MUSEUM "WAALSDORP"

Ultra short waves in the thirties

Around 1930 wavelengths below 10 m (or frequencies above 30 MHz) began to attract professional interest for applications. These frequencies were known not to be reflected by ionised layers and to be readily restricted to a relatively narrow angle ("beam"). An object within this angle comparable in size to the beam’s cross-section or greater however prevented transmission or reception. The first indications of interest of the Armed Forces in this area are found in 1933. The Measurements Building was then requested to assist in reception trials of a five meter transmitter in an air plane and the requirements for a future radio meteorograph for a weather balloon were discussed. Additionally the Measurements Building proposed the investigation of electromagnetic radiation below 10 m caused by the engine ignition of an air plane as basis for their location from the ground. The latter investigation was discontinued a few years later because suppression of this noise was needed for the plane’s radio receiving equipment and German planes began to be fitted with Diesel engines.

The difficulties in designing practical circuits some 70 years ago should not be underestimated. Each discrete component and even the wiring consisted of a mixture of resistance, capacitance and inductance. Skin effects and travel time of electrons in tubes played a role. In receivers the lack of effective means for either amplification or frequency transformation preceding demodulation was solved by the so called "super regenerative detection". This arrangement served amplification as well as amplitude demodulation and contained a periodically (e.g. 30 kHz) self interrupting circuit, fiercely oscillating at the frequency to be received with the main disadvantage of a high noise level in the absence of modulation.

Transmitters posed even greater problems in obtaining maximum high frequency power on the antenna. This necessitated careful high frequency isolation between the generator circuit and power supply as well as prevention of radiation leakage. Also temperature effects and frequency instability played a role. From 1934 onwards Von Weiler experimented with two (push-pull) and single tube transmitter designs down to one meter wavelength using high power directly heated triodes.

Ultra Short Wave receiver 1934
(full size30 KB)

Receiving 5 m air plane transmitter,
Von Weiler at the left; Van Soest at the right
(full size 20 KB)

Ignition noise measurements
on wingless air plane
(full size 37 KB)

Tube-heater "choke"
(tube with adjustable plunger)
(full size 13 KB)

"Temple" model transmitter
(push-pull circuit)
(full size 40 KB)

MuseumWaalsdorp@tno.nl