"The proximity fuse was a radio transmitter/receiver that detected an object in its path. When the object was close enough, about 30 feet, the fuse would go off. Before the proximity fuse, the range to the target had to be estimated and that range dialed into the shell. This was difficult against a maneuvering attack aircraft. The trick was to get the tiny radios rugged enough to withstand firing from an anti-aircraft gun." Interesting official secrecy act release film, and information cont... at: YouTube - The Radio Proximity Fuse‏
Actually it was time that was "dialed into the shell" not range. I have an earlier comment in another thread about this.