Often, small microwave dishes made of metal slats are mounted to the side of the tower. These slat dishes are mostly empty space, so they're less electronically efficient than a smooth metal dish would be. However, a smooth metal dish, being cupshaped, acts just like the cup on a wind-gauge, so if a strong wind-gust hits it, it will strain the tower violently. Slotted dishes are lighter,cheaper and safer.
Then there are horns. Horns are also microwave emitters. Horns have a leg-thick, hollow tube called a wave-guide at the bottom. The waveguide supplies the microwave radiation through a hollow metallic pipe, and the horn reflects this blast of microwave radiation off an interior reflector, into a narrow beam of the proper "phase," "aperture," and "directivity." Horn antennas are narrow at the bottom and spread out at the top, like acoustic horns. Some are conical, others rectangular. They tend to be mounted vertically inside the tower structure. The "noise" of the horn comes out the side of the horn, not its end, however.
One may see a number of white poles, mounted vertically, spaced parallel and rather far apart, attached to the tower but well away from it. On big towers, these poles might be half-way up; on shorter towers, they're at the top. Sometimes the vertical poles are mounted on the rim of a square or triangular platform, with catwalks for easy access by tower hands. These are antennas for land mobile radio services: paging, cellular phones, cab dispatch, and express mail services.
The tops of towers may well be thick, pipelike, featureless cylinders. These are generally TV broadcast antennas encased in a long cylindrical radome, and topped off with an aircraft beacon.
Very odd things grow from the sides of towers. One sometimes sees a tall vertically mounted rack of metal curlicues that look like a stack of omega signs. These are tubular ring antennas with one knobby stub pointing upward, one stub downward, in an array of up to sixteen. These are FM radio transmitters.
Another array of flat metal rings is linked lengthwise by two long parallel rods. These are VHF television broadcast antennas.
Another species of FM antenna is particularly odd. These witchy-looking arrays stand well out from the side of the tower, on a rod with two large, V-shaped pairs of arms. One V is out at the end of the rod, canted backward, and the other is near the butt of the rod, canted forward. The two V's are twisted at angles to one another, so that from the ground the ends of the V's appear to overlap slightly, forming a broken square. The arms are of hollow brass tubing, and they come in long sets down the side of the tower. The whole array resembles a line of children's jacks that have all been violently stepped on.
The four arms of each antenna are quarter-wavelength arms, two driven and two parasitic, so that their FM radiation is in 90-degree quadrature with equal amplitudes and a high aperture efficiency. Of course, that's easy for *you* to say...
In years to come, the ecology of towers will probably change greatly. This is due to the weird phenomenon known as the "Great Media Exchange" or the "Negroponte Flip," after MIT media theorist Nicholas Negroponte. Broadcast services such as television are going into wired distribution by cable television, where a single "broadcast" can reach 60 percent of the American population and even reach far overseas. With a combination of cable television in cities and direct satellite broadcast rurally, what real need remains for television towers? In the meantime, however, services formerly transferred exclusively by wire, such as telephone and fax, are going into wireless, cellular, portable, applications, supported by an infrastructure of small neighborhood towers and rather modestly-sized antennas.
Antennas have a glowing future. The spectrum can only become more crowded, and the design of antennas can only become more sophisticated. It may well be, though, that another couple of decades will reduce the great steel spires of the skyline to relics. We have seen them every day of our lives, grown up with them as constant looming presences. But despite their steel and their size, their role in society may prove no more permanent than that of windmills or lighthouses. If we do lose them to the impetus of progress, our grandchildren will regard these great towers with a mixture of romance and incredulity, as the largest and most garish technological anomalies that the twentieth century ever produced.
"The New Cryptography"
Writing is a medium of communication and understanding, but there are times and places when one wants an entirely different function from writing: concealment and deliberate bafflement.
Cryptography, the science of secret writing, is almost as old as writing itself. The hieroglyphics of ancient Egypt were deliberately arcane: both writing and a cypher. Literacy in ancient Egypt was hedged about with daunting difficulty, so as to assure the elite powers of priest and scribe.
Ancient Assyria also used cryptography, including the unique and curious custom of "funerary cryptography." Assyrian tombs sometimes featured odd sets of cryptographic cuneiform symbols. The Assyrian passerby, puzzling out the import of the text, would mutter the syllables aloud, and find himself accidentally uttering a blessing for the dead. Funerary cryptography was a way to steal a prayer from passing strangers.
Julius Caesar lent his name to the famous "Caesar cypher," which he used to secure Roman military and political communications.
Modern cryptographic science is deeply entangled with the science of computing. In 1949, Claude Shannon, the pioneer of information theory, gave cryptography its theoretical foundation by establishing the "entropy" of a message and a formal measurement for the "amount of information" encoded in any stream of digital bits. Shannon's theories brought new power and sophistication to the codebreaker's historic efforts. After Shannon, digital machinery could pore tirelessly and repeatedly over the stream of encrypted gibberish, looking for repetitions, structures, coincidences, any slight variation from the random that could serve as a weak point for attack.
Computer pioneer Alan Turing, mathematician and proponent of the famous "Turing Test" for artificial intelligence, was a British cryptographer in the 1940s. In World War II, Turing and his colleagues in espionage used electronic machinery to defeat the elaborate mechanical wheels and gearing of the German Enigma code- machine. Britain's secret triumph over Nazi communication security had a very great deal to do with the eventual military triumph of the Allies. Britain's code-breaking triumph further assured that cryptography would remain a state secret and one of the most jealously guarded of all sciences.
After World War II, cryptography became, and has remained, one of the crown jewels of the American national security establishment. In the United States, the science of cryptography became the high-tech demesne of the National Security Agency (NSA), an extremely secretive bureaucracy that President Truman founded by executive order in 1952, one of the chilliest years of the Cold War.
Very little can be said with surety about the NSA. The very existence of the organization was not publicly confirmed until 1962. The first appearance of an NSA director before Congress was in 1975. The NSA is said to be based in Fort Meade, Maryland. It is said to have a budget much larger than that of the CIA, but this is impossible to determine since the budget of the NSA has never been a matter of public record. The NSA is said to the the largest single employer of mathematicians in the world. The NSA is estimated to have about 40,000 employees. The acronym NSA is aptly said to stand for "Never Say Anything."