COLD RECEPTION OF AUDIO FREQUENCIES
(1973)
KLIK HIER VOOR DE NEDERLANDSE VERSIE
The simple idea of the audio frequency receiver.
The idea was simple
Connect an antenna cable to the microphone input of an amplifier and you will hear everything between 20 Hz and 15 kHz (audible audio range) received by the antenna. I was a poor student. No TV or Hi-Fi set with pickup. A good mono radio and an old bike, I did not want to have more. I did like to do simple experiments. To make a 0V0 receiver with a tube, to live a week without a radio, but only with a crystal receiver for the short-wave. And now ... this simple experiment, receiving audio frequencies!
I got the idea after getting an old tape recorder. I cleaned it and it worked fine again. An antenna wire in my room just below the roof was connected to the microphone input. What I heard was hum and mainly broadcast stations, caused by RF signals rectified by the input stage of the microphone amplifier. A BCI filter had to be built in the cassette recorder to suppress the RF signals. And there were some more modifications. The cassette recorder motor was switched off to save power and to prevent interferences. And it had to be possible to listen to the signals via the internal speaker. Very important was that the tape recorder had batteries and could work independently of the mains.
I got an old cassette recorder.
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It did eat the tapes and the high frequencies were gone.
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In my room in the civilized world
In my room I could only receive 50 Hz hum. And tone messages that were transmitted on the power line, for example to switch on the street lights. Also clicks of light switches and thermostats of for example refrigerators could be heard. So it was clear to me that when I would receive anything else than hum and "man-made noise", then I had to find a place far from the civilization! Admitted, we had skipped the lessons in the drawing of schematic diagrams and designs of printed circuit boards with pen. But that could happen, in 12 years we would have ORCAD!
The first test location was my nice students room.
Far away from civilization in a beautiful winter landscape
With Peter I went to Alex at the countryside to test his receiver, as in his appartment he did not have space for an antenna. Not only Alex, but I also walked barefoot in the snow! A poor student has no boots! The fresh snow was very soft, but also very cold! What a difference with Peter his expensive, warm boots and two pairs of socks! And had his Murphy B40 to be carried by two people, my 0V0 receiver and receiver for audio frequencies could be carried with one finger. Soon I had ice cold red toes and we had hanged up an antenna in the cold snow, made of wire and insulators for electric fence. But also there only 50 Hz hum could be received. There was a perfect deserted place about half a mile from Alex's home. The distance from the antenna to the power line would be about 100 times larger, the 50 Hz hum according to my studybooks 10000x or 80 dB lower and should disappear in the noise. We walked in a beautiful winter landscape, quite a challenge for my bare feet in the snow! But certainly a nice place to receive audio frequency signals undisturbed by hum. No interferences from electric fences because the land is not used during the winter.
A nice place in a beautiful winter landscape to receive audio frequency signals without any interference!
Of course we had to try to walk barefoot in the snow!
But never do that when it is colder than -3C to -4C (26F)!
What could be heard?
The antenna was hanged up in a treetop. Really many signals could be heard! All kinds of discharges like thunderstorms and many cracking sounds. We thought that it were discharges between air layers with different temperatures. And also very weak still some 50 Hz hum, probably from a power line a few miles away, although the addition of the three phases should be zero. But I did not hear short whistles around approximately 10 kHz. They should be there too. But perhaps that the other interfering signals were too loud or we did not listen long enough. During a quarter of an hour we listened to these signals, then we went back and talked about other cold things. Like that cold gramophone records sound better, you have to put them in the fridge before you play them!
Funny, ice cold red toes in the snow! What a difference with the indoor life in the city! It was great not only to see this beautiful winter landscape, but also to feel the cold snow with my bare feet! "Why should you wear boots if you can walk barefoot? You're feeling more comfortable and it's even cheaper!" said Alex. Although ... my bare feet needed a few terribly unpleasant minutes to get used to the cold! Peter told that "Barefoot Power" was low power and "With Boots" was high transmit power. I did not know much then, so I learned something more then! A few years later I was a fanatic radio amateur and I made nice QSO's with a simple 1 watt transceiver. "Little Toe Power!"
Audio limiter circuit for headphones.
Nice afternoon!
The audio frequency receiver, made with simple means was nice. I did not expect that we would receive so much and such strong signals!
Also the very simple 0V0 receiver with regeneration and one tube was a nice experiment. And you could receive much more than you should expect with such a simple thing. But the reception was not noticeable better than on my students room just under the roof.
Of course, the beautiful Murphy B40 was much better than my 0V0. When I tuned my generating receiver to the B40's reception frequency, Peter did really hurt his ears because of the noise! Later I made a limiter circuit for his headphones.
And I would walk barefoot in the snow more often, after a few times, getting used to the cold became much easier!
I gave the cassette recorder away, so no further audio-frequency reception experiments more.
Modern VLF reception
If you want to know more about receiving audio-frequency signals, please check the following link. They also sell a kit of a receiver with a high impedance antenna input:
https://theinspireproject.org/
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