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Research Paper - EVP Reception with Coherers - Basic considerations by Andres Ramos


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The use of the coherer effect is relatively new for ITC. The first occurrence l encountered in an article of the german VTF association. The carbon powder cell described in this article appeared very familiar to me as I had intensively studied the structure of coherers and thus the works of Branly, Marconi and J. C. Bohse. A coherer is an amazingly simply and effective early device for the reception of radio wave energy. It contains fine granular media, particulary metal filings of iron, silver and nickel and two electrodes in lose contact with the filings. The filings are normally covered with thin films of non-conductive media like oxide. In idle mode the coherer has a high impedance of several megaohms. If a radio wave hits the coherer there is something amazing happening. In a moment of some nanoseconds the impedance drops down to some 10 ohms and the coherer may switch a relais or drive a lamp with current.

Apart from this amazing effect I discovered that a dc current running through the coherer, provided a proper configuration of the filings, generates a strong noise! To scrutinize this effect I fabricated coherers myself. What you need for this is a file and different metals. I used iron, aluminum and silver

Coherer-6.jpg

Coherer-5.jpg

I used a vise to fix the material and then filed it down until I had enough filings to fill a small glass tube with them.

Coherer-4.jpg

 

I made a simple device to apply a voltage and an adjustable current to the coherer.

 

1_coherer_evp_receiver_mark_ii.png

 

On a lathe I cut a plexiglas rod and drilled in a cavity with an electrode at one end to fill it with filings. The second electrode was a screw thus I could adjust the pressure on the filings very accurately.

Coherer-1.jpg

 

Coherer-2.jpg

 

Coherer-3.jpg

Coherer-8.jpg

Experiments with the coherer setup

I made tests with iron, aluminum, a nickel-silver mixture and graphite powder (used for lubricating locks from a hw store). Iron gave average results. The noise was not very agile. Aluminum was very eager to noise with huge amplitude changes but it was unstable. The noise ripped off very fast and the coherer needed to be readjusted. An outcome of the fast oxydization of aluminum in air. See below two signal examples from my tests.

 

Aluminum noise signal

Aluminum noise.png

 

Graphite Powder signal

Carbon noise spikes.png

 

I analyzed the spectra in Audacity. They were more or less the same. For me that was evidence that the signal quality was an outcome of the naked effect and not the material that emitted the effect. It seems that any conductive material in lose contact would do. What really differed between the materials was the stability of the noise. From this point of view nickel-silver and graphite were superior over the rest.

Typical spectrum of a coherer signal

Spectrum carbon.png

Principally the voice quality is rather bad, however there is a steady stream of voices and thus some of them have better quality if you catch the right moment where the actual spectral composition of the noise was at maximum. I needed to do a lot of post processing, mainly denoising and filtering to achieve acceptable results.

A collection of audio sample exports as mp3 can be found here.

Later i made a heavy simplified version of the coherer that also worked excellent. It contained a piece of plastic tube and two screws as electrodes.

Coherer-9.jpg

See the attached test report for more details on the coherer tests.

 

ITC-Report 2019-G-001.pdf

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  • Keith J. Clark changed the title to Research Paper - EVP Reception with Coherers - Basic considerations by Andres Ramos

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