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Moon radar experiment (Zoltán Bay, 1946)

Moon radar experiment (Zoltán Bay, 1946)

In 1946 a Hungarian scientist was the first person to detect radar echoes from the Moon.



radar, Zoltán Lajos Bay, radar signal, antenna, radio frequency, frequency, radar station, radar echo, wave, sign, measuring instrument, reflection, distance measurement, receiver, transmitter, Moon, Earth, speed of light, mechanics, experiment, physical

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  • At what speed did the transmitted signals travel?
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  • How many signals were transmitted over 50 minutes, i.e. the duration of the experiment?
  • Which of these is not true of\nZoltán Bay?
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  • The experiment resulted in the creation of a new branch of science. Which one?
  • Which of these is NOT true of the radar station used during the experiment?
  • In which company’s research laboratory did the famous experiment take place?
  • According to legend, from where did Bay want to touch the Moon in his childhood?
  • Which device was used for storing the signals?
  • In which century was Zoltán Bay born?
  • In which century did Zoltán Bay work?
  • The distance between Earth and which celestial body did Bay’s research team measure?
  • Is it true that Zoltán Bay worked on the development of gas and light tubes?
  • Who was NOT a contemporary of Zoltán Bay?
  • Who was a contemporary of\nZoltán Bay?


Experimental radar station

Radar station

The radar station is a 6 m x 8 m steel frame, fixed on a massive turntable. The radar’s angle of elevation is adjustable and the frame is equipped with 36 dipole antennas. The radar was installed on the roof of the research laboratory and the instruments were placed in two rooms on the laboratory’s 2nd floor, below the radar.

Radar antenna

  • steel frame 6 x 8 m
  • turntable
  • adjustable angle of elevation
  • 36 dipole antennas

Measuring instruments

  • electrically shielded cage
  • receiving unit
  • transmitting unit
  • power source
  • coulometer
  • switch

Measurement apparatus

Signals were sent to the Moon by the transmitter and their echo was detected by the receiver. Detecting the echo, however, was a challenge because the returning signals were very weak and electrical noise of external sources suppressed them. Nevertheless, Bay and his colleagues were able to find a creative solution to this problem.

Each measurement cycle lasted for 3 seconds. First, they transmitted a radar signal to the Moon then stored the echo about every 0.3 seconds. The measurement cycle was repeated several times and the signals detected in the corresponding phases were summed. As the sum of the continuously changing random noise increased more slowly than the sum of the useful signal, which had a constant strength, the useful signal reflected from the Moon could be distinguished from the background noise by integrating a sufficient number of measurement cycles. (This method is still in use today.)

The process was synchronised by a rotating switch that rotated at a constant speed. It sent out the signals, then directed the returning signals into 10 coulometers, with a time difference of 0.3 seconds. One measurement cycle lasted for 3 seconds and the switch operated for 50 minutes. This meant a total of 1,000 measurements.

Signal integration was performed by the coulometers. Electric signals received by the antenna were amplified and the current was used to split water into hydrogen and oxygen by electrolysis. The resulting hydrogen was fed into thin, liquid-filled tubes where it caused the columns of liquid in the tubes to rise. The level of the liquid showed the sum of integrated signals. Finally, they selected the tube with the highest liquid level; this was the one that indicated the time when the reflected radar echo returned.

Radar echo from the Moon

  • 2.5 s

Distance measurement

The measurement apparatus was installed near the end of December 1945. The experiments were carried out mainly at night because this way less electrical noise interfered with the operation of the machine. At last, on the 6 February 1946, the coulometer showed a signal above noise level.

Distance measurement with radio waves

  • 375,000 km



The illustrious Hungarian physicist Zoltán Bay was born in the last year of the 19th century. From 1936 he carried out his scientific experiments in Budapest, both in the research laboratory at the Tungsram Trust, and at the Technical University. His main research interests focused on vacuum, gas, and light tubes, and he also conducted numerous experiments on radio technology.

It was the latter field in which he achieved outstanding results. He led a research team whose mission was to measure the distance between Earth and the Moon using radio waves. The experiments began in 1945 and were successfully completed the following year.

On 6 February 1946 the team officially announced that radar signals had been sent to the Moon and that signals reflected from the surface of the Moon had also been detected. The key to the success of Bay’s experiment was the implementation of the formula for the recurrence and summation of signals. The experiment brought about a radical change in distance measuring equipment and led to the creation of a new branch of science, radar astronomy.

The microwave signals produced by the impulse generator located in the building were transmitted, via a transmission tube, to the antenna installed on the roof. The coulometer, developed by Bay’s colleagues, made it possible to store and aggregate the signals.

1,000 signals were transmitted over a period of 50 minutes, i.e. one signal every 3 seconds. According to the measurements, the signals returned after 2.5 seconds. Thus, the researchers calculated that the distance between the Earth and the Moon is 375,000 km.

The successful experiment was an answer to one of Bay’s childhood questions. I saw the Moon passing behind the tower and I asked the adults: If I climb the tower will I be able to touch the Moon?

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