OTB - Below 535, The Cape Breton Stations

Below 535
The Cape Breton Stations Of The Marconi Transatlantic Radio Service - Part 1
All photos courtesy The Marconi Company, Ltd.

by Henry Bradford
Site 1, Comp. A-O, R.R. #2 Wolfville, NS B0P 1X0 Canada
Email: henry.bradford@ns.sympatico.ca Please include SASE for reply

Henry Bradford is a student of the historical aspects of radio transmission in Nova Scotia, Canada, where Marconi established a station for his first commercial transatlantic service. No discussion of activity below the AM Broadcast Band can ignore where it all started many years ago. Our columns for this and the following issue will be devoted Henry's brief conversational visit to Marconi's Nova Scotia site. It goes without saying that the original LF stations for the transatlantic radio service were monster structures. I am sure you will enjoy the unique photographs of the Marconi installation included by our author.--FJL

Fig. 1. Marconi Towers station near Glace Bay, Nova Scotia, circa 1910.

Introduction

The first regular commercial transatlantic wireless telegraph service began in October 1907 between Cape Breton, Nova Scotia, Canada, and Clifden, Ireland. The stations used spark transmitters and crystal detector or Fleming valve (diode detector) receivers until about World War 1 (1914-18). During that War, triode amplifier tubes (valves) were gradually introduced. The innovations found their way into the Marconi stations on both sides of the Atlantic. This article will describe the pre-WW1 transmitting facilities at the Cape Breton station. The receiving equipment will be covered in a future article.

Fig. 2. Basic Spark Transmitter Circuit.

The Original Hertz Transmitters

The first spark radio transmitters were built by Heinrich Hertz of Germany in the 1880s. Their purpose was to study the properties of radio waves. These very early transmitters included a battery power supply, a high voltage induction coil (transformer) with a buzzer-type interrupter in the transformer's primary circuit, a spark gap connected across the secondary, and an UHF dipole antenna connected across the spark gap. The transmitted frequency was around 400 MHz. In the mid-1890's Marconi and others developed apparatus for wireless communications for use over long distances. Their transmitters were based on the Hertz transmitters with the following significant design departures:

1. Generator power supplies for more power.
2. Transformer coupling, rather than direct coupling, between the spark circuit and the antenna for improved transmitter tuning.
3. Gradual lowering of the operating frequency to the LF and VLF bands in an attempt to increase range.
4. Large antennas consisting of aerial arrays and a ground connection or ground screen. Initially, the lowering of the operating frequency was the result of using larger antennas in the quest for greater ranges using the assumption that "bigger is better." As time went on, the lower attenuation of the ground wave at longer wavelengths was noted. (The increase in wavelength towards the VLF band did have a problem: physically large antennas. The early generation of LF and VLF antennas was a construction disaster. The life expectancy of the wood lattice and pole antennas was at best a few years.--FJL)

Fig. 3. DC generators at Marconi Towers, circa 1910.

The Early Marconi Transatlantic Stations

In December, 1901, using primitive portable receiving equipment, Marconi received a transatlantic test signal at St. John's, Newfoundland from his station at Cornwall, England. In 1903, he had to rebuild the station at Glace Bay on Cape Breton Island, Nova Scotia. The St. John's station had been only a receiving station.

In December 1902, Marconi transmitted the first transatlantic wireless telegraph messages from Glace Bay to Cornwall, England. The transmitter frequency was intended to be around 150 kHz. However, the transmitter's broad tuning and the use of poorly calibrated measuring instruments made the determination of the actual frequency doubtful.

These first stations proved to be inadequate for a reliable transatlantic service. In 1905 a second set of stations was built. One was just south of Glace Bay (Figure 1). The local people referred to this station as "Marconi Towers." The other station was built at Clifden, Ireland. In 1907, Marconi opened a regular commercial transatlantic wireless telegraph service between these stations.

Fig. 4. The suspended batteries at Marconi Towers, circa 1910.

The Transmitter Power Supply

Figure 2 shows a simplified schematic for the basic spark transmitters used at Marconi Towers and Clifden around 1910. The circuit was simple, but the components were huge due to the low frequency, the need to use of easy to obtain local materials, and the need for high transmitted power.

The power supply consisted of three 5-kilovolt DC generators in series (Figure 3). Connected in parallel was a standby battery. The battery consisted of 6,000 two-volt, 40 ampere-hour cells in series. If the generators broke down, the battery could provide about 500 kilowatt-hours at 12 kV. Each battery cell was built in a porcelain container. The batteries were suspended from steel rafters by porcelain insulators. The battery supply filled a very large room (Figure 4).

Fig. 5. Looking down at the "condenser" plates, condenser building, Marconi Towers, circa 1910. The power supply charged a capacitor made from 288 sixty foot by twenty foot wide metal sheets separated about 6 inches from each other (Figure 5). The sheets were suspended from rafters at the top of the building and hung vertically down almost to ground level. This capacitor (or "condenser," to use the original terminology) occupied most of the 160 foot long transmitter building. Thus, the building became known as the condenser building.

This great array of plates provided only 1.7 microfarads with a voltage rating of 15 kilovolts. An "air insulated" design was chosen rather than a more compact glass dielectric design because it was relatively-trouble free and easy to construct from locally available materials. If a draft caused the plates to move and short out, the "spot welded" plates were simply knocked apart.

Part 2 (conclusion) of this article will appear in the November, 2000 OTB.

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