Matching Networks and Phasing

Any AM antenna system consists of the antenna itself, some sort of transmission line to convey the RF energy from the transmitter to the antenna, and some sort of impedance matching network. Directional antenna systems consist of multiple radiating elements, transmission lines and matching networks. In this paper, we will examine in building-block fashion the components that make up such systems. In addition to the theory behind them, we will discuss practical aspects of network design and construction. 1.00 Matching Networks A vertical radiator, whether it is a simple non-directional radiator or an element of a multi-tower directional array, presents a complex impedance to the source feeding it. Transmission lines are used to couple the RF from the transmitter output to the antenna, and coaxial transmission lines have a characteristic impedance based upon, among other things, the diameter and spacing of the inner and outer conductors. The modern coaxial transmission lines commonly used in AM antenna systems typically have a characteristic impedance of 50 ohms. To properly and efficiently couple RF from the transmission line to the antenna, the impedance of the load must be transformed or “matched” to the impedance of the transmission line. In the simplest of situations, wherein the resistance of the antenna is 50 ohms, all that may be needed is a reactive component of an equal value and opposite sign of the antenna reactance to “tune out” the reactance of the antenna. This would leave only the resistive component of the antenna impedance, thus presenting a “match” to the transmission line. Seldom do such situations occur, however. A typical radiator will exhibit a complex base or driving point impedance that has a resistance of some value other than 50 ohms along with a reactance. It is thus necessary to use a matching network to transform the resistance to 50 ohms and eliminate the reactance. This can be done with any of several types of networks: L, Tee or Pi. Pi networks are seldom used in AM antenna systems. L networks are wellsuited to use in non-directional antennas where the phase shift through the network is not important. Tee networks are useful where control of phase is necessary in addition to impedance transformation. A matching network will either advance (lead) the phase of the current through it or retard (lag) it. The configuration of leading and lagging L networks is shown below: