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Guglielmo Marconi: Application of Radio Waves to Communication, 1918

Improving Wireless Telegraphy

The road to improving wireless telegraphy involved: (1.) strengthening and refining the transmitted signal and (2.) increasing the sensitivity of the detecting apparatus.

The strength of the sending signal was mostly dependent on the power of the spark source. Marconi built power plants with increasing capacity, reaching 25 kW at his Poldhu station. Parabolic reflectors were used to concentrate and redirect the signal. Increasingly high, precariously located antennas were installed in the mistaken belief that antenna height exponentially increased signal transmission distance. The steel masts at the Poldhu station were 250 ft high.

The Branly coherer was the first instrument used to detect radio signals. It relied on the phenomenon that loose metal filings will merge and become a conductor in the presence of a weak electric current caused by radio waves. The coherer consisted of a glass tube containing nickel and silver filings with terminals fused into each end of the tube. A hammer arrangement was also attached to disrupt the metal after each passage of current, making it ready to react to the next signal received. Marconi built on the coherer principle to build and patent increasingly sensitive magnetic detectors.