- Published in issue 7/1992 on page 46
Wideband active telescopic antenna
Most VHF/UHF amplifiers for use with a symmetrtcal antenna (such as an open or closed dipole) have a balun at the input. A balun (short for 'balanced to unbalanced') is an inductive device that converts the symmetrtcal (balanced) RF signal into an asymmetrical (unbalanced) signal that can be applied to the base ofa transistor. Unfortunately, baluns have an inherent loss of 2-3 dB, while most input transistors (typically a BFR91 or similar) have noise figures not better than about 2 dB.This explains the rather poor overall noise figure of 4-5 dB of this type of input stage.