The effectiveness of ecm jamming is related to the specific radar being jammed. To jam all radars is not possible. different radars run on different frequencies and newer radars have the ability to change frequencies (since Vietnam). The jammer is used to target a specific radar with known frequencies that it uses. Most self contained radar jammers in fighter aircraft have limited frequency range and power. Remember that ecm is used for self defense not attack (There are aircraft designed that specific roll) (to bad none are on DCS world). ECM jammers are used to overwhelm a radar (such as a self guided missile). Basically it's a signal to noise ratio. Jammers create noise, but if your aircraft has a strong signal return , the jammer becomes useless. And last but not least, as soon as you turned it active, you are now announcing your location to everyone. That is why there are aircraft built for this role so bombers and fighters can go do their job.