In the design of RF power amplifiers it is useful ( and important) to know how the output power of the amplifier gets influenced by changes of the the load impedance under varying conditions. In order to get an understanding of this, a useful technique is “load pull analysis”. It is a graphical ( usually) technique that uses the Smith Chart to plot the contours of the load impedance for fixed constant powers. It provides valuable information to the engineer/user about the performance of the amplifier for reasons of assessment of the quality of the amplifier, conditions of operation, design fit or various other parameters. A technical article on the technique has been released by Signal Processing Group Technical staff and is available for perusal by interested parties in www.signalpro.biz>complementary menu>RF amplifier design 2,  item.