frequencies (known as "barrage" or "frequency diversity" broadcasting) to the already heavily overcrowded shortwave bands to such an extent that many broadcasters not directly targeted by the jammers (including pro-Soviet stations) suffered from the rising levels of noise and interference. There were also periods when China and the Soviet Union jammed each other's programmes. The Soviet Union also jammed Albanian programmes at times. Some parts of the world were more impacted by these broadcasting practices than others Eurasia (worst affected, including mediumwave frequencies particularly 720 kHz used by RFE) North Asia, Americas and Sub-Saharan Africa (partly affected) Australasia, South America (rarely affected) Meanwhile, some listeners in the Soviet Union and Eastern Bloc devised ingenious methods (such as homemade directional loop antennas) to hear the Western stations through the noise. Because radio propagation on shortwave can be difficult to predict reliably, listeners sometimes found that there were days/times when the jamming was particularly ineffective because radio fading (due to atmospheric conditions) was affecting the jamming signals but favouring the broadcasts (a phenomenon sometimes dubbed "twilight immunity"). On other days of course the reverse was the case. There were also times when jamming transmitters were (temporarily) off air due to breakdowns or maintenance. The Soviets (and most of their Eastern bloc allies) used two types of jamming transmitter. Skywave jamming covered a large area but for the reasons described was of limited effectiveness. Groundwave jamming was more effective but only over a small area and was thus used only in/near major cities throughout the Eastern Bloc. Both types of jamming were less effective on higher shortwave frequencies (above 15 MHz); however, many radios in the Soviet Union didn't cover the higher bands. Skywave jamming was usually accompanied by morse signals in order to enable (coded) identification of the jamming station in order that Soviet monitoring posts could assess the effectiveness of each station. In 1987 after decades of generally refusing to acknowledge that such jamming was even taking place the Soviets finally stopped jamming western broadcasts with the exception of RFE/RL which continued to be jammed for several months into 1988. Previously there had been periods when some individual Eastern bloc countries refrained from jamming Western broadcasts but this varied widely by time and country. In general outside of the Soviet Union itself Bulgaria was one of the most prolific operators of jamming transmitters in the Eastern bloc with East Germany the