http://www.worldwar-two.net/weapons/barrage_balloons/
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The balloons were huge (on average, about 18.9 meters long and 7.6 meters in diameter), fixed to the things that being protected or from the back of lorries with a winch. By 1944 the balloons were moved to make up a ring around south London to combat the V-1 menace with a fair degree of success - as many as 100 V-1s snagged themselves on the balloons' cables. It was not all plain sailing, however. Some of the balloons were struck by lightning while others were shot down - 50 were shot down in one day when they were set up round Dover. (The Scottish physicist Arthur Vestry [1869-1959] later devised a method for protecting barrage balloons from lightning.)
During the Battle of Britain and throughout the war, the balloons proved their value. Besides protecting strategic cities and ports, barrage balloons mounted in boats defended estuaries against mine-laying aircraft. A declassified wartime report assessed their performance: Following the aerial sowing of mechanical mines, the reallocation of various units of the balloon barrage system to places like the Thames Estuary, and certain other channels, has resulted in effectively reducing the aerial mine sowing operations of the German Air Force. Barrage balloon cables also successfully frustrated German attempts to achieve surprise, low-level penetration at Dover.
The Dover incident deserves elaboration because it provided, in the words of Air Marshal Gossage, a clear indication of their [the Germans'] respect for the British barrage balloon. In an attempt to clear the balloons from Dover, the Germans launched a major effort in late August 1940. They destroyed 40 balloons but lost six aircraft in the process. Much to the Germans' misfortune, 34 new balloons appeared the very next day. Air Marshal Gossage commented on the action: The protective balloons still fly over Dover. The attack on the barrage has proved too costly... In general, major attacks on balloon barrages have ceased, the enemy realized that the game was not worth it. The fact, however, that he hoped to destroy our balloons is in itself proof of the utility of the barrage. During the Blitz, 102 aircraft crashed in the cables, resulting in 66 crashed or forced landings.
After the Battle of Britain, balloons continued to prove their effectiveness in combat. Because of heavy losses during the day, the Germans switched to night attacks. Defensive night fighters were still in their rudimentary stages of development, so guns and balloons had to do most of the work against German bombers. Even after advances in night-fighter technology, it was the opinion of London that balloons and guns were still essential. Two examples illustrate London's sentiments. First, an installed aerial barrage at Norwich that surprised the Germans and diffused their bombardment by forcing them to attack above 2,400 metres. Second, the barrage balloons at Harwich saved that city from an attack by 17 bombers because the Germans went after their secondary target at Ipswich-Felixstowe, a place not protected by balloons. Overall, balloons decreased the severity of night raids on England by deterring point-blank bombing. Incidentally, they also had some tangible results in February and March of 1941, in that seven enemy aircraft crashed after striking cables in various parts of Great Britain.
Given the limited range of Dragon breath weapons, barrage balloons around Ft Salby or other Sharonan portal fortifications to deter further Dragon dive bombing attacks are a worthwhile investment.