Lecture 6

Wireless Technology: Electromagnetic Fields and Radiation

March 17, 2004



Visualizing the Invisible - Electric and Magnetic Fields:

Goal: To establish a viable model for characterizing information flow through space by means of electromagnetic radiation.

To accomplish this goal we need to come to grips with Faradays essential notion of "lines of electric and magnetic force."   In thinking about sonic radiation there are tangible connection between the sender and receiver.  A very good microscope would show us in detail how the medium provides the mechanical connection - see ISVR's Wave Basics (local copy) or our MATLAB simulation snd.

In the case of electromagnetic radiation is that there no connecting medium!  There is no hiding of this fundamental conceptual problem, but Faraday's field concept (and the related notion of lines of force) gives us an important way to bridge the problem and to think about electromagnetism.   To begin with, let's look at the characteristics of the electrostatic force with the aid of the applet Charges and Fields 

By studying the applet Electric Force Fields, we can see how the idea of electrical lines of force has evolved.

Pictures of typical electrostatic force fields can be found here

By studying the applet The Electric Force, we can see how one charge influences the motion of another.

But the applet Wiggling Charges and Electromagnetic Waves clearly relates cause and effect.  We here that an oscillating charge can entrain the motion of a remote charge through the mediation of an electric field.

It is a short step from these ideas to a picture of radiating lines of force from an oscillating charge.  A dynamic version of these pictures may be seen in the applets Overview and Oscillating Charge.

The applets Dipole Radiation and Far fields give a more detailed view of the radiating field and how it varies with distance (viz.,the radiated electric and magnetic fields vary inversely with the distance)
 

Visualizing Magnetic Fields:

Pictures of typical magnetostatic force fields can be found here

The Maxwell-Hertz Revolution


James Clerk Maxwell
(1831-1879

Heinrich Rudolf Hertz
 (1857-1894)
With the publication in 1864 of his treatise A Dynamic Theory of the Electromagnetic Fieldand his now famous set of equations, James Clerk Maxwell extended and completed the Faraday-Henry theory of induction and theorized that electrical disturbances should travel at the speed of light.
In the words of the Master, "This velocity is so nearly the velocity of light, that it seems we have strong reason to conclude that light itself (including radiant heat and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws." (Maxwell Homepage)

In 1879 the Berlin Academy of Sciences offered a prize to the first to demonstrate experimentally that a changing electric field generates a transient magnetic field, and vice-versa.  Among those who took up this challenge was Heinrich (Rudolph) Hertz (1857-1894), a professor at the Karlsruhe Polytechnic.  In 1887 through a remarkable series of experiments, Hertz establishes the validity of Maxwell's theoretical analysis by showing that a field generated by an electric spark can travel through space as waves and that these waves have the same physical properties as light.  In particular, he demonstrated that the velocity of these electromagnetic waves is equal to that of light.  Hertz never tried to use electromagnetic waves for communication and even denied the practicability of such an undertaking.

Let's take a look at Herz's famous experiment.  (See On Resonance for a little background the all important subject of resonance.)

Professor Fu-Kwun Hwang's (Dept. of Physics, National Taiwan Normal University) applet Propagation of Electromagnetic Waves gives us a most precise visualization of electromagnetic radiation.

Electromagnetic spectral information:

SCIMEDIA: Electromagnetic Spectrum

Allocation of Radio Spectrum in the United States

Maxwell's theory, of course, unifies our general understanding of all electromagnetic radiation through the common relationship

(frequency) x (wavelength) = velocity (3x108 m/sec).

However, the propagation characteristics vary considerably over radio frequency spectrum.

Marshall Brain's How the Radio Spectrum Works is light hearted but informative.

(Source)

The Evolution of Wireless Technology: (reference)

Before Hertz clarified things, there were a number of attemps to achieve "wireless" communication with some limited successes -- see, for example, Faux Wireless.

In order to put Guglielmo Marconi's work into its proper place, it is necessary to mention not only his activities, but those of Lodge and Jackson in Britain, of Popov in Russia, and Slaby in Germany many more people than this, too numerous to mention, actively experimenting at that time. In the United States, for instance, De Forest, Fessenden, Stone and Shoemaker took out hundreds of wireless telegraphy patents, shortly after the original idea had been demonstrated by Marconi.  But Marconi was certainly the pace setter and a truly successful entrepreneur.

A look at Marconi's early technology

Early wireless transmitters: (reference)

On Collecting old radio literature radio books and radio journals

Spark gap technology dominated the early years -- see A "Quenched" Spark Gap of Unknown Make
http://www.vistech.net/users/w1fji/spark.html
http://www.physics.otago.ac.nz/ursi/belrose/spark.html

But it was Reginald Aubrey Fessenden (biography) who was the most outspoken advocate for the development of  continuous sources.

In 1902, Valdemar Poulsen, a Danish engineer, invented an arc converter as a generator of continuous-wave radio signal.  Beginning in 1904, Poulsen used the arc for experimental radio transmission from Lyngby to various receiving sites in Denmark and Great Britain.  Poulsen-arc transmitters were much used internationally until they were superseded by vacuum-tube transmitters.  The Poulsen Arc Transmitter: is an abstract of a thesis by Hans Buhl which describes the invention and development of the Poulsen System in Denmark, England and the United States.  For background information on "negative resistance" arcs see William Du Bois Duddell and the "Singing Arc"(1899) and The Telefunken/Sayville Wireless.

The Alexanderson radio alternator (two picture of Alexanderson alternators) was a high-power, radio-frequency source which provided reliable transoceanic radiotelegraph communication during and after World War I.  Ernst F.W. Alexanderson (1878-1975), a General Electric engineer, designed radio alternators with a frequency range to 100 kHz and a power capability from 2 kW to 200 kW.  These machines, developed during the period 1904 to 1918, were used in research on high-frequency properties of materials as well as for international communications. (reference)

Early wireless receivers:
A discussion of "coherers"

A discussion of "valves and diodes"
 


This page was prepared and is maintained by R. Victor Jones
Comments to: jones@deas.harvard.edu.

Last updated March 12, 2004