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1920s thoughts regarding radiotelegraphy

Ever wonder how we got to 5G? Many earlier discoveries, experiments analyses led to wireless communication, but let’s look at what helped spur wireless radio 90 to 100 years ago.

Let’s start with Radiotelegraphy, a means of radio communication typically via Morse code. The radio carrier could be Amplitude Modulated (AM), Frequency Modulated (FM), Phase Modulated (PM) or another type of coded signal.

The recovered signal at the receiver used a demodulator that handled the particular kind of carrier frequency and converted it to symbols. In those early days the symbols were generated by a manual telegraph key, but today a computer is used to generate these kinds of signals.

Radiotelegraphy used a narrow frequency bandwidth, via point-ti-point communication between fixed points on the globe, which enabled signals to be sent and received pretty accurately even in the presence of interference and noise. This means of communications revolutionized ship-to-shore communication especially in emergency situations. Vice-versa, National Maritime services were now able to send out sea-state information along with weather details. This technique also led to the rise of amateur radio or ham radio communication.

In 1918 there were two phenomena used for radiotelegraphy: Impact excitation and multi-spark systems. In order to create an impact excitation, a proper gap must be determined and the radio frequency circuitry needed to be designed with the appropriate constants for this form of energy transfer.

Back then, there were three forms of gap which were pretty interchangeable:

1 The Aluminum-copper gap in an air-tight chamber in which alcohol was fed via a wick and converted to a vapor by the heat of that gap. This method had excellent quenching properties. This system could operate well with a primary circuit of far-less desirable constants, meaning a “stiffer” circuit, than an Impact excitation would need. This had a pretty ‘regular’ operation, but was less efficient than the other two types. The gap could run with an opening of 0.0006 to 0.014 inches.

2 A Copper-Copper gap or Silver-Silver gap of similar construction. The gap could run with an opening of 0.001 to 0.003 inches.

3 Two electrodes were both of thin Tungsten, welded to Copper backs operated in air. The gaps had to be fairly identical regarding their electrical properties. These were fairly regular in operation as well although Copper required a higher voltage. The gap could run with an opening of 0.001 to 0.003 inches.

The gaps of these devices were connected to a primary condenser and the primary inductance of the coupling coil. The circuit had very low persistence, i.e. could not be sustained very long.

In the 4A, 0.5 kilowatt set, the condenser has the value of 0.16 uf. The inductance is one single turn of heavy copper tubing giving a value of about 1.2 microhenries. In order to get maximum energy transfer, this primary circuit should have a free period of from 1.2 to 1.7 that of the secondary. The primary condenser is connected to a source of potential, either direct or alternating, having a value of from two to four hundred volts. If direct current is used, a fairly large iron core inductance should be inserted in this line. If an inductor alternator is used, the inductance of a machine is found sufficient. The condenser charges up until it has reached a potential sufficient to break down the gap; it then discharges through the gap in a single loop or half cycle which sets the antenna into oscillation. The condenser immediately begins to charge again, and when it has reached a potential almost sufficient to break down the gap, the slight counter e.m.f. induced in the primary by the still oscillating secondary adds just sufficient increment to “trigger” the gap off in the proper phase relation to maintain smooth antenna oscillations. If direct current is employed, this process continues at regular intervals and as the value of the feed current is increased the gap discharges more and more frequently. The number of antenna oscillations which occur between the discharges of the gap is called by the “inverse charge frequency” .(Reference 1)

The following photos were taken with the Braun tube, the earliest version of the CRT, at an antenna frequency of 500,000 cycles per second. Figure 1 shows the E-I characteristic of the gap-the current vertical, voltage horizontal. A large residual charge is shown in the primary condenser.

Figure 1

The E-I characteristic of the gap: The current is
the vertical image, and the voltage is the horizontal image. A large residual charge is
shown in the primary condenser (Image courtesy of Reference 1).

The E-I characteristic of the gap: The current is the vertical image, and the voltage is the horizontal image. A large residual charge is shown in the primary condenser (Image courtesy of Reference 1).

Reference 1, written in 1918, provides a really nice progression of Braun tube oscillograms with changes in the number of secondary oscillations per primary discharge showing an amazing regular gap-functioning over hundreds of kilocycles (kc). The Reference 1 also discusses the fabrication of such transceivers and the need for an alternator which helped give a highly non-sinusoidal wave to feed the gap transformer.

The system used a modified tone or ‘concentration circuit’ across the gap which increased receiving station audibility per ampere in the transmitter antenna. Finally, Reference 1 shows how 0.5 kW and 2 kW radio sets were designed on the transmitter and receiver sides. Finally, some antenna designs are shown.

Reference 2 discusses how the aim of 100 miles for wireless communication was the goal at that time and ultimately wireless men pushed for the goal of ΩR, where Ω is the cyclic number and R is the radius of the Earth. They were ‘dreaming of wireless communication with Antipodes, just like some are dreaming now of rocker airship traffic with the Moon’ (The Antipodes are one of New Zealand’s volcanic islands in the sub-Antarctic region of the Earth—not very hospitable to life) Remember, this Reference 2 was written in 1929 and only 40 years later we did finally reach the Moon.

Readers need to get a copy (maybe on IEEE Xplore—you will need to register and pay for it I think) of Reference 2 because if lists examples of what science owes to radiotelegraphy; one of these examples is the Physics of the Atmosphere with regard to propagation of radio waves and electron density at different altitudes. This effort has similarities to the understanding the influence on the propagation of radio waves of solar activity.

Another example was in physiological acoustics using radiotelegraphy using a Reiss microphone, a resistance-capacity coupled multi-stage amplifier, and a Siemens oscillograph (Again, this paper was written in 1929) which could reproduce time curves of a vowel such as a as in “are” and e . as in “Ellen”. See Figure 2.

Figure 2

Time curves of the e and a vowels (Image courtesy of Reference 2)

Time curves of the e and a vowels (Image courtesy of Reference 2)

1 ON THE ELECTRICAL OPERATION AND MECHANICAL DESIGN OF AN IMPULSE EXCITATION MULTI-SPARKGROUP RADIO TRANSMITTER BY BOWDEN WASHINGTON (RADIO ENGINEER, CUTTING AND WASHINGTON, CAMBRIDGE, MASSACHUSETTS), February 28, 1918

2 Proceedings of the Institute of Radio Engineers Volume 17, Number 1, January, 1929 THE IMPORTANCE OF RADIOTELEGRAPHY IN SCIENCE BY JONATHAN ZENNECK (President, Institute of Technology, Munich, Germany)

3 RadioTelegraphy.net

4 New England Wireless & Steam Museum: Ship Radio Room from the 1920s

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