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I6KZR > TECH 04.01.17 18:15l 175 Lines 7484 Bytes #999 (0) @ WW
BID : 18296I6KZR
Read: GUEST
Subj: Re: Coherer Use
Path: IW8PGT<IR2UBX<SR1BSZ<OK0NBR<F3KT<F1OYP<GB7COW<GB7COW
Sent: 170104/1756 @:GB7COW.#44.GBR.EURO Sally 7.0.077 $:18296I6KZR
In un suo messaggio I6KZR ha scritto:
> The construction of the coherer was the subject of much experimentation.
> Often details of the construction of various coherers were kept secret
> from the competition because the operation of the coherer gave the key to
> success or failure of a radio communications system.
>
> A coherer that operated well would provide a significant advantage in any
> trials as the detector, of which a cohere was a form, was often the
> limiting element in the successful transmission and reception of a
> signal.
>
> Basis of operation of the coherer
>
> The coherer was a form of detector whose operation was applicable to the
> very early radio transmissions that were being used at the end of the
> nineteenth and beginning of the twentieth centuries.
>
> These transmissions used signals generated by a continuous spark which
> was turned on and off, or keyed to give the Morse code characters
> required. In this way, the operation of the early detectors such as the
> coherer only needed to detect the presence or absence of a signal.
>
> There are two types of coherer, and the operation of them is slightly
> different. The two types are:
>
> Branly coherer
> Imperfect junction coherer
>
> The basis of the coherer operation and construction is different for both
> types. The Branly coherer operation is what may be termed as the basic
> coherer operation and it will be covered first.
> Branly coherer operation & construction
>
> This type of coherer was the first to be invented and used and was used
> by many companies for inclusion in their radio communications systems.
> Typically it consists of a glass tube containing two electrodes and some
> metal filings of power. Often a mixture nickel and silver in equal parts
> was used.
>
> The coherer was constructed using a glass tube that was sometimes
> evacuated. The two electrodes within the glass tube were also brought
> close to each other. In the case of the Marconi coherer, this was to
> within a couple of millimetres of one another.
>
> When radio signal impacted on the coherer, the particles would cohere or
> join up and stick together. The mechanism behind this is a micro-weld
> phenomenon. This is caused by the radio frequency current that is caused
> to flow across the small contact area between the individual particles.
> Imperfect junction coherer
>
> The second type of coherer is known as the imperfect junction coherer.
>
> There are several variations to the imperfect junction coherer which is
> quite different to the Branly coherer. Essentially the imperfect junction
> coherer consists of a small metallic cup which contains a pool of
> mercury. This is covered by a very thin insulating film of oil. A small
> iron disc is then suspended above the surface of the oil film.
>
> An adjusting screw is then used to lower the disc until its lower edge
> just touches the mercury with a pressure small enough not to puncture the
> film of oil.
>
> Its principle of operation is not well understood but detection occurs
> when the radio frequency signal breaks down the insulating film of oil,
> allowing the device to conduct. This, in turn is made to operate the
> receiving sounder. The imperfect junction coherer is self-restoring and
> needs no decohering.
>
> The disadvantage of the imperfect junction coherer is that it is very
> delicate to set up.
>
> Coherer circuits
>
> The radio antenna and earth were applied directly across the contacts of
> the coherer so that the radio signal would be developed across the
> electrodes. When the radio signal was received, the coherer would become
> conductive.
>
> The electrodes for the coherer were also attached to a second DC circuit.
> This was decoupled using an inductor. This circuit would then either be
> presented to a set of headphones, in which a click would be heard, or it
> could be used to drive an inker to record the signal.
>
> Disadvantages of coherers
>
> The coherer was never a very satisfactory form of detector. It suffered
> from several disadvantages:
>
> Insensitive: The signal level had to be high to cause the particles
> in the device to cohere.
> Not resilient to interference: All forms of signal would cause the
> coherer to be activated. It was therefore difficult to operate under
> noisy conditions, especially when levels of static were high.
> Expensive: With all the materials used and the need for specialised
> glass tubes, coheres were expensive to produce.
> Only able to detect Morse signals: With the introduction of
> amplitude modulated signals for sound transmissions it was not possible
> to use the coherer as its mode of operation was not suitable for this.
>
> In view of its limitations and the fact that better detectors were
> introduced, the coherer fell into disuse for radio communications systems
> after about 1907.
>
> 73 Scott KF5JRV
> KF5JRV.#NWAR.AR.USA.NA
>
>
Hi Scott,
about the "copy and paste" function in scientific inventions.
Temistocle Calzecchi Onesti (December 14, 1853 – November 25, 1922) was an
Italian physicist and inventor born in Lapedona, Italy, where his father,
Icilio Calzecchi, a medical doctor from nearby Monterubbiano, was
temporarily working at the time.
His mother, Angela, was the last descendant of the ancient and noble Onesti
family. His first name is the Italian version of the Athenian general
Themistocles.
Calzecchi demonstrated in experiments in "1884 through 1886" that iron
filings contained in an insulating tube will conduct an electric current
under the action of an electromagnetic wave.
This discovery was the operating principle behind an early radio wave
detector device called the coherer, developed about 6–10 years later by
Oliver Lodge, Edouard Branly, and Guglielmo Marconi, which was influential
in the development of radio.
Since 1884 Calzecchi had been researching the properties of metal powders,
finding high electrical conductivity due to various excitations such as
extra current, lightning, electrostatic induction, etc.
Calzecchi's experiments with tubes of metal filings led to the development
of the first radio wave detector, the coherer, in 1890 by Edouard Branly.
This unit consists of a glass tube containing powder and nickel silver with
traces of mercury, placed between two steel electrodes in vacuum.
When it is hit by electromagnetic fields, the high resistance of the powder
becomes relatively low until it is "percussed"—that is, hit, to break the
welded bonds between the particles.
These studies by Calzecchi predate by nearly six years those of the French
physicist Edouard Branly and Oliver Lodge in England, although they are
largely credited with the discovery.
At the time, Calzecchi saw this as an invention for detecting lightning and
as a seismic detector, but a lively debate followed when Branly, Lodge and
Marconi used the coherer for radio.
The world is full of wolves. Alexander Graham Bell docet!
Frase del giorno:
When smoke descends, good weather ends.
73 Ciao a tutti Renzo.
Fermo li, mer 04 gennaio 2017 17:40 (+1.00)CET
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