More like distortion.
Given the wingspan of Il-76 of 50 m. The biggest "rotodome" it can take is 15 m. Looks big but you might want to also consider the weight of the radarset. For 3 faced array, the available aperture width is about 73% of the available Diameter, thus the width can be about 11 m and 2.1 m in height (There is rule of thumb that AEW rotodome is 20-30% of the wingspan in diameter while the thickness is about 20% of the diameter)
Eliptical 11 x 2.1 m give 18.14 sqm aperture area. Assume COTS TRM from say, Shenzen Yonlit (yes they do sell L-band module) with frequency of 1430 MHz or about 0.2 m wavelength. Using triangular antenna grid. Each array face would have 1243 TRM. Each TRM in that frequency can weigh about 4 Kg. Thus each face can weigh 4.9 metric tonne. 3 Faces would weigh almost or 15 tonne.
a Phased array radar can have ratio between antenna to entire radar set weight of about 9-70%. 9% from Russian Zhuk-AE, 70% is from Czech republic concept of a ground based air transportable radar which got lost to Israeli Radar, middleground is 15% but this is for PESA (AEGIS, Irbis, Zaslon). If we take that middleground, the entire radarset would weigh 100 tonne, which Il-76 cannot carry. 70% would be ideal, tho i dont think the structural supports and other radar support electronics for airborne example and moving platform can be equal to those stationary radar.
So yeah, going beyond 10 m diameter is, i think bit hard to achieve especially if the Il-76 being used is the Il-76TD with payload capacity of some 46-48 metric tonne.
10 m diameter rotodome still give adequate performance with manageable weight of some 43 metric tonne. range is about 312 vs 2 sqm target with Probability of Detection of 90%.
