# Multifunction RF Systems for Naval Platforms
## 3 MFRF System Design Considerations
### Practical Limitations and Trade-Offs
#### 3.2.1 Combined vs. Separate Transmit/Receive Arrays
In this scenario, only the Tx channels of the T/R modules for one subarray and only the Rx channels for the other one would be employed. Consequently, the EM coupling between the two subarrays would be the only contributor to leakage from the Tx to the Rx channels. EM simulation results at C-band have been reported for this type of configuration, involving two small subarrays separated by about 2 m on an array of wideband flared notch elements [28]. The simulations indicated acceptable isolation between the subarrays of at least 70 dB, modelled with the Tx subarray beam pointed at boresight and the Rx subarray beam scanning over \(\pm 60^{\circ}\) in both azimuth and elevation. However, a potential problem with this approach is that given the fixed size of the array, the only way to increase subarray separation to achieve the isolation required for simultaneous transmission and reception is to reduce the size of the subarrays. This leads to lower EIRP (due to a smaller numbers of T/R modules in the subarray and lower subarray gain) and larger widths for the Tx and Rx subarray beams. As a result, the subarray EIRP and beamwidths may fail to meet the requirements of the RF functions for which the subarrays are to be used. Another potential advantage of using separate Tx and Rx arrays is that it affords greater design flexibility than that which is possible with a combined Tx/Rx array. For example, antenna gain and beamwidth, which are functions of the AESA size, can in general be different for Rx and Tx operations. Separated arrays allow for the possibility of differently sized Rx and Tx arrays to be used to achieve this. Also, with separated Tx and Rx arrays, different technologies in principle can be more easily employed for fabrication of the Tx and Rx modules with which the respective arrays would be populated. For example, older gallium arsenide (GaAs) technology could be used for Rx modules since it is well suited for the fabrication of low-noise wideband LNAs. On the other hand, newer gallium nitride (GaN) technology, while currently more expensive than GaAs technology, is attractive for use in Tx modules, since it allows HPAs to be fabricated with five times more power output than GaAs HPAs within the same chip footprint [29]. The obvious disadvantage of employing separate Tx and Rx arrays in a MFRF system is that the number of required antenna arrays would be greater than that needed for the case where combined Tx/Rx arrays are used. The general rule of thumb is that AESAs provide useful coverage over scan angles of \(\theta_{0}=\pm 60^{\circ}\) relative to boresight. This results from the practical fact that impedance matching of AESA elements over large scan angles becomes increasingly difficult, and also from the theoretical observation that for any AESA antenna, the antenna gain varies as \(\cos\theta_{0}\) and the beamwidth varies as \(\left(\cos\theta_{0}\right)^{-1}\). At \(\theta_{0}=\pm 60^{\circ}\), the resulting antenna gain drop of 3 dB and the beamwidth increase factor of 2 start to become significant. Consequently, if combined Tx/Rx arrays are used, at least three such arrays are required to provide hemispherical coverage, and often four are preferred to minimize performance degradation at the edge of the scan patterns. Now considering the case of separate Rx and Tx arrays, these numbers would be doubled, adding at minimum another three topside antennas to provide the required coverage volume. Thus, the use of combined Rx/Tx arrays mitigate to a larger extent the contribution of topside antennas to overall ship RCS, and may simplify antenna installation due to the fewer number of antenna arrays involved. It is worthwhile to note that for stand-alone naval radar systems or communication systems, the problems with combined Tx/Rx arrays discussed in this section are not necessarily relevant. For naval radars, which typically use a pulsed waveform with a relatively low duty cycle, there is no requirement to receive while transmitting.