# Multifunction RF Systems for Naval Platforms
## 3 MFRF System Design Considerations
### Practical Limitations and Trade-Offs
#### 3.2.2 Wideband vs. Multiband Operation
Furthermore, the employment of relatively narrowband radiating elements in the AESA yields the attendant benefit of being able to readily source narrowband components with the required performance characteristics for the associated Rx and Tx channels. Figure 3: Dual polarized AESA element based on flared notches. Another AESA-related issue is the separation between elements on the array. As indicated in Section 3.1, in order to prevent grating lobes within the full operating frequency range and scan angle sector of \(\pm 60^{\circ}\), this spacing should be \(0.54\lambda_{\mathrm{g}}\). However, assuming a wideband AESA with a 10:1 bandwidth, this means that for all but the highest frequency within the supported bandwidth, the number of elements populating the AESA would be more than required, by a factor between one and 10. This is a concern because the cost of an AESA implementation is largely proportional to the number of elements used, with the cost of the Rx and/or Tx channel electronics associated with each element being the main cost driver. To appreciate the scale of the problem, consider a MFRF system utilizing an AESA with a 10:1 instantaneous bandwidth covering 1-10 GHz. Referring to Table 1, if the radar volume search function is being conducted in L-band at 1 GHz, then the one-way beamwidth requirement of \(2^{\circ}\) for that RF function dictates that the array size is about 7.6 m per side. If the element spacing is then set to \(0.54\lambda_{\mathrm{g}}\) to avoid grating lobes at 10 GHz, then a total number of 220,000 radiating elements is required for the AESA. One way to reduce the overall element count is to further divide the frequency range of interest into multiple smaller bands, each band with its own AESA. This allows the AESAs covering lower frequency bands to utilize less elements as a result of larger allowed element spacing. However, this solution carries with it all the previously mentioned disadvantages of additional antenna apertures that need to be installed on the ship. An alternative multiband approach that maintains use of the single wideband array with the same overall size and bandwidth is based on the implementation of different element spacing in various zones on the AESA. This idea has been referred to as a wavelength-scaled array [32]. The concept is depicted in Figure 5 which indicates the element locations within the different zones on a wavelength-scaled AESA. In this example, the element spacing in Zone 3 is twice that of Zone 2, which, in turn, is twice the element spacing of Zone 1, where the Zone 1 element spacing is chosen to ensure that there are no grating lobes at the highest frequency of operation. The outer dimensions of each zone are successively doubled in progressing from Zones 1 to 3. Assuming that the AESA is required to operate over a 10:1 bandwidth, say from 1-10 GHz, then Zones 1 to 3 support grating lobe-free operation within frequency ranges of 1-10 GHz, 1-5 GHz and 1-2.5 GHz respectively. This implies that RF functions operating in the 1-2.5 GHz band may use the full array, namely, Zones 1 to 3; those functions operating at frequencies between 2.5-5 GHz are restricted to use of elements in Zones 1 and 2; and RF functions active in the 5-10 GHz band may only employ the Zone 1 elements. The total element count for the AESA in this case is about six times less than that which would be required if the array was fully populated with  
Figure 4: Geometry of dual-band array. elements at spacing d = 0.54\(\lambda_{\text{g}}\). In addition to lower costs, this reduced element count also results in less AESA weight. However, the disadvantage of the wavelength-scaled array approach is that it reduces flexibility in configuring the AESA of a MFRF system. For example, the decreasing size of the array area available to RF functions as their operational frequency band increases leads to formed beams that have approximately the same minimum widths for all functions.