User:LaurentianShield/sandbox/Spark-gap transmitter

This article exists, but has a bunch of problems. Foremost is that I think the editors who have been working on it don't understand a couple of key aspects of the operation of the device, and so therefore can't put the rest of the article in its proper perspective. This coincides with the fact that the article is lacking in-line citations. Much of what needs to be fixed is the history, but I don't want to start with that because I believe that putting the history in the proper perspective follows from a sound understanding of how the technology works.

Note that I had an important breakthrough today in my own understanding, which is that "in the day" when scientists and engineers used the term "induction coil" they meant a coil/interrupter system. Nowadays an induction coil is just the coil set, such as the ignition coil on a car. With the integrated interrupter, the induction coil automatically provides the AC signal and boosts the low voltage (say 12 volts) up to the spark breakdown potential. In what most modern engineers would think of as an induction coil, the provision of "alternation" (so to speak) is provided externally.

Meanwhile the circuit schematic in the article is simplistic to the point of being wrong, does not allow for the important improvement by Braun wrt the coupled antenna, and does not allow for a proper explanation of the use of and need for quenching (even though quenching is mentioned) let alone synchronous quenching.

I think (as I write this 2015-09-18) I am going to stick with the general structure of the "operation" section, but not even going to use any of its current text as a starting point.

Operation
The circuit show in Figure 1 depicts a canonical spark-gap transmitter as first used by Marconi (and which followed directly from Hertz). It is important to note that the induction coil was actually a coil with an integrated interrupter, so that the signal at the primary had a time-varying aspect, a requirement for the induction coil to be able to boost the voltage from the primary to the secondary. This interrupter used the magnetic field of the coil set to pull a hammer in tension with a spring, and when the hammer contacted the primary it briefly shorted the coil to ground and then sprung back. The repetition rate of these interrupters were in the range of 60 to 100 cycles per second (the modern unit is hertz),

As long as the key was pressed, these 100 hertz current rise-and-break cycles raised the secondary voltage high enough to cause a spark across the gap shown as G in the figure. Upon discharge, the current in the secondary would oscillate at a rate determined by the reactive elements in the secondary, mainly the reactive equivalent circuit of the antenna itself (in the sense of a so-called "lumped" equivalent circuit of the antenna). Since the antenna was part of the resonant circuit, it automatically radiated the radio-frequency signal.

A big problem historically with this initial design was the fact that the signal died out very rapidly in each spark cycle, reducing the transmission range. While a a number of contributors could claim a degree of simultaneous invention, it was Ferdinand Braun who came up with the main improvement to this arrangement, which is shown in Figure 2. Here an additional circuit is added whereby the oscillations in the secondary of the induction coil were coupled to the antenna. Shown is an inductive coupling, but other couplings including direct and capacitive, were also used. In this scheme the oscillations in the secondary of the induction coil and spark gap were coupled to the resonant antenna, the latter not suffering from useless damping in the spark gap and therefore radiating the energy much more efficiently. The 1909 Nobel prize in physics was jointly awarded to Marconi and Braun because the committee recognized that bothe Marconi's original insight and this key improvement by Braun were both required in order for the technology to become truly useful (to be able to transmit more than just a few miles).

Figure shows a more or less complete system with practical improvements that while not fundamental did help in several respects. The induction coil was usually replaced by an alternator, and the voltage boosting was done via a transformer (although of course the induction coil is really just a transformer in the first place, so it is a matter of semantics). The alternator allowed for much higher power transmission than a battery could provide. It was also easily sychronized to a rotary spark gap (see below). Other improvements to the circuit included RF chokes (L3 and L4), which prevented the RF signal from adversely affecting the AC source.