The first transatlantic wireless telegraph from US was sent 19th Jan 1903 from an oceanfront location in South Wellfleet, Cape Cod, nowadays called Marconi Beach. I visited the site on my holiday trip this Summer.
The weather was not so good on Cape Cod that day, so my family did not object too much to make a short diversion from the highway to see the site. We took a few pictures. The Marconi Site was not the place for the very first transatlantic communication over radiowaves, which happened a few months earlier from New Foundland, (then British) Canada, but it was the first from US soil.
There is not much original left on the site, but there is a display model and descriptions of the site layout, antennas and the spark transmitter. Some remains of the the foundations of the center building and one tower base were left.
The site had had 4 wooden towers, each 210 feet (ca 64m) high. The antenna was erected between the towers as an upside down pyramid consisting of 200 wires. There were no indication of what kind of counterpoise was used, only a mention on the transmitter schematic. There is no indication on what frequency the site worked, nor the receiving apparatus used. Elsewhere I have found that the site may have used 1500m or 200kHz wavelength. One might try EZNec or similar to model and check how well this kind of antenna system might have worked. The spark transmitter had a resonant circuit and the transmission was "modulated" by rotary spark gap.
The power was supplied from an AC "alternator" (generator, upper right). This charged 110V batteries, too. 110 DC was obviously needed for the keying relay ( "N", left from the automatic keying tape box "K" in the middle) and the rotary spark gap motor ("M" lower left). Alternator supplied 60Hz/2000V. This was transformed to 20000V. When key was down, the high voltage charged a main capacitor "C". The capacitor may have been huge. The sparks over the gap shorted the capacitor with coil "H" (bottom left) to form a resonant circuit. The energy was inductively coupled to the antenna/counterpoise system. There must have been huge keyclicks, at least the sparks were head some 4 miles away ;o). However even then they have been thinking of RFI as the diagram shows two RFCs between the power supply and the resonant circuit.
The Site had a callsign "CC" (later "WCC") and even communicated with the sinking Titanic in 1912. The Marconi Site was operative from 1902 until 1917 when it was closed due to wartime security. 1920 the equipment were salvaged (where are they now?), towers dismantled and the buildings were left to the sea. The site is now part of the Cape Cod National Seashore. Interesting piece of wireless history.