Home › Forums › temp › OTR goodie: Old copy of The Radio Amateur’s Handbook online › Negative resistance in glow tubes and spark gaps
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I’ve seen glow tubes used in RC relaxation oscillators at audio frequencies. I don’t know how high in frequency they can go. I have not heard of anyone driving an LC circuit with a glow tube. I suspect that the response time of glow tubes is not fast enough to operate at higher than the lower radio frequencies, but I don’t know that for sure.
I should point out that there are different kinds of spark gaps. There is the original spark gap used by Marconi, which produces wideband damped oscillations. Then, there is the far more stable spark gap of the arc lamp, which was used by Frederick Collins, who is the author of the edition of the ARRL Handbook discussed in this thread. In 1899, Collins, as he describes in the Handbook he wrote, used a microphone to modulate the spark of an arc lamp. Collins’s transmitter produced waves that were nearly sinusoidal, and had low enough noise to allow AM to be transmitted.
In the arc lamp transmitter, there is a delay in the change in ionization due to a change in the current through the arc. So, the arc lamp AM transmitter could be used only at the lower radio frequencies. This was fine during the early days of radio, because groundwave was used for long-distance communications, and groundwave transmission is best at the lower frequencies.
Radio amateurs were at one time restricted to frequencies above 1500 kHz, which were considered to be unsuitable for long-distance radio communications. Thanks to the pioneering efforts of these radio amateurs, methods of achieving reliable long distance communications using skywave propagation, were developed.