[Prev][Next][Index][Thread]

Old ways of RF generation

I was reading a book about the history of radio, and I stumbled upon some
interesting (?) things. First, about spark-gap transmitters. It seems that
to improve quenching, Marconi started using a 400-600 BPS rotary spark gap
in 1904, and at about the same time, Telefunken started using a series gap
much like what we call the TCBOR/RQ gap now. Of course the construction was
very different way back then, but the idea is still the same. 

Also, Marconi seemed to have the cap across the voltage source, and the gap
in series within the oscillating circuit, while Telefunken had the gap across
the voltage source.

Then there is a stranger invention - an "arc oscillator" invented by William
Duddell in 1900, and improved upon by Valdemar Poulsen. In 1902, he placed
the arc in hydrogen gas to make the frequency higher, 100-1000 kHz range.
In 1906 in Denmark there was a 250 km wireless telegraph connection using such
a transmitter, at a power of 2.8 kW. Also during WW I, in America there were
several very efficient transmitters working on the principle, in the range
of 100 - 500 kW. I only wonder what makes it oscillate, otherwise it would
be quite a good high-power CW source for a tesla coil, if the efficiency is
right. 

Even better (and IMHO stranger) is the alternator, already worked upon by
Tesla and Thomson, but perfected by Alexanderson and Fessenden. It's a
big steel disk spinning around, with segments of brass and steel alternating;
then a magnet is placed on one side, and a coil on the other side, the amount
of magnetic flux varies according to which material is between the coil and
the magnet, hence a radio-frequency current in the coil. The top of the line
was a 200 kW, 12.5 - 28.57 kHz model during WW I. Of course it is mechanically
a very hard construction to make at home, so it is not a realistic power
source for TC's. The edge of the steel rotor in this "best" model was spinning
at a tangential velocity of about 500 mph... And the frequency tends to be a
bit low, unless really huge coils are driven.