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.