the beginning of the 20th century, radio communication comprised
only "wireless" telegraphy. Ship mounted spark transmitters up
to 2 kW operated on wavelengths (as the unit then was) between 300 and 600
m. Land based stations up to 400 kW used progressively longer waves up to
18 km according to increasing ranges to be , relying on the propagation
path following the curved earth surface. These long range stations used
heavy rotating generators and extensive tuned antenna systems.
This situation had drastically changed by the time the Measurements Building was founded in 1927. The development of the triode vacuum tube had resulted in a different type of wave generation enabling modulation of the transmitter and amplification in the receiver. Experiments, initially by amateurs, in the early twenties ("transatlantic tests") with relatively low power at wavelengths below 300 m had demonstrated the existence of reflecting layers surrounding the earth and day - night differences in the transmission. So "wireless" had begun the development for as well commercial as public services (broadcasting) including speech- and music-transmission using short wavelengths down to a few tens of meters requiring smaller antenna systems and much reduced power. As to receivers the direct current (DC) anode voltage supply by a battery was gradually replaced by rectified alternating current (AC) and the indirectly heated triode for AC-filament supply had just appeared on the horizon. The first tetrode was introduced, to be followed in the subsequent years by multigrid- and more function- tubes adapted to more refined receiver circuitry. The growing interest, at the time again by amateurs only, in wavelengths below 10m is dealt with in ultra short waves in the thirties.
Transmitter Kootwijk Radio
ca. 1918 for telegraphy with the
Dutch East Indies.
Wavelength 17 km (17,6 kHz),
power 400 kW.
(full size 66 KB)