Small transmitters, ranging in size from a thimble to a matchbox, are available to permit eavesdropping and tracking, either to gain evidence or intelligence or for the wearer's safety. Most can transmit up to a quarter mile and many operate continuously for weeks. Doubling the voltage of a transmitter can double its output and range, but may result in more rapid burnout. Fresh batteries should be used in both transmitters and receivers operated by the backup team. Transmitters can be disguised as innocent objects to prevent detection. The signal from voice transmissions may be detected through feedback, interception, or by a radio frequency detector. Antennas may include those that look like boxes and combination helical/telescoping antennas. Recording equipment may include cassette, reel-to-reel, miniature, and standard-size recorders. Undetectable and voice-activated recorders are available, and several permit identification of tampering and precise location of desired tape segments. Wiretapping or telephone intercepts have become more complicated as a result of telephone company reorganization, computer applications, and technological advances both in telephones and transmission media. Pen registers and dialed number recorders are available for providing logs of telephone calls.