Actually, something quite strange is that modern ARH generally use the Ku-band. This is actually advantageous; for example, PAC3 or 9M317M have extremely small beamwidth, Maybe only 2-3°. And AAM with smaller radar antenna apertures, such as the AAM-4B or MICA-EM, can also achieve a main lobe width of around 7° using the Ku band, which can significantly improve the target SNR.
However, the AIM-120 has always used the X-band since its inception. Perhaps the band used by RIM-174 remains unchanged. Even using the upper limit of 12 GHz for calculations using the IEEE X-band, the beamwidth of RIM-174 might still be as high as 6°.