"Thanks but that chocolate person while comparing f15 with su30 talked about that close the engines lesser is the plane signature to battlefield radar??"
I beleive she is refering to the surface area of a fuselage, and while the F-15 has engines that are in close proximity it's fuselage appears to be just as wide as the SU-30. For instance, the F-15's intakes are wide but it's engines are not, or in other words they merge together. so it's not how close the engines are, it's how much surface area a fuselage has. Although i will say this, the F-15 does has a flatter fuselage which may have a smaller radar retun.
So this is how it goes, from the frontal section an aircraft has a small RCS but once that aircraft shows it's fuselage, or goes vertical the RCS will increase. So does the F-15 have a smaller RCS from the fuselage? Perhaps but there is no way to tell, even if it did it would be negligible. Even more so if the aircraft has a combat load and drop tanks, and in combat aircraft are armed. Even if the aircraft is in a clean configuration it still has a jammer hanging underneath the fusalage.