Last update 25 Jan 2016
Suppose you had access to a radio antenna that would enable you to tune in signals from the past.
Staring a 1940 U.S Navy type RAX Receiver
(I needed something relatively simple, so museum visitors could operate it themselves.)
It's been my contention for some time now that cold silent radios have very little attraction to the casual museum visitor.
Our current Radio Technology Museum installation amounts to cable radio. A good broad-band antenna above the building captures low-, medium-, and high-frequency signals that are boosted by a distribution amplifier, and routed to radios throughout the museum via 75-ohm coaxial cable. Our local oldies station, 1080 - WALL, also feeds the amp, and provides appropriate vintage programs from a CD or MP3 player that are not available out of the ęther.
As I started to study Edwin Howard Armstrong's early experiments it became apparent that it would be interesting to hear what he heard when he discovered regeneration. He describes tuning in the Marconi press station on Cape Cod, MCC, and how the volume and then the timbre of the sound changed when he adjusted the tuned circuit he had added to the "wing" of of Deforest's Audion receiver. We all think we know how a regenerative receiver sounds. You get a whooping beat note as you tune past each AM carrier on the broadcast band. The problem is MCC was a spark station, there was no carrier, and there's hardly anyone alive that actually heard spark on the air through a regen.
So was born the MCC simulator. A 120 hertz square wave was used to excite a parallel tuned circuit at about 800 KHz. This produced trains of damped waves with a "spark rate" of 240 hertz, the reported sound of MCC. Output was about a microwatt, which represents a pretty strong signal when applied directly to the antenna terminal of even a primitive receiver. The transmitter was keyed by a Morse signal generated by a personal computer to make the original recording.
Sure enough it was just like Howard said:
“All the old timers remember CC, later known as MCC and WCC, the Marconi press station at Wellfleet, Mass. This station was the one-hundred percent reliable testing standby of all experimenters, and on MCC the first tests were made. A standard audion detector system was set up and tuned in, and a tuning inductance introduced into the plate circuit of the audion.
Then various things began to happen. As the plate inductance was increased, the signals were boosted in strength to an intensity unbelievable for those days, the more inductance the louder the signal, until suddenly the characteristic tone of M. C. C. -- the tone which any of the old timers, if they heard it on Judgment Morn, would recognize instantly -- disappeared, and in its place was a loud hissing tone, undeniably the same station, but recognizable only by the characteristic swing and the messages transmitted.
A slight reduction of the plate Inductance and the old tone was back again, -- and then the placing of the hand near a tuning condenser, and the hissing tone reappeared.
It required no particular mental effort to realize that here was a fundamentally new phenomenon, as obscure as the principle of the operation of the audion itself, but which opened up an entirely new field of practical operation.”
That phenomenon was RF oscillation.
Hear it at: http://www.ar88.net/arm4/images/regen.swf
Armstrong's demonstrations of his receiver in the Winter of 1913-14 revealed that the regenerative receiver was not only the best receiver available of spark radio telegraphy, but also worked extremely well on continuous-wave Morse signals. So there were three classes of signals to be heard, buzzing spark, on-off keyed CW from the high-frequency alternators, and frequency-shift keyed CW from the Poulsen arc stations. During a demonstration to representatives from AT&T their chief engineer observed that the latter sounded like "an inebriated flute player."
So the simulated spark transmitter has been joined by the Nauen, Germany, Slaby-Arco alternator and the San Francisco, Federal Telegraph, Poulsen Arc. The three devices are on different frequencies, and their signals are combined so that the demonstration receiver can be tuned across the band as it sounded in 1914. Thus, Virtual Ęther has been attained.
Each transmitter is keyed by a separate Arduino micro-controller board. The text used has been derived from the signals recorded by David Sarnoff in his report to Marconi America following the January 1914 demonstration at Belmar..
Experimental Armstrong Kiosk content - including more videos.
The Radio Technology Museum website.
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