Bruce Sterling
bruces@well.com

Literary Freeware:  Not For Commercial Use

From THE MAGAZINE OF FANTASY AND SCIENCE FICTION:  
March 1994.  F&SF, 143 Cream Hill Road, West Cornwall, CT 
06796.  $26/yr; $31 outside USA

F&SF Science Column # 11:  "Spires on the Skyline"

	Broadcast towers are perhaps the single most obvious 
technological artifact of modern life.  At a naive glance, 
they seem to exist entirely for their own sake.  Nobody 
lives in them.   There's nothing stored in them, and they 
don't offer shelter to anyone or anything.   They're 
skeletal, forbidding structures that are extremely tall 
and look quite dangerous.  They stand, usually, on the 
highest ground available, so they're pretty hard not to 
notice.  What's more, they're brightly painted and/or 
covered with flashing lights.

	And then there are those *things* attached to them.   
Antennas of some kind, presumably, but they're nothing 
like the normal, everyday receiving antennas you might 
have at home: a simple telescoping rod for a radio,  a 
pair of rabbit ears for a TV.  These elaborate, 
otherworldly appurtenances resemble big drums, or sea 
urchin spines, or antlers.

	In this column, we're going to demystify broadcast 
towers, and talk about what they do, and why they look 
that way, and how they've earned their peculiar right to 
loom eerily on the skyline of every urban center in 
America.

	We begin with the electromagnetic spectrum.  Towers 
have everything to do with the electromagnetic spectrum.  
Basically, they colonize the spectrum.   They legally 
settle  various patches of it, and they use their 
homestead in the spectrum to make money for their owners 
and users.

	The electromagnetic spectrum is an important natural 
resource.  Unlike most things we think of as "resources," 
the spectrum is immaterial and intangible.  Odder still, 
it is limited, and yet, it is not exhaustible.  Usage of 
the spectrum is controlled worldwide by an international 
body known as the International Telecommunications Union 
(ITU), and controlled within the United States by an 
agency called the Federal Communications Commission (FCC).

	Electromagnetic radiation comes in a wide variety of 
flavors.  It's usually discussed in terms of frequency and 
wavelength, which are interchangeable terms.  All 
electromagnetic radiation moves at one uniform speed, the 
speed of light.  If the frequency of the wave is  higher, 
then the length of the wave  must by necessity become 
shorter.

	Waves are measured in hertz.   One hertz is one cycle 
of frequency per second, named after Heinrich Hertz, a 
nineteenth-century German physicist who was the first in 
history to deliberately send a radio signal.

	The International Telecommunications Union determines 
the legally possible uses of the spectrum from 9,000 hertz 
(9 kilohertz) to 400,000,000,000 hertz  (400 gigahertz).   
This vast legal domain extends from extremely low 
frequency radio waves up to extremely high frequency 
microwaves.   The behavior of electromagnetic radiation 
varies considerably along this great expanse of frequency.  
As frequency rises, the reach of the signal deteriorates; 
the signal travels less easily, and is more easily 
absorbed and scattered by rain, clouds, and foliage.

	After electromagnetic radiation leaves the legal 
domain of the ITU, its behavior becomes even more 
remarkable, as it segues into infrared, then visible 
light, then ultraviolet, Xrays, gamma rays and cosmic 
rays.

	From the point of view of physics, there's a strangely 
arbitrary quality to the political decisions of the ITU.   
For instance, it would seem very odd if there were an 
international regulatory body deciding who could license 
and use the color red.  Visible colors are a form of 
electromagnetism, just like radio and microwaves.  "Red" 
is a small piece of the electromagnetic spectrum which 
happens to be perceivable by the human eye, and yet it 
would seem shocking if somebody claimed exclusive use of 
that frequency.   The spectrum really isn't a "territory" 
at all, and can't really be "owned," even though it can 
be, and is,  literally auctioned off to private bidders by 
national governments for very large sums.  Politics and 
commerce don't matter to the photons.   But they matter 
plenty to the people who build and use towers.

	The ITU holds regular international meetings, the 
World Administrative Radio Conferences,  in which various 
national players jostle over spectrum usage.  This is an 
odd and little-recognized species of diplomacy, but the 
United States takes it with utter seriousness, as do other 
countries.  The resultant official protocols of global 
spectrum usage closely resemble international trade 
documents, or maybe income-tax law.  They are very arcane, 
very specific, and absolutely riddled with archaisms, 
loopholes, local exceptions and complex wheeler-dealings 
that  go back decades.  Everybody and his brother has some  
toehold in the  spectrum: ship navigation, aircraft 
navigation, standard time signals, various amateur ham 
radio bands, industrial remote-control radio bands, ship-
to-shore telephony, microwave telephone relays, military 
and civilian radars, police radio dispatch,  radio 
astronomy, satellite frequencies, kids' radio-controlled 
toys, garage-door openers, and on and on.

	The spectrum has been getting steadily more crowded 
for decades.  Once a broad and lonely frontier, inhabited 
mostly by nutty entrepreneurs and kids with crystal sets, 
it is now a thriving, uncomfortably crowded metropolis.  
In the past twenty years especially, there has been 
phenomenal growth in the number of machines spewing radio 
and microwave signals into space.   New services keep 
springing up:  telephones in airplanes, wireless 
electronic mail, mobile telephones, "personal 
communication systems,"  all of them fiercely demanding 
elbow-room. 

	AM radio, FM radio, and television all have slices of 
the spectrum.  They stake and hold their claim with 
towers.  Towers have evolved to fit their specialized 
environment: a complex interplay of financial necessity, 
the laws of physics, and government regulation.

	Towers could easily be a lot bigger than they are.  
They're made of sturdy galvanized steel, and the 
principles of their construction are well-understood.   
Given four million dollars, it would be a fairly simple 
matter to build a broadcast tower 4,000 feet high.   In 
practice, however, you won't see towers much over 2,100 
feet in the United States,  because the FCC deliberately 
stunts them.   A broadcast antenna atop a 4000-ft tower 
would hog the spectrum over too large a geographical area. 

	Almost every large urban antenna-tower, the kind you 
might see in everyday life,  belongs to some commercial 
entity.  Military and scientific-research antennas are 
more discreet, usually located in remote enclaves.  
Furthermore, they just don't look like commercial 
antennas.   Military communication equipment is not 
subject to commercial restraints and has a characteristic 
appearance:  rugged, heavy-duty, clunky, serial-numbered,  
basically Soviet-looking.  Scientific instruments  are 
designed to gather data with an accuracy to the last 
possible decimal point.  They may look  frazzled, but they 
rarely look simple.   Broadcast tower equipment by 
contrast is designed to make money, so it looks cheerfully 
slimmed-down and mass-produced and gimcrack.

	Of course, a commercial antenna must obey the laws of 
physics like other antennas, and has been designed to do 
that, but its true primary function is generating optimal 
revenue on capital investment.  Towers and their antennas 
cost as little as possible, consonant with optimal 
coverage of the market area, and the likelihood of 
avoiding federal prosecution for sloppy practices.  Modern 
antennas are becoming steadily more elaborate, so as to 
use thinner slices of spectrum and waste less radiative 
power.   More elaborate design also reduces the annoyance 
of stray, unwanted signals,  so-called "electromagnetic 
pollution."

	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 
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.