# Multifunction RF Systems for Naval Platforms
## 3 MFRF System Design Considerations
### Practical Limitations and Trade-Offs
#### 3.2.2 Wideband vs. Multiband Operation
This outcome may not have a significant impact though, as Table 1 indicates that beamwidth requirements for all RF functions are comparable. Another resulting restriction in AESA use occurs with formation of transmit beams. Because RF functions operating in higher frequency bands of the supported bandwidth are constrained to use a smaller area of the AESA, the ability to generate multiple simultaneous transmit beams from different parts of the array to accommodate these RF functions may be adversely affected. Most of the analog components in the Tx and Rx channels have bandwidth limitations which may be additional drivers in a decision to use several AESAs to cover multiple frequency bands within the 0.5-40 GHz range of interest. These limitations are described as follows:  
* Circulator: For a combined Tx/Rx array that requires use of a circulator, currently available microstrip circulators typically are designed to have bandwidths of 0.5-4 GHz and isolation values of 20-30 dB. There are a few wider-bandwidth circulators available, such as the UIYModel UIYBMC1212A (Shenzhen, China) [33]. This circulator has a bandwidth of 10 GHz, extending from 8-18 GHz, but only provides about 13 dB isolation between ports, which may be inadequate. Connectorized circulators can have bandwidths of up to 12 GHz [34], but these circulators are likely too bulky to be included in T/R modules, and have isolation values of only about 15 dB as well. * Limiter: There are limiters available that cover the full frequency range of interest. An example is the 1 GC1-8053 (Keysight Technologies, Santa Rosa, CA, USA) which is a MMIC diode limiter that covers 0-65 GHz, with power limiting beginning at 10 mW [35]. * LNA: A number of wideband GaAs MMIC LNAs are commercially available with good performance specifications, although there appears to be none that cover the full operational frequency range. For instance, the Analog Devices HMC1049 LP5E maintains a gain of 15 dB with a noise figure of less than 4 dB over 0.3-20 GHz [25], while the Analog Devices HMC-ALH445 operates over 18-40 GHz with 9 dB gain and acceptable noise figure of less than 5 dB [36]. Figure 5: Wavelength-scaled array concept. * HPA: The Table 2 lists some key specifications for several commercially available MMIC HPAs with different bandwidths and operating frequencies [37; 38]. The first HPA listed uses GaAs technology, while all of the others in the table are GaN devices. As mentioned earlier, GaAs HPAs cannot produce power outputs as high as similar-sized GaN-based HPAs, but this one is included because a GaN device with a similar ultrawide bandwidth could not be found. The quantity PAE indicated in the table is _p_ower-_a_dded _e_fficiency, calculated as \(\text{PAE}=(P_{RF\_\text{out}}-P_{RF\_\text{in}})/P_{DC\_\text{in}}\), where \(P_{RF\_\text{out}}\) is the maximum HPA RF power output, \(P_{RF\_\text{in}}\) is the maximum RF power input, and \(P_{DC\_\text{in}}\) is the DC supply power required by the HPA. PAE represents the percentage of the DC supply power that is converted in the HPA to useful RF output power, with the remainder being dissipated as heat. All else being equal, a low value of PAE for the HPAs implies that a larger DC power supply must be provided for the AESA, and that the thermal cooling design for the array becomes more challenging. The table indicates that both HPA output power and PAE generally decrease as bandwidth and operating frequency increase. While the first HPA in the table covers the full frequency range of interest, its output power of 0.25 W and PAE of 10% are by far the lowest of the HPAs listed. As mentioned in Section 3.2.1, an output power of at least 10 W per element is generally required to accommodate high EIRP RF functions like naval radar, so this HPA would be unsuitable. The other HPAs listed may be appropriate MFRF system candidates, although they cover smaller bandwidths. The TGA2813 and TGM2635-CP (Qorvo, Greensboro, NC, USA) are designed specifically for S-band and X-band radars.