I Was Wondering When Someone Would Do This

It's a fact that from its beginnings, the 7-inch diameter AIM-120 AMRAAM has outperformed its 8-inch diameter predecessor, the AIM-7 Sparrow, (30 miles for the final version of the Sparrow as versus 40+ for the first version of AMRAAM) even though it weighs less than half as much, and contains less far less fuel than its predecessor.

Part of this is due to improvements in propulsion, new propellants and whatnot, and part of it is because the aerodynamic configuration (tail steering rather than the Sparrow's mid body control surfaces), but most of this is from an optimized flight path, because with its active seeker and sophisticated autopilot, the AMRAAM does not need to keep its nose constantly on target.

I've always wondered why someone hasn't applied active radar homing on a Sparrow sized missile.

Well, it looks like Japan is looking at doing this (paid subscription required):

Japan already has bought Raytheon AIM-120 Amraams, so why is it spending ¥36 billion ($468 million) to upgrade about 60 F-2 fighters with the Mitsubishi Electric Corp. AAM-4B missile?

Although the benefits to Japanese industry are obvious, details of the upgrade and the missile itself suggest that the program is giving an enormous boost to the Mitsubishi Heavy Industries fighter’s ability to counter enemy aircraft. The weapon has at least one advanced feature that other such missiles lack: a seeker with an active, electronically scanned array (AESA) radar.

The program will move into high gear in the financial year that begins April 1. The work is progressing in two parallel programs: integration of the AAM-4B missile, and upgrade of the J/APG-1 radar to a more powerful standard called J/APG-2. The improved radar, needed to exploit the new missile, will incidentally raise the capabilities of the aircraft by offering greater detection ranges.

Both systems have been developed by the Japanese defense ministry’s Technical Research and Development Institute with considerable help from contractors, Mitsubishi Heavy Industries for the missile integration and Mitsubishi Electric for the radar. The same companies are contracted to do the installation work. Ministry officials tell Aviation Week that development went smoothly and is now complete.

Early in the development program, in 2001, the ministry gave rough indications of the AAM-4B’s capabilities. It could be launched at a 20% greater range than could the then-current AAM-4 and at least as far as an “AIM-120B+,” a standard that was expected to appear around 2004. The crucial claim was that the AAM-4B could switch to autonomous guidance at a 40% greater range than either of the other two missiles and would similarly outperform what was expected to be the 2009 standard of the Russian R-77 (AA-12 Adder). In a 2010 paper, the ministry attributed the seeker’s greater performance to the higher transmitting power available from the AESA.

The implication is that an F-2 firing AAM-4Bs can stop tracking the target for missile guidance much sooner than an unmodified F-2 can—and officials tell Aviation Week that the key aim of the project is indeed to increase the range at which an F-2 can turn away.

The AAM-4B won't make any international sales, of course, because the Japanese constitution, and it won't fit in the F-22 or F-35 weapons bays, but I can see a larger being an equalizer on other aircraft.