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Radio Telegraphy
345
The transmitter Marconi used to demonstrate his directive transmission system for extremely short wave worii. The table supports a pivoted disc which carries the transmitting antenna and the parabolic reflector. The direction of transmission is altered by orientating the reflector. The upright rods in the reflector are insulated from each other and are of one and one half the transmitting wavelength. Directional transmitting of this nature 6n 15 or 20 meters would present great difficulties for the amateur to overcome due to the complexity of building the reflecting system.. However, where the transmitter could be installed on the roof of a building and a higher building was adjacent to it, the reflecting wires could be suspended and
insulated from the latter
has to be increased to any considerable extent.
RECENT DEVELOPMENTS
IT WOULD be really interesting to know how much the increased number of C. W. transmitters, the development in directional reception, and the improvements in tuning that has taken place during the last few years have really increased our speed of readability and reliability over given distances.
As the development has been gradual, the tendency is toward pessimism, but I think we are now able at the same expense to work at about 8 to 10 times the effective speed that we were able to work at in 191 2 under the same atmospheric conditions.
Interference from other stations has, of course, enormously increased, and this has perhaps somewhat checked the increase of speed, but fortunately prevention of interference from other radio stations is a very much easier problem than the prevention of the disturbances caused by natural electric waves, or static.
Amongst the different types of tube amplifiers used in modern radio receiving stations, the tuned high frequency and audio frequency amplifier is probably the one which excites the greatest technical interest. In fact, its selective qualities, combined with the comparatively better ratio of signal strength to interference which it secures, justifies such interest.
If those researches were generally not quite successful in regard to preparing or fixing the