# 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 practice, a wideband microstrip circulator that may be used in a T/R module does not completely suppress signals travelling in the reverse direction around the circulator. At best, currently available circulators can provide about 30 dB of such suppression between circulator ports, as exemplified by the JC2S8000T12K0G2 microstrip circulator (JQL Electronics, Rolling Meadows, IL, USA) [24] which operates over 8-12 GHz. This is sufficient for isolation of the HPA from signalsreturned by the array element, but not for isolation of the LNA from the HPA output. The issue is that for a typical wideband LNA, exemplified by the HMC1049 LP5E LNA (Analog Devices, Norwood, MA, USA) for instance, the input power corresponding to the 1-dB gain compression point is about 1 mW [25]. The 1-dB compression point on the LNA gain curve indicates the point at which the LNA starts to become saturated. With a circulator providing 30 dB of isolation between Tx and Rx channels, the peak HPA output power must consequently be less than 1 W to ensure that leakage into the Rx channel does not drive the LNA into saturation if transmitting and receiving simultaneously. For some naval RF functions with a high EIRP requirement, such as radar, this is an onerous constraint. Normally, naval radars employ T/R modules with peak powers of at least 10 W to achieve the required EIRP with a reasonable number of modules. Consequently, usage of lower power T/R modules to accommodate simultaneous transmitting and receiving would imply the need for many more such modules to meet EIRP requirements, leading to increased cost, size and weight of the array. A potentially more serious issue relates to power reflected back from the radiating element during transmission due to impedance mismatches between the antenna element and free space. This reflected power emerges unattenuated from the circulator into the Rx channel, and as such, represents another source of leakage from the Tx channel. In a traditional mechanically scanned antenna, where the residual impedance mismatch is only a function of frequency, this mismatch can be electronically tuned out near the antenna to mitigate the problem. However, with an AESA, mutual coupling between radiating elements in the array result in an element impedance mismatch that varies both with frequency and scan angles, making it much harder to control [2]. This leads to element reflection coefficients that can be as high as \(-6\) dB when measured over a \(\pm 60^{\circ}\) scan sector and across a wide range of frequencies [26]. Assuming the same LNA characteristics as in the previous paragraph, and a \(-6\) dB element reflection coefficient, the peak HPA output power must be less than 4 mW to ensure that this reflected power does not saturate the LNA during simultaneous transmitting and receiving. Along the lines of the discussion in the previous paragraph, this would clearly impose a problematic limitation on AESA design with regard to those RF functions with high EIRP requirements. If separate Tx and Rx arrays are used, Tx and Rx channels are largely electrically disconnected. In this case, the principal source of leakage becomes electron/magnetic (EM) coupling between the two arrays, which can be reduced to an acceptable level simply by increasing the physical separation between them. For example, with a separation of a few metres between edges of a Rx and Tx array in the same plane, isolation values of greater than 80 dB can be readily achieved over a wide range of frequencies and array scanning angles [27]. With this level of isolation, high-power HPAs can be used on the Tx array without affecting simultaneous reception on the separate Rx array. It should be pointed out that a combined Tx/Rx array, meaning an array that uses T/R modules to enable sharing of each array element by a Rx and Tx channel, can be configured to emulate separate Tx and Rx arrays by utilizing two subarrays on the same aperture with a separation between them.