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Early radio: transmission and reception, 1900 - 1914

Ship's radio room before 1920

Image: Ship's radio room before 1920

Source: Museum Victoria

Introduction

The period from 1900 to the beginning of the First World War saw the key events in the birth of radio as a communication technology.

The following sections briefly describes the transmission and receiving processes for this early period; each stage includes a listing of relevant item(s) in Museum Victoria's collection.

The transmission process
This process could involve high or low powered systems; the latter were used in emergencies.

High powered system - Stage 1:
A morse code signal is keyed into the system

ST 015474, Morse Key - Telefunken, Radio Spark Transmitter, 1910-1920
ST 018549, Morse Key - Marconi's Wireless Telegraph, circa 1899
ST 015473, Morse Key - Marconi's Wireless Telegraph, circa 1902

High powered system - Stage 2:
High powered systems use alternating current at around 500 Hz from a rotary converter via a high voltage transformer to charge the main capacitor.

ST 021405, Rotary Converter - G. Weymouth, Radio, circa 1915

High powered system - Stage 3:
The signal then passes through a keying relay, which protects the morse operator (and the key) from the high voltages and currents required by the transmitter. The morse key could not stand up to the high energies involved in controlling the sparks.

ST 015472, Keying Relay - AWA, Radio Transmitter, circa 1914

High powered system - Stage 4:
A coil then maximises the transfer of energy from the alternating voltage source (the rotary converter) to the main capacitor

ST 021406, Inductance Coils - AWA, Low Frequency, 1913-1916.

High powered system - Stage 5:
The voltage is then stepped up to a high enough value to break down the spark gap using a transformer.

ST 021407, Transformer - G. Weymouth, Radio circa 1913-1916

High powered system - Stage 6:
Choke coils isolate the spark circuit from the secondary of the transformer.

ST 021409, Coils - AWA, High Frequency Air Core Choke, 1913-1916
ST 21409.1, Coil - AWA, High Frequency Air Core Choke, 1913-1916
ST 21409.2, Coil - AWA, High Frequency Air Core Choke, 1913-1916
ST 021413, Coil - AWA, High Frequency Inductance, 1914-1920

High powered system - Stage 7:
The signal in the form of the keyed high voltage developed on the main capacitor is discharged via a spark gap (discharger).

ST 015462, Discharger - Marconi, Radio Transmitter, circa 1912
ST 015463, Rotary Discharger - AWA, Spark Radio Transmitter, circa 1914
ST 015471, Discharger - Shaw, Radio Transmitter, circa 1913
ST 021414, Rotary Discharger - AWA, Radio Transmitter, 1913-1916
ST 029225, Spark Gaps - Unknown Manufacturer, Quenched, c 1915
ST 015470, Capacitor - AWA, Radio Transmission, circa 1914


High powered system - Stage 8:
The energy in the main capacitor excites the oscillatory transformer to produce a damped oscillation at the resonant frequency.

ST 021412, Oscillatory Transformer (Jigger) - AWA, Radio Transmitter, 1913-1916

High powered system - Stage 9:
The energy of the main oscillatory circuit is matched to aerial by the aerial tuning inductance

ST 021415, Aerial Tuning Inductance - AWA, Radio Transmitter, 1913-1916

High powered system - Stage 10:
A tune indicator is used to allow the operator to adjust the aerial tuning inductance to maximise the radio frequency current in the aerial, indicated by maximum brightness of the lamp. In a typical ship's installation an earth arrestor is used to isolate the receiver from the transmitter during transmission, allowing the receiver to share the same aerial.

ST 021416, Tune Indicator - AWA, Spark Transmitter,1913-1916
ST 021417, Earth Arrester - AWA, Spark Transmitter,1913-1916

Low powered transmission systems
A low powered system can work in two ways. In the earlier and simpler system, the spark coil is connected directly to the aerial, producing a broadband signal, which is likely to be picked up ships in the area regardless of the listening frequency. The second system, the tuned coil system, the spark coil is connected to the main capacitor and creates a spark across the existing discharger which excites the aerial in the normal way.


Low power or emergency transmitters use an induction coil as a high voltage source for the spark. They are controlled directly by the morse key.

ST 018551, Induction Coil - Marconi, Radio, circa 1910

Capacitors are used across the interrupter of the induction coil to minimise sparking.

ST 015470, Capacitor - AWA, Radio Transmission, circa 1914
ST 018550, Condenser - Marconi, Coil, Radio, circa 1910


The Receiving Process

Stage 1:
An incoming signal from an aerial passes through a multiple tuner, which selects the required receiving frequency. A multiple tuner has more than one tuning circuit. The multiple tuner contains a tapped inductor and variable capacitor to tune the aerial followed by two further tuned circuits to increase selectivity and to match the output to the low impedance of the signal coil of the magnetic detector with which the multiple tuner was designed to work.

ST 015459, Multiple Tuner - Marconi, Radio Receiver, circa 1908

Stage 2:
The output from the tuner is fed to a detector, which was initially a coherer, later to be replaced by a magnetic detector, crystal detector and thermionic valve.

HT 26365, Coherer Detector - Unknown Manufacturer, 1900-1910
ST 015414, Coherer - Marconi, Radio Receiver, circa 1900
ST 15415, Coherer - Gesellschaft fuer drahtlose Telegraphie System Telefunken, Radio, circa 1905
ST 18546, Coherer Based Detector Assembly - Marconi, Radio Receiver, 1906-1912
ST 025918, Coherer - Radio Telegraphy, Jenvey, 1901
ST 15464 Magnetic Detector
ST 015475, Detector - Shaw, Crystal, 1913

Stage 3:
When a magnetic detector was used, its output was connected to earphones (known as telephones at the time).

An adjustable capacitor, known as a telephone condenser, was placed across the output of the magnetic detector in parallel with earphones. The capacitor is adjusted to resonate with the inductance of the output circuit at the repetition frequency of the transmitter, making the signal easier to hear.

ST 015469, 'Telephone' Condenser - Marconi, Radio, circa 1908

Where a coherer was used, a morse inker could be used to produce a permanent record of the signal. Coherers were more suited to shore-based stations than ships.

ST 018547, Morse Inker - Marconi, Radio Receiver, circa 1905

Dates

Dates of items are approximate, given the lack of definite information. If an item was manufactured by AWA, the date must be 1913 or after as the company was formed in July 1913. AWA commenced the manufacture of equipment in 1913. Prior to 1913, manufacturers were Marconi, Telefunken, Shaw and Balsillie. As of 2013, there are no examples of Balsillie in Museum Victoria's collection.

 

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