Every circuit in electrical engineering can basically be traced back to the diagram shown in Figure 1.1. A source Q feeds a load V via a transmission element U. The power Pi output from the source Q, and absorbed by the transmission element at the input is, as a rule, different from the power P₂ emitted by the transmission element at the output and absorbed by the load. To describe a system of this kind, three items are required:1) First, an understandable, i.e. easily visualised and at the same time physically correct, model of the processes at work in the system. 2) Secondly, characteristic values that are of physical significance and therefore capable of being measured, as well as being suited for the quantitative description of the components. 3) Thirdly, an algorithm to link the characteristic values on the basis of the model. Electrical engineering knows several models of this type, which can be used from case to case according to suitability or requirements.

Striplines are being used increasingly as conducting elements in circuits in the frequency range from about 300 MHz to about 30 GHz, the reason for this being the trend towards circuit miniaturisation. The technology of striplines has today improved to the point where comparatively cheap microwave circuits can be made, capable of good replication, which can completely fulfil the demands made on them. To a large extent, in the microwave range below 30 GHz, hollow conductors or other types of conductors only continue to be used in cases in which high transmission capacities are required, or in which stringent requirements are placed on the low attenuation properties of the lines.

Ever since its invention by Shockley and Bardeen in 1947, the transistor has acquired a scope of application in the field of electronics as an amplifying component which would originally have been undreamt of. In addition to the large number of small signal circuits, this semiconductor element has also established itself in the sectors of high-frequency power devices such as transmitters, industrial generators, medical microwave equipment, and radar systems. While transmitter amplifiers up to 10 kW were, until a few years ago, still the exclusive domain of tubes, there has now been an enormous and revolutionary swing in favour of HF power transistors. True, there is as yet still no semiconductor amplifier element which can produce this power; but, with the possibilities offered by modern circuit engineering, and in particular with the technology of wideband power distributors and combiners, it would be possible to connect together any desired number of individual transistors with a power rating, for example, of 100 W. In Germany, VHF transmitters have been developed with 10 kW output power which use, for example, about 20 transistors in the output stage. Japan has produced a 20 kW television transmitter with 100 transistors in the output stage; modern pulse transistors for radar applications produce up to 1 kW in a housing, and expensive travelling wave amplifiers are being replaced more and more by solid-state arrangements.

Until a few years ago, high investment costs meant that computer aided design, or CAD for short, was almost exclusively the prerogative of large commercial concerns. Essentially, until then the term CAD had been understood to mean computer-aided development and design in, for example, the mechanical engineering sector or for resolving PCB tasks in electrical engineering. Thanks to the rapid pace of development in the software sector, a large number of specialised areas have since arisen for which the expression computer assisted engineering (CAE) can be applied as an umbrella term for all computer-assisted activities in the development sector. A number of highly efficient programs have been developed for the analogue and digital high-frequency technology sectors, which make it possible to achieve the synthesis, analysis and optimisation of an electronic circuit, right through to the automatic generation of a layout.

HF systems such as telecommunications equipment, radar systems, and navigational systems contain a large number of modules which are connected to one another by lines. The individual modules themselves, and the lines which connect them, are subject to match faults, which cause reflectances and multiple reflectances, and could interfere with the function of the entire system. The smallest possible faulty matching is desirable for the exact functioning of a system for each module used, and for the connection lines. The production of HF systems would, however, be substantially more expensive, and as a result certain matching errors are permissible in the specifications for the system as a whole and for the individual subsystems. The modules are monitored with appropriate matching measuring equipment. In this context, it is extremely important to know the error limits of the measuring devices being used to ensure on the one hand that the specifications are maintained, and on the other that no disproportionately high expenditure needs to be incurred during the examination process. Different measuring processes and measuring arrangements also need to be compared with regard to their degree of precision.