Edge Localized Modes (ELMs) are known to affect dramatically the coupling of the Ion Cyclotron Range of Frequencies (ICRF) antennas. In order to investigate the influence of transient effects on the operation of the ICRF systems, data acquisition systems with high time resolution are used on JET and ASDEX Upgrade. It wasfound on both machines, that the response of the four toroidally distributed antennas to ELMs is not simultaneous. The observations indicate constraints for design of an efficiently working ELM-resilient ICRF system. The observed asymmetry can also provide additional information on ELM physics. This paper presents studies of the delays in appearance of the perturbation on RF signals due to Type I ELMs for different ICRF antennas on JET and ASDEX Upgrade.