062

Poldhu,
Cornwall, England

50° 1′ 46.30″ N, 5° 15′ 47.80″ W

The First Transatlantic Radio Transmission

In the southwest corner of the UK lies the county of Cornwall. It was there that the first transatlantic radio transmission was made, from the remote and windswept Poldhu Cove on December 12, 1901.

The rugged Lizard Peninsula, which contains Poldhu Cove and Poldhu Point, is the southernmost part of Great Britain and was chosen by Italian radio pioneer Guglielmo Marconi as a site for experimental radio communication with ships at sea. Geologists will enjoy the presence of green serpentine rock, and it was here that titanium was first discovered in 1791.

The wireless station built by Marconi and his company, Marconi’s Wireless Telegraph Company, was not only the site of the first transatlantic transmission but also the place where, in 1910, the first SOS (dot dot dot, dash dash dash, dot dot dot in Morse code) was received in Britain from a ship at sea. And in 1962, the Goonhilly Downs (see Chapter 48) area of the peninsula was chosen as a ground station for the first transatlantic satellite, Telstar.

But 61 years before Telstar’s ability to send television pictures, Marconi’s message was just the letter S sent in Morse code (dot dot dot). It was transmitted continuously between 1500 and 1900 local time on December 12. Marconi (with assistant George Kemp) had traveled to St. John’s, Newfoundland, to listen for the signal using a kite trailing a 155-meter-long wire as an antenna. He had originally planned to use antennas installed on the site, but these were destroyed in a storm. The same fate had befallen the antennas installed at Poldhu, and the first transatlantic transmission was achieved with makeshift equipment for transmission and reception.

The transmitting equipment consisted of an enormous condenser made of metal plates and capable of providing 1500V after being charged up by a steam-powered generator. This large voltage was applied to a “spark gap” (literally a pair of electrodes with an air gap between them). The condenser charged up until a spark was able to jump between the electrodes, creating the radio signal.

Listening through the background static, Kemp and Marconi claimed to hear the S signal transmitted from the UK some 2,100 miles away on three occasions (at 1230, 1330, and 1430 local time). A few months later, Marconi repeated the experiment, sailing west on the SS Philadelphia and recording signals from the Poldhu station up to 2,099 miles away. By the end of 1902, Marconi had sent transmissions in both directions between Poldhu and Nova Scotia. For his many wireless achievements, Marconi received the Nobel Prize in 1909.

Today the entire Poldhu area is owned by Britain’s National Trust, and you can have a superb day out walking along the cliff tops and visiting the restored Lizard Wireless Station and the Marconi Centre. The 800-year-old town of Helston is nearby, and is an ideal starting point for wireless explorers. A suitable end to the day is a traditional cream tea—consisting of warm scones, clotted cream, and strawberry jam—in one of Helston’s many tea rooms.

For the complete wireless experience, and because it’s a good base for exploration, stay at the Housel Bay Hotel, where Marconi himself stayed in 1900 while scouting for a location to set up his wireless telegraphy station. The hotel is just 200 meters from the spot where Marconi first received signals from the Isle of Wight (some 186 miles away). The hotel is also close to Poldhu.

Practical Information

To visit the Poldhu site, check in with the National Trust; its website can be found at http://www.nationaltrust.org.uk/main/w-vh/w-visits/w-findaplace/w-thelizardandkynancecove.htm.

Marconi’s Transmitting Equipment

In early wireless experiments by the German physicist Heinrich Rudolf Hertz, there was no antenna at all: the entire transmission was achieved by a spark jumping across an air gap. This sent out a wide range of frequencies and could be detected using another spark gap connected to a coil of wire. You can still hear these types of transmissions by listening to the radio during a lightning storm: the cracks coming from the radio are “transmissions” from the lightning bolts.

To achieve more reliable transmission, Marconi and others used a simple method of tuning the output of the spark to certain frequencies and adding an antenna. In his earliest experiments, Marconi’s transmitters were nothing more than an antenna plus a spark gap connected to the ground (i.e., the Earth).

But by 1901 he had improved the circuit to include the equivalent of an LC resonant circuit. That type of circuit consists of an inductor (a coil of wire wrapped around some ferromagnetic material, such as an iron bar) and a capacitor (called a condenser in Marconi’s day, and typically consisting of a pair of metal plates separated by some non-conducting material called the dielectric). The capacitor C and inductor L are connected as shown in Figure 62-1.

Figure 62-1. LC resonant circuit

When electricity is applied to this circuit, the capacitor charges up, acting like a battery, until it cannot hold further charge and starts to discharge through the inductor. The inductor turns the current into a magnetic field, since the current flowing through the coil induces a magnetic field in the bar it is wrapped around. Once the capacitor is empty (completely discharged), the inductor will start to recharge the capacitor using the energy stored in its magnetic field (the magnetic field now induces a current in the coil of wire). This circuit continues to oscillate between capacitor and inductor at a frequency determined by the two components.

Using such a circuit meant that Marconi could coarsely tune his transmitter. The circuit diagram in Figure 62-2 shows a simplified version of the transmitter he used in 1901.

Figure 62-2. Simplified circuit diagram of Marconi’s transmitter

Marconi’s circuit worked by charging the spark gap until the voltage became large enough to cause a spark to jump across it. The jumping spark had the effect of closing the LC circuit consisting of capacitor C and inductor L and providing a jolt of power to it; the circuit then started to oscillate. L actually consisted of a pair of coils with one coil connected to the antenna. The magnetic field of L induced a current in the second coil, which passed into the antenna and created the radio signal.