FH still occurs with in a certain spectrum. Indian EW system will save the frequency being transmitted at it by Pakistani Radar, and send it back to the radar.
They under estimated PAF's EW capability and didnt have a credible EW system of their own. The shot down Mig-21 was supposed to be carrying ELTA-EW pod 82xx series.
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An unsecured (un encrypted) radio can be intercepted and jammed. This could have been the case with Abhinandan Mig-21 as i read some news last year that IAF will revamp radios, cant exactly remember.
This is why PAF uses secure (encrypted) radios. IAF is now relying on Rafale and its EW technology. Somethings look good on paper. Open the IAF Wikipedia page, you will see IAF as a paper tiger, yet IAF lacks a dedicated EW aircraft such as EA-18G, J-16D, Tornado ECR etc.
It'll be a matter of time IAF plugs the gap and we'd back on the drawing boards again.