Starting from the pioneering researches of Oersted, Henry, Faraday, Ampère, in 1864 J. C. Maxwell developed his “Dynamic Theory of the Electro-Magnetic Field”, demonstrating mathematically the existence of the electro-magnetic waves. (Fig. 1)

Fig. 1 - Set of differential equations developed by Maxawell baseed on t he experimental work of Ampère, Henry, Faraday and others, which led him to predict tlhe properties of electro-magnetic waves long before t heir existence was suspected..
(Technology Review)

Further on the formulation of Maxwell theory, Henrich Hertz working in the universities of Kiel and Bonn, in Germany, proved experimentally the existence of the electro-magnetic waves.
Ressonators, originally used by Hertz to demonstrate the existence of electromagnetics waves. (Electronics)
Heinrich Hertz

Hertz's historic researches in electro magnetism led to new horizons in the wireless communications by gathering together many inventors and innovators.
Among them are:

Oliver Lodge
A physicist at the University of Liverpool, designed in 1894 an effective system of wireless reception of the electro magnetic waves whose main innovation consisted to use the coherer, originally invented by E. Branly, instead of the Hertz’s loop. (Fig. 3)

Fig. 3
Electro-magnetic waves wireless transmission and reception system in the beginning of XX century. The spark gap in the induction coil can be switched on and off by a telegraphic key. Oscillatory currents from the spark gap excite the coherer, which becomes conductive. When the coherer is placed in series with a battery and a telephone receiver, it switch the currents on and off in synchronization with the telegraphic key in the transmitter.

a) Telegraphic key

b) Spark gap

c) Coherer

d) Telegraphic receiver

e) Telephone receiver

Fig 4 - Replica of a spark gap used in early transmitter system.

Fig 5

Diagram of the receiving station developed by Oliver Lodge in 1894 where:

a) Coherer

b) Trembler

c) Battery

d) Relay

e) Inker

Fig. 6 -Replica of the cohesor originally invented by E.Branly.

Guglielmo Marconi
Educated in Italy primarily by tutors, Marconi early developed an absorbing interest in Physics and Chemistry. In the beginning of his career he improved on the Hertzian oscillator by constructing transmitting apparatus, which, from an elevated antenna, discharged across a spark gap to earth. He also improved on Lodge’s coherer by evacuating the tube with more selected metal fillings as well grading the connection plugs.
In 1896 he moved to England when he started his commercial and technical activities on the behalf of wireless transmission and reception of electro magnetic waves. On December 12, 1901, Marconi was working in his receiving station located in Newfoundland and was able to hear the historical feeble signal of the Morse telegraphic letter “S” transmitted from England.

Ernst F. W. Alexanderson
A native from Sweden had graduated as an electrical and mechanical engineer from the Royal Technical University in Stockholm. Later on he studied under Professor Sabli in Berlin and decided to come to America to work under supervision of Steimetz at GE. In 1904 he was assigned by the company to supervise the manufacturing of a large alternator, capable to generate frequencies up to 100,000 cycles, originally ordered by Reginald Fessenden, when he introduced several technical modifications. During this development stage of the high frequency alternator, one of Alexanderson main targets was the investigation the problem of modulating the powerful currents of the apparatus by minute energies of the voice. Alexanderson was responsible also for others important developments such as: the magnetic amplifier as well as the multiple-tuned antenna, which materially increased the efficiency of the radiation.

Illustration of a radio station transmitter using the Alexanderson alternator circa 1916. Alexanderson working at General Electric laboratories introduced several innovations in order to improve even more the machine's performance. One of the most important innovation consisted to replace the apparatus' wooden armature by an iron made one.
Close up view of the rotor used in an alternator for generating electromagnetic waves circa 1915.

Morse code training course circa later 1910’s.

Reginald Fessenden
of the University of Pittsburgh in 1900 did one of the first practical demonstrations on the speech transmission by means of the electro-magnetic waves using two masts 15 m high and 1600 m apart. Starting from the early researches made by Nikola Tesla, Fessenden believed he could design an equipment to transmit and receive Morse code signals across the Atlantic.
His first high-frequency alternator for producing 10000 cycles was built to his specification by Steimetz of the General Electric Company in 1903. After several attempts, he developed in the same year a new receiving device known as the electrolytic detector much more sensible to radio frequencies when compared with the Branly’s coherer.
Therefore, it was to take still several years for his theory to transmit signals by means of continuous waves to become possible as it was related mainly due to the innovations introduced by Alexanderson in the alternator, which certainly demonstrated it was far superior to the primitive spark gap transmitting method.
Interior view of a Fessenden wireless telephone station.

A Fessenden wireless telephone station at work.

H.J. Round
In order to improve the selectivity of the feeble radio frequency signals, in 1911 he invented a balanced crystal circuit for reception. He employed two crystals working in opposition, and so set that one was sensitive to the signal whereas the other only operated when a disturbance exceeded the value to which it had been adjusted.

As a professor of the University of Kronstadt, in Russia, in 1895 improved further on the Lodge’s electro magnetic reception system. In his system the coherer was protected from the local effects of local sparks at the relay contact by inserting chocking coils in the wires along which the waves from such sparks could run. In lieu Hertz’s loop of wire he used a long vertical wire, insulated at its upper end and connected to earth through the coherer at its lower end.
Alexander S. Popoff