n cases like this a balun must be used. This can be a radio frequency transformer used to isolate the signals from a direct connection to earth. Another method adopted on some antennas is to coil the coax, or use ferrite beads to prevent radio frequency energy propagating along the outside of the feeder. 92 Newnes Guide to Radio and Communications Technology Balanced feeder In some instances balanced feeders need to be used in antenna systems. Often called twin or open wire feeder this type of feeder is not nearly as widely used as coax but has a number of advantages in many applications. Cross-section Top view Conductors Plastic dielectric to act as spacer and protective insulation for the wires Conductors Figure 4.12 Twin feeder A balanced or twin feeder consists of two parallel conductors as shown. The currents flowing in both wires run in opposite directions but are equal in magnitude. As a result the fields from them cancel out and no power is radiated or picked up. To ensure efficient operation the spacing of the conductors is normally kept to within about 0.01 wavelengths. This type of feeder can take a variety of forms. An 'open wire' feeder can be made by having two wires running parallel to one another. Spacers are used every 15 to 30 centimetres to maintain the wire spacing. Usually these are made from plastic or other insulating material. The feeder may also be bought as a 300 ohm ribbon consisting of two wires spaced with a clear plastic. This is often used for temporary internal VHF FM antennas. It can also be bought with a black plastic dielectric with oval holes spaced at intervals. This type gives a better performance than the clear plastic varieties that absorb water if used outside. Like coaxial cable, the impedance of twin feeder is governed by the dimensions of the conductors, their spacing and the dielectric constant of the material between them. The impedance can be calculated from the formula given below: Antenna systems 93 where D is the distance between the two conductors d is the outer 