As he has done each December for the past several years, Brian Justin,
WA1ZMS, of Forest, Virginia, will transmit a program on 486 kHz, under
authority of his FCC Part 5 Experimental License WG2XPN to commemorate
the accomplishments of wireless pioneer Reginald Fessenden. The
Canadian-born inventor, experimenter, and entrepreneur has been credited
as the inventor of radiotelephony. He claimed to have made his first
voice — and music — broadcast on Christmas Eve in 1906 from Brant Rock,
Massachusetts, although his account is disputed.
Fessenden’s transmitter was most likely a high-speed “dynamo” or
alternator — a predecessor to the later Alexanderson alternator. He
modulated the signal by placing a carbon microphone in series with the
antenna feed line to create an amplitude modulated signal. Fessenden a
few years earlier had limited success with voice transmissions using a
rotary spark gap transmitter. Fessenden fed his signal into a
substantial antenna system erected in Brant Rock for his experiments.
Accounts say that, on Christmas Eve 1906, he transmitted recordings of
two pieces of music and read a verse from the Bible.
Justin will transmit for at least 24 hours starting at around 2000 UTC
on December 24, with a repeat transmission on New Year’s Eve likely,
“keeping in step with what Fessenden was reported to have done on both
nights in 1906,” Justin explained. He will use equipment of a somewhat
more modern design — a home-brew master oscillator, power amplifier
(MOPA) transmitter based on a classic design from the early 1920s. It
uses a UV-201 oscillator tube driving a VT-25 tube — a modern equivalent
to a UV-202 — to generate “a few watts” on 486 kHz. His modulator
consists of another VT-25, which uses a large inductor in the RF
amplifier’s plate supply to serve as a Heising modulator. The audio
program comes from a laptop computer.
“Heising modulation was used in World War I as an easy way to achieve AM
in rigs such as those used in aircraft,” Justin said. “My particular
Heising modulator can deliver only around 60% modulation, so an audio
processor is used to help boost the average volume level ahead of the
modulator tube.”
Justin uses far more modern technology to boost “the few watts” of
modulated RF to drive a modified Hafler 9505 solid-state 500-W audio
amplifier. “The idea for the amp came from W1TAG and W1VD,” he said,
“and information on using such an amp on the 630 and 2200-meter ham
bands can be found on the web.” After a multi-pole low-pass filter, the
carrier output is 150 W.
Justin’s antenna is a Marconi T, crafted from a 160-meter dipole some 60
feet above ground and fed with open-wire line, which is shorted at the
transmitter end. A homebrew variometer — constructed from 14-gauge wire
wound on a piece of 4-inch diameter PVC pipe — is placed in series to
resonate the antenna, which is fed against an extensive ground system.
Jimmy Vance
NA5D
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