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 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.