Jamming
Conical scan radars can be easily jammed. If the target knows the general operating parameters of the radar, it is possible to send out a false signal timed to grow and fade in the same pattern as the radar lobe, but inverted in strength. That is, the false signal is at its strongest when the radar signal is the weakest (the lobe is on the "far side" of the antenna compared to the aircraft), and weakest when the signal is the strongest (pointed at the aircraft). When added together with the "real" signal at the radar receiver, the resulting signal is "always strong", so the control system cannot make an accurate estimate as to where in the lobe pattern the target is located.
Actually accomplishing this in hardware is not as difficult as it may sound. If one knows that the signal is rotated at 25 RPM, as it was in the Würzburg radar, the jammer is built to fade from maximum to zero at the same speed, 25 times a second. Then all that is needed is to sync the signals up, which is accomplished by looking for the low point in the signal (which is generally easier to find) and triggering the pattern at that point. This system, known as inverse gain jamming, was used operationally by the Royal Air Force against the Würzburg radar during WWII.
It is possible to arrange a radar so the lobes are not being moved in the broadcaster, only the receiver. To do this, one adds a second antenna with the rotating lobe for reception only, a system known as COSRO, for Conical Scan on Receive Only (compare to LORO, a similar system used against lobe switching radars). Although this denied lobing frequency information to the jammer in the aircraft, it was still possible to simply send out random spikes and thereby confuse the tracking system (or operator). This technique, called SSW for Swept Square Wave, doesn't protect the aircraft with the same sort of effectiveness as inverse gain, but is better than nothing and often fairly effective.
Read more about this topic: Conical Scanning