Towers fall under the aegis of not one but two powerful bureaucracies, the FCC and the FAA, or Federal Aviation Administration. The FAA is enormously fond of massive air-traffic radar antennas, but dourly regards broadcast antennas as a “menace to air navigation.” This is the main reason why towers are so flauntingly obvious. If towers were painted sky-blue they’d be almost invisible, but they’re not allowed this. Towers are hazards to the skyways, and therefore they are striped in glaring “aviation white” and gruesome “international orange,” as if they were big traffic sawhorses.
Both the FCC and FAA are big outfits that have been around quite a while. They may be slow and cumbersome, but they pretty well know the name of the game. Safety failures in tower management can draw savage fines of up to a hundred thousand dollars a day. FCC regional offices have mandatory tower inspection quotas, and worse yet, the fines on offenders go tidily right into the FCC’s budget.
That orange and white paint costs a lot. It also peels off every couple of years, and has to be replaced, by hand. Depending on the size of the tower, it’s sometimes possible to get away with using navigation-hazard lights instead of paint, especially if the lights strobe. The size of the lights, and their distribution on the tower structure, and their wattage, and even their rate and method of flashing are all spelled out in grinding detail by the FCC and FAA.
In the real world — and commercial towers are very real-world structures — lights aren’t that much of an advantage over paint. The bulbs burn out, for one thing. Rain shorts out the line. Ice freezes solid on the high upper reaches of the tower, plummets off in big thirty-pound chunks, cracking the lights off (not to mention cracking the lower-mounted antennas, the hoods and windshields of utility trucks, and the skulls of unlucky technicians). The lights’ power sometimes fails entirely.
And people shoot the lights and steal them. In the real world, people shoot towers all the time. Something about towers — their dominating size, their lonely locales, or maybe it’s that color-scheme and that pesky blinking — seems to provoke an element of trigger-happy lunacy in certain people. Bullet damage is a major hassle for the tower owner and renter.
People, especially drunken undergraduates in college towns, often climb the towers and steal the hazard lights as trophies. If you visit the base of a tower, you will usually find it surrounded with eight-foot, padlocked galvanized fencing and a mean coil of sharp razor-wire. But that won’t stop an active guy with a pickup, a ladder, and a six-pack under his belt.
The people who physically build and maintain towers refer to themselves as “tower hands.” Tower engineers and designers refer to these people as “riggers.” The suit-and-tie folks who actually own broadcasting stations refer to them as “tower monkeys.” Tower hands are blue-collar industrial workers, mostly agile young men, mostly nonunionized. They’re a special breed. Not everybody can calmly climb 2,000 feet into their air with a twenty-pound tool-belt of ohmmeters, wattmeters, voltage meters, and various wrenches, clamps, screwdrivers and specialized cutting tools. Some people get used to this and come to enjoy it, but those who don’t get used to it, never get used to it.
While 2,000 feet in the air, these unsung knights of the airwaves must juggle large, unwieldy antennas. Quite often they work when the station is off the air — in the midnight darkness, using helmet-mounted coal-miners’ lamps. And it’s hot up there on the tower, or freezing, or wet, and almost always windy.
The commonest task in the tower-hand’s life is painting. It’s done with “paint-mitts,” big soppy gloves dipped in paint, which are stroked over every structural element in the tower, rather like grooming a horse. It takes a strong man a full day to paint a hundred feet of an average tower. (Rip-off hustlers posing as tower-hands can paint towers at “bargain rates” with amazing cheapness and speed. The rascals — there are some in every business — paint only the underside of the tower, the parts visible from the ground.)
Spray-on paint can be faster than hand-work, but with even the least breeze, paint sprayed 2,000 feet up will carry hundreds of yards to splatter the roofs, walls, and cars of angry civilians with vivid “international orange.” There simply isn’t much calm air 2,000 feet up in the sky. High-altitude wind doesn’t have to deal with ground-level friction, so wind-speed roughly doubles about every thousand feet.
Building towers is known in the trade as “stacking steel.” The towers are shipped in pieces, then bolted or welded into segments, either on-site or at the shop. The rigid sections are hauled skyward with a winch-driven ‘load line,’ and kept from swaying by a second steel cable, the ‘tag-line.’ Each section is bootstrapped up above the top of the tower, through the use of a tower-mounted crane, called the ‘gin pole.’ The gin pole has a 360-degree revolving device at its very top, the ‘rooster head.’ Each new section is deliberately hauled up, spun deftly around on the rooster head, stacked on top of all the previous sections, and securely bolted into place. Then the tower hands detach the gin pole, climb the section they just stacked, mount the ginpole up at the top again, and repeat the process till they’re done.
Tower construction is a mature industry; there have not been many innovations in the last forty years. There’s nothing new about galvanized steel; it’s not high-tech, but it’s plenty sturdy, it’s easy to work and weld, and it gets the job done. The job’s not cheap. In today’s market, galvanized steel towers tend to cost about a million dollars per thousand feet of height.
Towers come in two basic varieties, self-supporting and guyed. The self-supporting towers are heavier and more expensive, their feet broadly splayed across the earth. Despite their slender spires, the guyed towers actually require more room. The bottom of a guyed tower is tapered and quite slender, often a narrow patch of industrial steel not much bigger than the top of a child’s school-desk. But the foundations for those guy cables stretch out over a vast area, sometimes 100 percent of the tower’s height, in three or four different directions. It’s possible to draw the cables in toward the tower’s base, but that increases the “download” on the tower structure.
Towers are generally built as lightly as possible, commensurate with the strain involved. But the strain is very considerable. Towers themselves are heavy. They need to be sturdy enough to have tower-hands climbing any part of them, at any time, safely.
Small towers sometimes use their bracing bars as natural step-ladders, but big towers have a further burden. It takes a strong man, with a clear head, 3/4 of an hour to climb a thousand feet, so any tower over that size definitely requires an elevator. That brings the full elaborate rigging of guide rails, driving mechanism, hoisting cables, counterweights, rope guards, and cab controls, all of which add to the weight and strain on the structure. Even with an elevator, one still needs a ladder for detail work. Tower hands, who have a very good head for heights, prefer their ladders out on the open air, where there are fewer encumbrances, and they can get the job done in short order. However, station engineers and station personnel, who sometimes need to whip up the tower to replace a lightbulb or such, rather prefer a ladder that’s nestled inside the tower, so the structure itself forms a natural safety cage.
Besides the weight of the tower, its elevator, the power cables, the waveguides, the lights, and the antennas, there is also the grave risk of ice. Ice forms very easily on towers, much like the icing of an aircraft wing. An ice-storm can add hugely to a tower’s weight, and towers must be designed for that eventuality.
Lightning is another prominent hazard, and although towers are well-grounded, lightning can be freakish and often destroys vulnerable antennas and wiring.
But the greatest single threat to a tower is wind-load. Wind has the advantage of leverage; it can attack a tower from any direction, anywhere along its length, and can twist it, bend it, shake it, pound it, and build up destructive resonant vibrations.
Towers and their antennas are built to avoid resisting wind. The structural elements are streamlined. Often the antennas have radomes,
plastic weatherproof covers of various shapes. The plastic radome is transparent to radio and microwave emissions; it protects the sensitive antenna and also streamlines it to avoid wind-load.
An antenna is an interface between an electrical system and the complex surrounding world of moving electromagnetic fields. Antennas come in a bewildering variety of shapes, sizes and functions. The Andrew Corporation, prominent American tower builders and equipment specialists, sells over six hundred different models of antennas.
Antennas are classified in four basic varieties: current elements, travelling-wave antennas, antenna arrays, and radiating-aperture antennas. Elemental antennas tend to be low in the frequency range, travelling-wave antennas rather higher, arrays a bit higher yet, and aperture antennas deal with high-frequency microwaves. Antennas are designed to meet certain performance parameters: frequency, radiation pattern, gain, impedance, bandwidth, polarization, and noise temperature.
Elemental antennas are not very “elemental.” They were pretty elemental back in the days of Guglielmo Marconi, the first to make any money broadcasting, but Marconi’s radiant day of glory was in 1901, and his field of “Marconi wireless” has enjoyed most of a long century of brilliant innovation and sustained development. Monopole antennas are pretty elemental — just a big metal rod, spewing out radiation in all directions — but they quickly grow more elaborate. There are doublets and dipoles and loops; slots, stubs, rods, whips; biconal antennas, spheroidal antennas, microstrip radiators.
Then there’s the travelling-wave antennas: rhombic, slotted waveguides, spirals, helices, slow wave, fast wave, leaky wave.
And the arrays: broadside, endfire, planar, circular, multiplicative, beacon, et al.
And aperture variants: the extensive microwave clan. The reflector family: single, dual, paraboloid, spherical, cylindrical, off-set, multi-beam, contoured, hybrid, tracking…. The horn family: pyramidal, sectoral, conical, biconical, box, hybrid, ridged. The lens family: metal lens, dielectric lens, Luneberg lens. Plus backfire aperture, short dielectric rods, and parabolic horns.
Electromagnetism is a difficult phenomenon. The behavior of photons doesn’t make much horse sense, and is highly counterintuitive. Even the bedrock of electromagnetic understanding, Maxwell’s equations, require one to break into specialized notation, and the integral calculus follows with dreadful speed. To put it very simply: antennas come in different shapes and sizes because they are sending signals of different quality, in fields of different three-dimensional shape.
Wavelength is the most important determinant of antenna size. Low frequency radiation has a very long wavelength and works best with a very long antenna. AM broadcasting is low frequency, and in AM broadcasting the tower is the antenna. The AM tower itself is mounted on a block of insulation. Power is pumped into the entire tower and the whole shebang radiates. These low-frequency radio waves can bounce off the ionosphere and go amazing distances.
Microwaves, however, are much farther up the spectrum. Microwave radiation has a short wavelength and behaves more like light. This is why microwave antennas come as lenses and dishes, rather like the lens and retina of a human eye.
An array antenna is a group of antennas which interreact in complex fashion, bouncing and shaping the radiation they emit. The upshot is a directional beam.
“Coverage is coverage,” as the tower-hands say, so very often several different companies, or even several different industries, will share towers, bolting their equipment up and down the structure, rather like oysters, limpets and barnacles all settling on the same reef.
Here’s a brief naturalist’s description of some of the mechanical organisms one is likely to see on a broadcast tower.
First — the largest and most obvious — are things that look like big drums. These are microwave dishes under their protective membranes of radome. They may be flat on both sides, in which case they are probably two parabolic dishes mounted back-to-back. They may be flat on one side, or they may bulge out on both sides so that they resemble a flying saucer. If they are mounted so that the dish faces out horizontally, then they are relays of some kind, perhaps local telephone or a microwave long-distance service. They might be a microwave television-feed to a broadcast TV network affiliate, or a local cable-TV system. They don’t broadcast for public reception, because the microwave beams from these focused dishes are very narrow. Somewhere in the distance, probably within 30 miles, is another relay in the chain.
A tower may well have several satellite microwave dishes. These will be down near the base of the tower, hooked to the tower by cable and pointed almost straight up. These satellite dishes are generally much bigger than relay microwave dishes. They’re too big to fit on a tower, and there’s no real reason to put them them on a tower anyway; they’ll scarcely get much closer to an orbiting satellite by rising a mere 2,000 feet.
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 waveguide 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 halfway 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 “broadcas
t” 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.
Essays. FSF Columns Page 12
