ADVANCEDTEMPLATE.MZT

ADVANCED LESSON 52

LEARNING OBJECTIVES and NOTES
Antenna Matching Units
5e.1 Understand that AMUs (ATUs) can “tune-out” reactive components of the antenna system feed-point impedance (before or after the feeder) and can transform impedances to an acceptable resistive value. Often called an Antenna Tuning Unit (ATU) this box of wide spaced large variable capacitors, large inductors and switches is used to match the antenna feeder to the input/output of a transceiver. It carries out several functions: It tunes out any inductive and/or capacitive reactance on the feedline leaving a pure resistive load. It matches the load on the line to the 50 ohm impedance required by most transceivers. It can help to reduce the transmission of harmonics As it doesn't actually tune the antenna it is better to call it an AMU (antenna matching unit). It is placed between the antenna and SWR meter and the transceiver. Ideally it should be placed at the antenna end of the feeder, but this is only possible if it is either a single band AMU or an auto ATU controlled by relays over a control line. In practice ATUs tend to be placed in the shack next to the transceiver so the operator can easily adjust it without going outside!
Understand that if the AMU is located at the transmitter, it will have no effect on the actual SWR on the feeder between the AMU and antenna. If the AMU is placed next to the transmitter, then it does not alter the impedance of the transmission line from the antenna. It "sees" an impedance at the end of the feedline and is adjusted to match this impedance to that of the transceiver.
Identify typical AMU circuits (i.e. T, Pi and L circuits). There are different ways of arranging the inductors and capacitors to make an AMU. The three main types are named after their shapes: A T-match looks like a letter T with an inductor forming the upright and two variable capacitors forming the top of the T. The T match will match a wide range of impedances, but can have more than one matching point. Some of the matches possible are more efficient than others. A Pi-match looks like the Greek letter Pi with a variable capacitor forming the horizontal top and a variable capacitor at each end. An L matching network only requires one inductor and one capacitor and is formed like an upside down letter L with an inductor forming the horizontal top and a variable capacitor forming the vertical section. The capacitor can be at either side. At the antenna side it is good at matching high impedance antennas and at the opposite side it is good at matching low impedance antennas. All of these are designed to match coax cable. They can all have a balun placed before the AMU to enable them to be used with balanced line. This is not considered ideal, but it does work well and a number of commercial AMUs have built in baluns.