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In the field of radio link and waveguide systems the A.T.&T. group in theU.S.A. has been paramounl. It is perhaps especially in this field that theenormous American resources have achieved their most unique results. Thetable 2 presents the data of the systems of current interest. The chief elementsthat have made possible the new TH link are travelling-wave tubes with 5 Woutput, ferrite isolators and the horn reflector antenna. Practically all othercomponents have at the same time been greatly improved.As regards the waveguide systems, the critical question is whether mmwave generators can be built sufficiently cheaply (klystrons or backward-waveoscillators). There are likewise certain problems involved in the travellingwavetubes for the regenerative amplifiers. It is possible that tunnel diodesmight be used for this purpose.During the fifties the requirement of broad band transmission over longerdistances past geographical obstacles led to the development of the scattercircuits. The scatter technique has been made possible by the development ofpowerful transmitter tubes and large antennae for transmitters and receivers.In the late fifties came parametric amplifiers and masers, as a result of whichmore sensitive receivers can now be built.Table 3. Basic data for scatter and satellite systems1950s1960sDesignationFrequencyBase bandNumber of telephone channels(frequency multiplex) . .Number of TV channels. . .ModulationModulation indexTransmitter outputAntennaeNumber of satellitesSatellite diameterNoise temperature of receiverTransmissionRangeEssential new technique. . . .Scatter circuit2000 Mc/s600 kc/s600FM21 kW10 m parabola3000° Kscatter300—400 kmSatellite system6000 Mc/s5 Mc/s6001FM with negativefeedback0.110 kW30 m parabola24 in 5000 km orbit100 m25° Kfree space propagation6000—8000 kmmasers, satellitesThe aspiration to enter outer space has, as we all know, also led to epochalresults in its bearing on long distance communication. Satellites for this purposemay be divided into two categories: active, with the same functions as a"normal" relay station, and passive, which function simply as reflectors. Asystem with active satellites and global coverage requires three satellites infixed positions at an altitude of 36,000 km, while a system with passivesatellites requires 24 satellites in a roughly 5,000 km orbit. Obviously a systemwith active satellites requires very much more of the satellite and its equipmentin respect to useful load and operational reliability than a system of passivesatellites in which the actual satellite is of the simplest imaginable constructionand also very light. The active satellite will also be very difficult to steer intoits proper orbit. Presumably, too, it would be easily disturbed by a powerfulradar beam. At least during the first half of the sixties, the passive satellitemust be expected to be the predominant type for purposes of civil telecommunications.109