There was an intense countermeasure followed by counter-counter measures war fought between the Germans and the British, particularly in regards to the night bombing campaign. The Germans tried to use Allied radar emissions to their own advantage, rather than try and jam them in most cases. For example, when heavy use of Window made tracking Allied aircraft more difficult using the Giant Wuerzburgs, the Germans would simply follow the progress of the bomber streams by following the bombers own radar emissions, so they didn't want to jam them. The Germans also developed passive beam following devices that could help them hone in on the enemy radar beams and actually target enemy using the enemys own radar. An example of this was Flensburg that honed in on the tail gun firecontrol radars of bombers.
The Germans did jam radars using active noise jamming when required though. In the Med the Germans utilized powerful noise jammers that shut down almost all of the many Allied radars operating on 150cm. The US Army's Flak radars were shut down completely at Anzio, but after a couple of weeks the Germans quit using jamming, because the Allies would continue bussiness as usual although their radars were in fact ineffective.The Germans assumed that the jamming hadn't worked very well. The Germans used the same tactic. The Germans would sometimes also pretend that the jamming had worked on their radars when in fact it had not. They successfully lead the Allies to assume that their noise jammers to be used vs the Freya and Seetakt radars were very effective when in fact they were not. After the war statistical analysis indicated that bomber intercepts and losses did not drop off until after the forward Freya location swere overran by Allied troops, and the Allied experts had only thought that they were jamming the Freyas.
The Germans successfully used jamming vs the all the known British radars during the channel dash. They gradually introduced jamming in the days proceeding the dash; increasing the power by degree and mimicking the natural noise of atmospherics and the noise caused by solar radiation. By the time of the actual channel dash the British radars were having all kinds of "atmospherics" problems. Some British techs smelled at rat, but by then it was already happening.
Steve is correct to point out that passive jamming was usually more effective vs search radars, although active noise jamming can significantly reduce the effective range of an enemy radar by degrading the signal to noise ratio, even if it doesnt jam the radar completely. However, the Wuerzburg could still detect targets with a S/n ratio at or below unity, so Window, or foil cut to 1/2 the wave length, was a better counter measure-for awhile. The Germans also knew about foil, or chaff, and they called it Dueppel. The Germans didn't use it earlier because they didn't want the British to find out. Once the British demonstrated that they also knew about chaff, by using window against Wuerzburg, the Germans began to use Dueppel to good effect on Allied radars. Even as early as 1943 U-boats could release balloons that would then release chaff cut to 5cm. Another trick used by the Germans were fake schnorkels in the form of floating reflectors that reflected radar signals much like a schnorkel would.
Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.