RadCom Basics May 2022, Edition 28

RadCom Basics 17 Why do you need to use a balun? By Lee, G4EJB If there’s one topic that seems to receive attention these days, par- ticularly if you’ve been through the great training programmes availa- ble, it’s the use of a balun. It even receives a mention or two many of the books available to us, from the excellent Foundation Licence Manual to the highly acclaimed HF Antennas for all locations. So why so much emphasis on a device that connects a balanced load to an un- balanced coaxial line? Let’s consider a bal- anced antenna - the half- wave dipole.The antenna has equal and opposite currents in each half and doesn’t need a connec- tion to earth to work. It has a low impedance feed point suitable for coaxial cable. What a simple an- tenna system in an ideal world? But your radio has an unbalanced output - the outer of your SO239 socket is connected to the radio’s chassis and then to earth. And to cap it all, coaxial cable is also unbal- anced [1]. Then there’s the real life envi- ronment, the dipole is probably not at some amazing height above the ground and there may be buildings, metal poles, trees and other objects Figure 1: The ideal - showing electrical field lines only