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KF5JRV > TECH 20.02.17 13:17l 66 Lines 3291 Bytes #999 (0) @ WW
BID : 10783_KF5JRV
Read: GUEST
Subj: Poulsen Arc Transmitter
Path: IZ3LSV<IW2OHX<UA6ADV<CX2SA<N0KFQ<KF5JRV
Sent: 170220/1215Z @:KF5JRV.#NWAR.AR.USA.NA #:10783 BPQK6.0.13
An arc transmitter is different to a spark gap transmitter. The arc
transmitter, sometimes called the arc converter, or Poulsen arc transmitter
after Danish engineer Valdemar Poulsen who invented it in 1903.
The Poulsen arc transmitter was a variety of spark transmitter that used an
electric arc to convert direct current electricity into radio frequency
alternating current. It was used as a radio transmitter from 1903 until the
1920s when it was replaced by vacuum tube transmitters.
The key advantage of the Poulsen arc transmitter was that it was one of the
first transmitters that could generate continuous sinusoidal waves.
Unlike the basic spark-gap transmitter, the arc converter produced undamped
continuous waves signals. This enabled much narrower bandwidths to be used
along with much higher levels of efficiency.
Development of the arc transmitter
As with all developments there were a number of foundations that were set in
place before the main invention could be made. The same was true for the
Poulsen arc transmitter.
One of the first developments that was fundamental to the arc transmitter was
the development by Sir Humphrey Davy of carbon arc lighting in the 1840s. Then
in the 1850s outdoor arc lighting became popular as a result of its efficiency
and power.
Then around 1890 Tesla and JJ Thompson independently design high-frequency
alternators to eliminate carbon arc audio noise. This work was then extended in
the 1900s by by Ernst Alexanderson to make the first continuous-wave RF
transmitters operating around 50 kHz for Reginald Fessenden.
The development was taken further in 1892 by Elihu Thomson who discovered that
an arc powered by DC loaded with inductance and capacitance could produce
oscillations. This work was extended by the realisation that the frequency of
oscillation and frequency could be increased by applying a magnetic field
perpendicular to the arc.
The arc was actually a negative resistance device in which the voltage across
the device fell as the current went up.
What is a Poulsen arc transmitter?
The Poulsen arc transmitter consisted of an arc across which was placed a tuned
circuit.
The arc converter itself consisted of a closed chamber in which the arc burned
in hydrogen gas between a carbon cathode and a water-cooled copper anode. The
arc was subjected to a strong magnetic field that was typically provided by two
series field coils above and below the chamber that surrounded and energised
the two poles of the magnetic circuit. These poles projected into the chamber,
one on each side of the arc to provide a magnetic field. An antenna tuning
section was needed to suppress the harmonic output of the arc converter.
The Poulsen arc transmitter was typically a very low frequency form of
transmitter, which was most effective between frequencies of few kilohertz up
to a few tens of kilohertz. The arc transmitter could be adaptable and could be
sued for designs of one to five kilowatts for ship-based transmitters up to
shore stations that could be up to several hundred kilowatts.
The Poulsen arc transmitter was used in the USA where despite the frequency
limitations, the improved transmission reliability enabled high speed Morse
code transmissions to be made.
73 Scott KF5JRV
KF5JRV.#NWAR.AR.USA.NA
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