD beamforming element to be used by either Rx or Tx signals. The TTD beamforming circuit provided up to 124 ps of time delay (equivalent to 3.7 cm in free space) by switching between \"artificial\" transmission lines realized with inductor-capacitor networks. This approach was used because physical transmission lines would have occupied an unacceptable amount of chip area for the required delay. The delay was controlled with a 5-bit word, where the least significant bit was equivalent to 4 ps of delay. The attenuator in the Rx chain was included to allow for tapering of the Rx signal across the array, which might have been desired for sidelobe suppression during beamforming. The purpose of the wideband amplifiers in both the Rx and Tx channels (to the left of the TTD element in Figure 7)  
Figure 7: Simplified block diagram of M-AESA T/R module. was to compensate for insertion losses introduced by the switches, attenuator and, mainly, the TTD beamforming element.