At 400km it will be very hard to hit an AWACS. Radar horizon is 9km altitude at 400km. Bellow that the radar won't see anything. The Awacs won't fly near an s-400 that high.
The radar from an AWACS aircraft travels enormous distances... generally as a rule of thumb more than three times the detection range of the system it is easy to detect because the radar itself detects targets at the detection range which is doubled because the radar waves travel out to the target and have to travel back to the antenna for a detection to be made.
An AWACS plane could be sitting on a runway... if its radar is operating and it is doing its job of AWACSing then the S-400 will detect the emission and be able to triangulate its location accurately enough to determine range and launch a missile. Being ARH missiles they don't need line of sight... they just need to know where to look... and a 400km range ballistic missile moves at over mach 7... proven in Desert Storm with modified Scud missiles... so a steeply diving missile moving at more than 1km/s which may or may not be using its own radar from directly above to detect targets... one second before impact it will be a spot above you... one second later impact... even if you saw it two seconds before impact... which would be damn impressive because nose on it will be smaller than a basket ball at more than 2km range... what sort of evasive action could you perform in a 747? None. You can't dodge it... even in a fighter plane if you immediately pulled hard on the flight stick and pulled a 12 g turn the missile would still be close enough for its 150kg warhead of HE and metal fragments would shower your aircraft and turn it into a burning sieve...
Best option against awacs is a su-57 with GCI armed with r-37 (400km) or r-77M(170-200km). Those missiles can be used at max range against such big aircraft and at those ranges the su-57 will not be detected by the awacs or detected too late to send its escort protect him. GCI means it will fly radar turned off so totally stealth.
AWACS planes emit radar waves and are easy to detect.... just like HQs and Comms centres on a battlefield... just monitor the cellphone network traffic and you will work out the area the bad guys are and then monitor foot traffic and individual cell transmissions and you will quickly work out which building to hit... AWACS... airborne warning (radar) and control (command and communications)...
Waiting that Awacs comes inside firing range of you air defence means you already lost the war. If the awacs can operate freely outside that then other ELINT can too and they will have good picture of your air defence position allowing them to destroy your forces.
Tracking down AWACS can be done with no emissions by an S-400 system in stealth mode.... they wont know it is there unless they see the single missile launch... and then it will move away... back into the IADS it is a part of...
S-400s and soon S-500s will also soon roam around the worlds oceans too... S-500s will not be cheap but a 600km range shot at an AWACS would be worth a missile...
HATO uses AWACS aircraft to coordinate attacks and operations so it wont be held back deep in friendly territory... it needs to be up in enemy territory looking for enemy aircraft and ground forces to coordinate attacks and resources... imagine an anti radiation version of Kinzhal hanging under a MiG-31... a Mach 9 2,000 km range anti AWACS weapon....
JSTARS and AWACS are integral parts of HATO.... they feed targets and commands into the western war machine to direct it where to go and where not to go... take away such force multipliers and countries like Malaysia and India can easily beat US pilots when it is one on one... because although well trained US pilots are nothing special... they have excellent support from assets like JSTARS and satellites and AWACS platforms to direct and coordinate their moves to make them look good.
The myth was that the 40N6 was only useful against awacs, etc...type of targets.
Because of the range involved most targets will be big.... B-52s, AWACS, JSTARS, and of course inflight refuelling aircraft and transport types will be most vulnerable... but coming down at mach 7 there wont be any chance of dodging such a weapon without jammers and decoys and a seriously good ESM suite...
Interestingly from above even a B-2 probably wont be stealthy...
Why ? At 7 or 8km it can still see far away. Only things flying very low further away than 400km won't be seen.
At 2km altitude it wont see 400km as you point out due to the radar horizon, but the radar signals it is emitting will continue travelling thousands of kms... making locating the AWACS pretty straight forward for ELINT and SAMs and other assets...
Anyway, AWACS are overrated. Their detection range is not enough anymore even against su-30 armed with r-77-1 there is little they can do. A su-35 with r-37 is impossible to intercept for its escort.
They are going to be in trouble, but new technology might change things with newer radar designs like photonic radars etc...
The AWACS will never go above enemy territory if the friendly forces have achieved total air supremacy or at least destroyed all the top fighters. It will stay at 100-150km from the front coordinating the interception of enemy fighters.
That is close enough for the 250km range S-400 missiles to hit them... and the 140km range S-350s could have a go as well...
Do you really think a boeing or a A-50 emmitting shit tons of radar signals could survive more than 2 minutes obove a land full of anti air ? Certainly not.
That is what we are saying... but HATO relies on their AWACS platforms for command control and coordination... it is their C4IR mobile base of imperialism...
But IMO, looking at how russian army are turning their eyes to small drones, they will use suicide drones against awacs and tankers when they are parked in the airport. Or kalibr.
Maybe a bundle of Kalibr or Iskander delivered suicide drones for those targets on the ground... and cratering munitions so any that survive can't get airborne...
You need to understand that the s-400 and other radars can't be deployed in enemy territory so they will always be behind the front line and safe from potential artillery. That's the weak spot of an IADS, the geographical limits of deploying radars where enemy can play with altitude to surprise you and destroy slowly your IADS.
Not true... Russia can deploy a wide range of high flying aircraft that can collect target data on low flying threats deep in enemy territory and emitting platforms like AWACS wont even need that... they give their own positions away through normal use.
Some s-400 can wait in ambushes but the front will be scanned by optics, stalites and drones putting them in danger.
Radar and radio silent S-400 batteries operating near borders can be covered by multiple overlapping systems from S-350 and BUK-M3 through Pantsir-SM and TOR-M3... when a HATO AWACS scans for targets 200km behind the front line the S-400 truck can launch a missile or two and then withdraw... HATO does not have an unlimited number of AWACS aircraft and when they start bursting into flames they will disappear from the battlefield pretty quick... either like a face mask delivery during a pandemic or in flames...
Maybe A-50U can guide active radar missiles but I doubt normal a-50 or fighters could guide them. Never heard of. I heard french frigates can control weapons of rafales to use it against targets designated by the frigate and about e-2 guided some SM missiles but in the very last variant.
The new model fighters are part of the network... the Su-30 was used to provide target data for a MiG-31 with the old short range radar for an R-37M launch of 300km. The S-400 400km range missile uses ARH like the R-37M and does not need and could not get a lock on target before launch anyway... all they need is information about where the target is and they look for them themselves as they approach the target area provided by another platform... or in the case of an AWACS target... its own emissions.
The Su-57 could probably use Link 12 L band emissions from the AWACS to locate it and launch an attack totally passively...