1 Background Data: Military Communication and The Radio Game Randy H. Katz CS Division, EECS Dept. University of California, Berkeley Spring 2011 Quick History: Electricity and Magnetism in the 19th Century • Alessandro Volta (1745 - 1827) – Volt: the unit of electrical potential (“electromotive force”) – Metals are “movers” of electricity: arrange metals in an electromotive series based on ability to gain or lose positive charges by contact – Voltaic pile: dissimilar metals separated by membranes moistened by acid -- the electric battery E&M History • Andre-Marie Ampere (1775 - 1836) – Amp: unit of electrical current ("velocity of electrical flow") – Demoed that magnetic effects between two current carrying wires are the same as those between a wire and magnet – Electrodynamics: mechanical forces between current-carrying conductors -- critical for the design of telegraph apparatus, telephone receivers, and loud speakers E&M History • Georg Simon Ohm (1789 - 1854) – Ohm: the unit of electrical resistance – Ohm’s Law: I = V/R -- fundamental relationship between voltage (aka electromotive force), current, and resistance E&M History • Hans Christian Oersted (1777 - 1851) – Forces of nature--light, electricity, magnetism, gravity-- variations on one source. Magnetic field exists around a current carrying conductor. A deflecting force, such as magnetism, was something completely new. Connection between electricity and magnetism. Demonstrated this effect with the deflection of a magnetic compass needle. E&M History • Michael Faraday (1791 - 1867) – Farad: electrical unit of capacitance (stored electrical charge). – Built on Oersted: Demoed electric motor: wire carrying current rotated around a fixed electro-magnet. Wire rotates around magnet. Also showed how to make magnet rotate around wire.2 Electrical Telegraph • Telegraph — “Writing at a distance” • Built on work of Volta, Faraday, Oersted. Signaling by means of electrical current along a conducting wire. • Basic idea: deflect a needle by connecting and disconnecting a battery to a wire. By alternating the connection between the + and - poles of the battery, the deflection could be enhanced: needle reverses its swing. Wheatstone and Cooke • 5 magnetic needles, deflected in pairs, selecting one of 20 letters of the alphabet. Original system required 5 wires. Later reduced to 2 by using coding techniques. 20 km of line installed along railway routes in 1838. Grows to 4000 miles by 1852. Samuel F. B. Morse • 1835: “printing telegraph” -- key switches on current to electromagnet. Pencil makes mark as long as key is held down, drawing a long or short line on a moving strip of paper. • 1837: Morse code – dots and dashes encode alphabet based on frequency of occurrence in English text James Clerk Maxwell (1831-1879) • Maxwell's equations described fundamental relationship between electricity, magnetism, wave propagation. Underlies all radio and cable communications. Light and radio waves are the same phenomena. Provides theoretical explanation for why radio waves can be focused and reflected just like light. Electromagnetic Radiation Electromagnetic Radiation • Heinrich Hertz (1857 - 1894) – Hertz: unit of frequency: cycles per second. – Demoed experimentally wave character of electrical transmission in space. Developed apparatus that could transmit high frequency, meter length waves.3 Radio Communications • 1895: Guglielmo Marconi demoed electromagnetic radiation, created by spark gap, could be detected long distance. Main Cable Lines -- 1911 Radio Communications in WWI • Wireless plays key role! – Within hours of hostilities, British Navy cut Germany's overseas telegraphic cables; systematically attack German overseas radio stations and shut them down. – Germans similarly cut Britain's overland cables. … Marconi completes several radio stations under contract from the British government, to reestablish comms with overseas possessions. Techniques of communications intelligence (comint)—message interception, cryptoanalysis, direction finding, jamming, and intelligence gathering—developed rapidly. Airborne Radio Communications in WW I • "It became clear as we made our preparations ... was not going to be anywhere near as simple as it had sounded. The first problem was the shear weight of the wireless apparatus. The guts of the system was a marvelously archaic contraption called a spark-generator. This worked by creating an arch through the teeth of a brass cog-wheel spinning against an electrode. Every time a tooth passed the electrode a spark jumped across the gap, and in this way, when connected to the aerial, it would produce a hideous, rasping crackle -- barbed wire made audible. The principle of signaling was that the operator worked a Morse key to turn this excruciating noise into a signal: a crackle for a dash and a short one for a dot.” Airborne Radio Communications in WW I • "That part of the wireless alone weighed about thirty kilograms. But there were all the other accoutrements that went with it. Power was provided by a dynamo fixed on to a bracket under the aeroplane's nose and driven by a leather belt from a pulley-wheel on the propeller shaft: that weighed about seven kilograms. Then there was the aerial: twenty meters of wire with a lead weight at one end to trail behind us in flight, plus a cable reel to wind it in when not in use: about ten kilograms' worth in all. Other accessories comprised a signal amplifier, a tuning coil, an emergency battery, an ammeter, a set of signal rockets plus pistol and a repair kit. Altogether the wireless apparatus -- weighed about 110 kilograms. Or to put it another way, the weight of a very fat man as a third crew member." Portable Radio, circa WW I4 Radio Communications WW II • Command and control in “lightening warfare” (blitzkrieg) – Mechanized warfare demands leading from the front—radios in tanks let to quantitative edge – Rapid pace of operations does not allow for laying of wires—by-pass hard points to infiltrate behind the enemy • Combined arms coordination – Bring to bear infantry, tanks, artillery, air power, even naval power – “Man” pack radios – Planes and ships don’t have wires “A Bridge Too Far” “A Bridge Too Far” “A Bridge Too