Retrotechtacular: [Zoltán Bay’s] Moon Bounce Coulometer Signal Amplifier

coulometers and antenna

In the years before World War II it was theorized that shortwave radio waves could propagate through the ionosphere relatively undisturbed and allow for a signal to be bounced off the moon and returned. [Zoltán Bay] calculated that the return signal would be too faint to be detected above background noise with the radio receiving equipment of the day. To overcome this receiver dilemma he devised a new receiving element consisting of 10 coulometers sharing a common tank of a water solution. Each of the coulometers had a separate electrical connector and when current flowed through the electrode, hydrogen bubbles would form in an attached glass capillary column. By periodically sweeping through all 10 coulometers using a rotating switch attached to the radar receiver, any radar echo as well as random background noise would be readable by the amount of bubbles in the capillary columns. A single radar echo would be indistinguishable from random background noise in the columns of bubbles, but if the sweep is continued for 30 minutes any periodic radar echo would show as an increased accumulation of bubbles in a respective column. By reading these coulometers and knowing the switching period you could determine that you were receiving a true radar echo from the moon.

What an amazing apparatus to amplify a periodic signal above background noise! Nowadays we would call this a long-time integrator or persistence measurement and it’s a relatively simple task. You can download and read [Zoltán Bay’s] paper on “Reflection of Microwaves From the Moon” dated 1946 in PDF form. His integrator apparatus details start on page 17.

It took some years but in 1946 [Zoltán Bay’s] receiving apparatus was tested and did confirm reception from moon bounce. However, U.S. Army Signal Corps with better crystal frequency stabilized equipment was able to perform the same task earlier as seen in the below video without the use of an integrator. Even though the U.S. Army equipment was superior for this task [Zoltán Bay’s] apparatus enjoyed years of service in the field of planetary radar observation where such a high sensitivity scheme was still necessary.

Retrotechtacular is a weekly column featuring hacks, technology, and kitsch from ages of yore. Help keep it fresh by sending in your ideas for future installments.


  1. oodain says:

    this is a great find, thanks for bringing it to my attention.

  2. Tom the Brat says:

    Utterly astounding.

  3. DainBramage1991 says:

    This is very cool.
    Radio technology has come a long way. These days you can do Moon-bounce with a ham radio, an amplifier, and a directional antenna.

  4. Okian Warrior says:

    The linked paper in the description is all kinds of awesome.

    Such impressive statements as:

    “The success of the Moon experiment adds a most direct evidence to other experiments which confirm the validity of the Copernican view.”

    “It is only recently that the technique of microwaves has enabled Man to leave his terrestrial realm and to make celestial bodies the objects of his experiments. Until now our knowledge of celestial objects and of the Universe has been based only on observations.”

    “In view of the danger of air-raids, the micro-wave laboratory — dealing with principal problems of radio location —was removed by the higher military command to Nogradveroce, where during the summer of 1944 the first equipment for the Moon-experiment was constructed.”

    Interesting stuff.

  5. SYNTRONIKS says:

    This is similar to a lock-in amplifier as well.

  6. Roklobsta says:

    Interesting. I remember my professor in a Comms. Eng. class at uni talking about such an apparatus for receiving echos off Venus. Of course, we had not much interest at the time as id Software had just released Doom and we wanted to get back to the computer labs ASAP.

  7. This is basically the same technique I use for my DCF77 clock decoder ( . Great to see the original works of the guy who invented the basic approaches. Although I got a lot of the ideas from the works of Mr. Shannon ;)

  8. fartface says:

    And today ham radio operators use moon bounce on a regular basis for communications.

  9. Greenaum says:

    It’s AMAZING that for so much stuff nowadays, we’d think “ah, use a microcontroller”, or for RF stuff, “buy some modules and some super-sensitive op-amps”. It’s just brilliant, because they had the essentials of the components we have now, ie tubes, but so few of them. So they really really went to the back of the brain-cupboard to come up with stuff from every discipline, scientists were much more generalists back then.

    Am I getting it right that the actual energy reflecting from the moon was electrolysing water in those tubes? That’s theory right up next to practicality, naked theory in your hands.

    Imagine the Sparkfun catalogue of the 1930s! Wire sieves connected up to rotating KHz-frequency generators, magnetic amplifiers. Those telephone amplifiers that used a rotating chalk wheel in a bath of chemicals, and a current altered the “stickiness” of the wheel, which ground against something and made a loud and low-quality amplifier! Or those compressed-air audio amplifiers. And everything has to work at 1,200 volts because of the mercury rectifiers, or whatever!

    They really, really, used everything imaginable, and then invented stuff that you barely could imagine. Cobbling scientific principles together from across the board. It’s like genius was striking 3 times a week.

    Anyway yeah, the old days. Amazing. You could discover a property of physics and build a business empire out of it. Or have some savvy pirate build an empire on your work, if you were unlucky.

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