Even in the latest Chinese F-7MG copies of the MiG-21 (equipped with more advanced Israeli and Italian radars ) this is still said to be a significant limiting factor for radar use. Also, because of the vacuum tube electronics, the radar system (like most other early generation radars) had a low mean time between failures (MTBF). The radar lacked look-down/shoot-down capabilities (though this was not at all a common feature of radar systems at the time), meaning that it couldn't intercept targets that were flying under the MiG-21, because the radar system was unable to filter out "ground clutter" return. Another disadvantage of the new radar (and other avionics) was the weight increase in the MiG-21 PF, which reduced the baseline MiG-21's otherwise impressive flight characteristics. The radar was simple to use and repair and, because of vacuum tube technology, was able to withstand a powerful electro-magnetic pulse (EMP), a side-effect from nuclear explosion, radiation harmless to humans but very damaging to modern solid state (silicon transistor) electronics. Its capabilities, as limited as they may seem, were offset by the standard PVO doctrine: pilots were tied to a ground-controlled interception (GCI) system, which, through ground-based radars and data links, provided interceptors more extensive and more precise information. Over the years, the West became more acquainted the RP-21 Sapfir during the Vietnam war in Indochina during the Six-Day and the Yom Kippur Wars in the Middle East. Its combat record is not impressive, mostly due to the primitive and overestimated early generation R-3R radar guided missiles. Also, the radar was rather quickly compromised because of defections, and its weaknesses were well known to the west, giving way for the creation of effective electronic countermeasures (ECM). Western pilots learned to distinguish the RP-21's rhythmic three tone pulse. Still, the RP-21 and its developments, like the RP-22 radar (NATO codename "Jay Bird") of the later MiG-21bis series, had a long career and the latter was even used for export purposes in downgraded MiG-23S and MiG-23MS interceptors.    https://www.cia.gov/library/readingroom/docs/CIA-RDP82-00038R001800190001-3.pdf https://www.cia.gov/library/readingroom/docs/CIA-RDP82-00038R001800180001-4.pdf