Broadcast engineeringVHF FM, 88–108 MHzwith the AM contrast

From Microphone
to Mast

A radio tower is, structurally, a very tall bracket. Almost none of the interesting engineering is in the steel — it is in the chain of boxes that turns air pressure in a studio into a 100-kilowatt electromagnetic field leaving a stack of aluminium rings 300 metres up. Here is every stage of that chain, and every physical part that holds it in the sky.

Carrier: 98.7MHz
Transmitter power out: 19.0kW
Antenna gain: 7.8dBd
Effective radiated power: 100kW
Height above avg terrain: 599m
FCC class: C

Overview

The whole chain, end to end

Fifteen boxes stand between a presenter's voice and the field in your car. Five of them are in the studio, five in a windowless building at the base of the tower, and five are the plumbing that carries kilowatts of radio-frequency energy up the steel. The tower holds the last one high enough to see a long way.

The FM signal chain. Everything up to the exciter is audio; everything after it is radio. The dashed branches are the two paths that never go on air — a sampled fraction of the forward and reflected power feeding the alarm system, and a water-cooled resistor the transmitter can be switched into for testing.

The one sentence version

The studio makes the programme; the exciter writes it onto a carrier by wobbling the carrier's frequency; the amplifier makes that carrier enormous; the feedline carries it up; and the antenna converts the guided wave into a free one. The tower's only job is height and stability — except on AM, where the tower is the antenna.

01STUDIO

Stage one

Air pressure to composite baseband

A studio is a room full of gain stages and one very opinionated box. Microphones (large-diaphragm condensers on booms, or dynamic broadcast mics like the RE20 and SM7 that reject room noise) feed preamps in a console. The console is really a summing and routing engine: mic channels, telephone hybrids for callers, the playout automation computer running the music log and spots, network feeds, and a logger recording everything the station transmits.

The audio processor

Between the console and the transmitter sits the box that defines a station's sound: a multiband processor such as an Orban Optimod or Omnia. It runs wideband AGC, then splits the audio into four to six bands for independent compression, then a look-ahead peak limiter, then a composite clipper. It exists for two reasons. Commercially, it makes the station loud and consistent across a car radio's dynamic range. Legally, it guarantees the station never exceeds ±75 kHz deviation — overmodulation splatters energy into the adjacent channel and is a licence problem.

Pre-emphasis

FM noise rises with modulating frequency (the demodulator differentiates, so noise power goes as f²). Broadcasters compensate by boosting treble before transmission and cutting it identically in the receiver. The time constant is 75 µs in the Americas and South Korea, 50 µs almost everywhere else — a single-pole shelf that starts lifting around 2.1 kHz and reaches about +17 dB at 15 kHz. The processor's limiter has to work in the pre-emphasised domain, which is why heavily processed stations sound dull on cymbals: the limiter is fighting its own treble boost.

Building the composite

The stereo generator assembles a single baseband waveform — the multiplex or MPX signal — that carries mono, stereo, and data in one 99 kHz-wide envelope. Its structure is the reason FM stereo is backwards-compatible: a mono receiver simply low-passes everything above 15 kHz and hears L+R.

MPX(t) = (L+R) — 30 Hz to 15 kHz, 90% modulation budget + 0.09 · cos(2π·19000·t) — stereo pilot, 8–10% injection + (L−R) · cos(2π·38000·t) — DSB-SC, 23–53 kHz + RDS(t) · cos(2π·57000·t) — 1187.5 bit/s BPSK, 2–4% + SCA(t) — optional subcarriers at 67 / 92 kHz

The 38 kHz subcarrier is suppressed — transmitting it would waste power — so the receiver regenerates it by doubling the 19 kHz pilot, which also tells the receiver that a stereo signal is present at all (that is the light on your dashboard). RDS at 57 kHz is the third harmonic of the pilot and is phase-locked to it, carrying the station name, programme type, traffic flags and the scrolling song title.

The composite baseband, before it ever reaches a radio frequency. This entire 0–99 kHz waveform is what the exciter treats as "the audio." Everything above 15 kHz is invisible to a mono receiver, which is how colour was added to a black-and-white service without breaking it.

Also in the rack

EAS encoder/decoderMonitors assigned sources for alert tones, interrupts programme audio, inserts the header bursts and attention signal, and logs every alert it relays. Legally mandatory, and it has hard priority over everything else in the console.
Telephone hybridCancels the caller's own return audio so a phone line can be mixed live without howling.
Audio router and loggerA matrix that lets any source reach any studio, and a continuous archive for compliance and complaints.
Silence sensorWatches for dead air and switches to a backup playlist if the studio goes quiet for more than a few seconds.

02THE LINK

Stage two

Getting the programme to the hill

Studios sit downtown where guests and staff are. Transmitters sit on the highest ground within reach, often tens of kilometres away. The link between them — the STL — is the single most common cause of a station going silent, so almost every site runs two of them with automatic failover.

Licensed microwave

The classic: a small parabolic or Yagi antenna on the studio roof aimed at the tower, in the 944–952 MHz aural STL band (or 18/23 GHz for high-capacity links). Line of sight required. A composite STL sends the whole 0–99 kHz MPX signal as one FM-modulated carrier, preserving pilot phase exactly.

IP codec

Now the default. An audio-over-IP codec (AES67, or a hardware codec running Opus/AAC) sends the programme over fibre or bonded cellular. Cheap and flexible, but adds buffering latency and needs a jitter buffer deep enough to ride out the network.

Leased circuit or fibre

A dedicated telco path. Reliable, no line-of-sight requirement, and the recurring cost is the reason many stations moved to IP.

Satellite / off-air relay

Network programming arrives by satellite receiver at the site. As a last-ditch backup, some sites simply demodulate a sister station off air and rebroadcast it.

Where the processor lives matters

If the STL carries composite MPX, the processor and stereo generator are at the studio and the exciter just amplifies what arrives. If the STL carries plain L/R audio, the processor sits at the transmitter site instead. The second arrangement survives a lossy link better — you never want a data-reduced codec upstream of a composite clipper, because the clipper amplifies codec artefacts into audible distortion.

03TRANSMITTER

Stage three

Writing audio onto a carrier, then making it enormous

The exciter

The exciter is where audio stops and radio starts. A modern one is almost entirely digital: the composite baseband is sampled, and a direct digital synthesiser generates the carrier with its instantaneous phase advanced or retarded in proportion to the integral of the modulating signal. That is the definition of frequency modulation —

s(t) = A_c · cos( 2π f_c t + 2π Δf ∫ m(τ) dτ ) f_c = 98.7 MHz carrier, locked to a GPS-disciplined or OCXO reference (±2000 Hz allowed, ±100 Hz typical) Δf = 75 kHz maximum deviation at 100% modulation W = 15 kHz highest audio frequency in the mono channel Deviation ratio D = Δf / W = 75 / 15 = 5 Carson bandwidth B ≈ 2(Δf + W) = 2(75 + 15) = 180 kHz (channel allocation is 200 kHz)

Note what doesn't change: A_c is constant. An FM carrier has a flat envelope regardless of how loud the music is. That single fact is why FM transmitters are so much simpler and more efficient than AM or television transmitters — the amplifier chain never has to be linear, so it can be run hard in Class C or a switching class where 75–80% of the DC power becomes RF instead of heat. (Add HD Radio's digital sidebands and the envelope is no longer constant; those transmitters need a linear amplifier and pay maybe fifteen efficiency points for it.)

The amplifier chain

The exciter puts out roughly 20 watts. Getting to 19 kilowatts is three stages of gain, each roughly 15–20 dB.

Intermediate power amplifierA single driver module taking 20 W to several hundred watts, usually a pair of LDMOS transistors on a heat-spreader.
Final power amplifierIn a modern rig, eight to sixteen identical pallets — LDMOS field-effect transistors such as the BLF188XR, each good for 1–1.5 kW — bolted to a water block or a big extruded heatsink. Their outputs are summed in a Wilkinson or hybrid combiner. The design is deliberately redundant: lose one pallet and the station drops a fraction of a dB and keeps broadcasting, with an alarm at the remote control. Older sites still run a single grounded-grid tetrode or a Doherty-configured tube, which is one part instead of sixteen but needs replacing every few years and runs on several kilovolts of plate supply.
Power supplyRectifies three-phase mains to a regulated 50 V DC rail for solid-state, or 5–10 kV for a tube. This is the heaviest thing in the building.
CoolingA 19 kW transmitter at 72% efficiency dumps roughly 7 kW of heat into the room continuously. Either forced air through a duct to outside, or a glycol loop to a dry cooler on the pad. The building's HVAC is sized around this number, not around human comfort.

Between the amplifier and the antenna

Harmonic (low-pass) filterA Class C stage is a distortion generator by design; its output is rich in harmonics at 197 MHz, 296 MHz and beyond, which would land on aviation and public-safety bands. A cavity or lumped-element low-pass filter attenuates them by 60 dB or more.
Directional couplerA short section of line with two loosely coupled sampling ports, one responding to the forward wave and one to the reverse. It is the transmitter's only view of the antenna 300 m away.
Coaxial switchA motorised bat-wing or drum switch that can route the transmitter to the antenna, to the dummy load, or connect a standby transmitter to the antenna. Switching it takes a few seconds and must happen with RF off.
Dummy loadA precisely 50-ohm resistor rated for full power, air- or water-cooled, so the transmitter can be run and tuned without radiating.
CombinerWhere several stations share one antenna — normal on a tall commercial mast — a bank of high-Q cavity filters lets each transmitter feed the same line while presenting a very high impedance to its neighbours' frequencies. Combiners are lossy and fussy, but a second antenna on the same tower is worse.
Standby transmitterOften a lower-power unit that keeps the station on the air at reduced coverage rather than off the air entirely.
Remote control / transmitter site unitA Burk or Davicom box watching forward power, reflected power, PA current, exhaust temperature, tower light status, door contacts, generator state and line pressure. It calls, emails or pages an engineer, and it can cut power or switch antennas by itself.

Nothing here moves

Someone expecting machinery is usually surprised. The whole RF path is solid-state and completely static. The only moving parts on the entire site are fans, HVAC compressors, the coax switch, contactors and relays, the generator, and — on directional AM arrays — motorised variable capacitors and roller inductors in the phasor.

04FEEDLINE

Stage four

Three hundred metres of very expensive plumbing

At 19 kilowatts, a transmission line is not a cable — it is a pressurised waveguide made of copper tube, and it is one of the most expensive single items on the site.

Rigid coaxial lineTypically 3⅛-inch (or 4-1/16 or 6⅛ on the biggest installations): a copper outer conductor with a solid or tubular inner conductor held concentric by PTFE insulator pins every few inches. It ships in 20-foot lengths that bolt together with flanges and inner-conductor bullets. Characteristic impedance is 50 Ω, and the dielectric is air.
Nitrogen or dry-air pressurisationBecause the dielectric is air, moisture is the enemy — condensation inside the line causes arcing at full power. The line is sealed and held at 3–10 psi of dry nitrogen or dehydrated air. A dropping pressure alarm is how engineers find a leak before they find a burned connector.
Expansion jointsA 300 m copper line moves several centimetres between a −20 °C January night and a black-painted July afternoon. Sliding-contact expansion joints absorb it; without them the line tears its own flanges apart.
Hangers and hoisting gripsSpring hangers every few metres carry the line's own weight (a 3⅛-inch line runs about 3 kg per metre — nearly a tonne hanging on the tower) while allowing that thermal movement.
Ice bridgeA galvanised steel canopy on legs, running from the transmitter building wall to the tower base, carrying the coax and control cables. Its job is purely mechanical: it stops a kilogram of ice shed from 200 m up from crushing the feedline.
Entry panel and gas-tube arrestorsEvery conductor entering the building passes through a single bonded copper plate, with surge protection at the boundary. Lightning that gets onto the coax shield is diverted here rather than inside the transmitter.

Why VSWR is the number engineers actually watch

The antenna presents a load impedance. If it is not exactly 50 Ω — because a connector corroded, or a bay filled with ice, or a bullet worked loose — some of the forward power reflects and travels back down the line. Forward and reflected waves add and cancel along the line, producing standing waves whose peaks can be far higher in voltage than the travelling wave alone. That is what punches a hole through a PTFE insulator.

Γ = (Z_L − Z₀) / (Z_L + Z₀) voltage reflection coefficient VSWR = (1 + |Γ|) / (1 − |Γ|) standing wave ratio Return loss = −20 log₁₀|Γ| dB VSWR 1.10 → 0.23% of power reflected healthy VSWR 1.40 → 2.8% reflected typical foldback threshold — transmitter reduces power VSWR 2.00 → 11.1% reflected transmitter shuts down and alarms

Line loss matters just as much, because it is paid for every second the station is on. Roughly 0.55 dB over 300 m of 3⅛-inch line at 98 MHz means about 12% of the transmitter's output is warming the copper — a little over 2 kW, continuously, forever. Going up one line size is a five-figure capital cost that pays back in electricity.

05ANTENNA

Stage five

Where guided waves become free ones

The antenna is the only component that does the thing the whole site exists for: it converts a wave travelling inside a copper tube into a wave travelling through space. On FM it is a stack of near-identical elements — bays — mounted on the tower's face or on a pole above its top.

A six-bay side-mounted FM antenna. The gain comes entirely from geometry, not from any active component. Stacking six identical elements a wavelength apart makes their fields add along the horizon and cancel above and below it — the same 19 kW, aimed rather than scattered.

Circular polarisation

Early FM was horizontally polarised, which suited rooftop dipoles and was hopeless for car whip antennas — which are vertical. Since 1977 the FCC has allowed equal vertical power, and essentially every station now runs circular polarisation: each bay radiates a horizontal and a vertical component of equal amplitude, 90° apart in phase, so the field vector rotates once per cycle. The catch is that ERP is specified per plane, so a circularly polarised station needs twice the transmitter power of a horizontal-only one for the same licensed figure.

Beam tilt and null fill

Stacking bays produces gain by narrowing the vertical beam — a six-bay array concentrates energy into roughly ten degrees. Two problems follow. First, from 600 m up, the city is below the horizon, so a perfectly horizontal main beam overshoots it; the array is given a fraction of a degree of beam tilt, either mechanically or by progressively phasing the bays. Second, an ideal uniform array has deep nulls between its lobes, and those nulls land on the ground at specific distances — real dead zones a few kilometres out. Deliberately unbalancing the bay amplitudes and phases fills them in, trading a little peak gain for coverage that doesn't have holes in it.

Vertical-plane pattern of the six-bay array. Gain is bought by narrowing this beam, and the narrower it gets the more precisely it must be aimed. At a 600 m HAAT, the difference between 0° and 1.5° of tilt is the difference between covering the city and covering the stratosphere above it.

06THE TOWER

Stage six

The steel, and what it has to survive

Everything above is electronics. The tower is a civil-engineering problem: hold a few tonnes of antenna and cable at 300 m, in wind, in ice, for fifty years, in a structure slender enough that you can see through it.

A guyed lattice mast — the classic broadcast tower. None of this steel is electrically part of the antenna. The whole structure is bonded and grounded; it exists to put the six rings at the top in the right place and keep them there through wind, ice and lightning.

Three ways to build the structure

Guyed mast

A slender triangular lattice, 45 cm to 2 m across the face, held upright by three sets of guys at 120°. Cheapest per metre by a wide margin and the only practical way past about 200 m — but it needs a large circle of land, since the anchors sit at roughly 60–80% of the tower height from the base.

Self-supporting tower

A three- or four-legged tapered lattice that carries everything in its own foundation. No guy easement needed, so it fits a small plot, but the steel weight climbs steeply with height and it becomes uneconomic above roughly 120–150 m.

Monopole

A tapered steel tube in slip-jointed sections. Compact, quick to erect, visually acceptable to planning boards, and the usual answer under about 60 m — but it has almost no room inside for cable and its deflection at the top is large.

Someone else's structure

Plenty of stations simply lease space on a broadcast candelabra, a taller station's mast, or a downtown building. The engineering then becomes a combiner-and-isolation problem rather than a structural one.

What the structure is actually designed against

In the United States the governing document is ANSI/TIA-222, currently Revision I (2023), with Revision H still widely referenced in existing analyses. It sets the loads a tower must survive and the risk category it belongs to — and a broadcast tower serving emergency alerting sits in a higher risk category, meaning higher design loads, than a private radio mast.

WindDesigned to a 3-second gust speed taken from a map, typically 90–130 mph at ground level and higher aloft, applied to the projected area of the steel plus every appurtenance — antennas, cable ladders, mounts. Adding a tenant's dishes to an existing tower is a structural change requiring reanalysis, which is why tower owners insist on mapping every attachment.
Radial iceA design ice thickness, typically 6–38 mm, accreted on every member and cable, which multiplies both the dead weight and the wind area. Ice governs the design in most northern sites. It is also why bays get radomes and heaters, and why the ice bridge exists.
Torsion and twistAntennas with narrow patterns care about angular displacement, not just deflection. Serviceability limits are written in degrees of twist and sway, because a microwave dish 300 m up that rotates half a degree in a gust drops its link.
Guy tension and creepGuys are pre-tensioned to roughly 8–10% of their rated breaking strength. Too slack and the mast buckles; too tight and the compression in the mast rises. Steel strand creeps over years, so tensions are re-measured and adjusted on a maintenance cycle.
CorrosionHot-dip galvanising to ASTM A123 is the whole corrosion strategy. Anchor rods buried at the guy anchors are the classic failure point, because they corrode invisibly below grade; inspection means excavating them.
FoundationsA drilled pier or spread footing under the mast in compression, and anchor blocks resisting a large uplift force. Sized from a geotechnical report — the same tower on rock and on soft clay is a completely different foundation.

The exception

On AM, the tower is the antenna

Everything above assumes the steel is inert. On a classic medium-wave AM station it is not — and the reason is simply wavelength. At 98.7 MHz a half-wave element is about 1.5 m long, so you bolt six of them near the top of whatever structure you have. At 1010 kHz a quarter-wave is 74 metres. There is nothing to hang; the tower has to be the radiator.

The same steel, two completely different electrical roles. On FM the tower is scaffolding; on AM it is the circuit. An AM radiator's base sits at high RF potential, which is why the base insulator, the tuning hut and the buried radial field all exist — and why the fence is not optional.

The parts an AM site has that an FM site doesn't

Base insulatorA porcelain or fibreglass cylinder carrying the entire compressive load of the tower while standing off several kilovolts of RF. Series-fed towers sit on one; shunt-fed towers stay grounded and are driven through a sloping wire instead.
Antenna tuning unitThe small hut at the tower base — an L or T network of roller inductors and vacuum capacitors that transforms the tower's own feedpoint impedance (which might be 30 − j200 Ω) to the 50 Ω the transmission line expects. Also called the doghouse.
Ground radial systemTypically 120 buried copper wires, each a quarter-wavelength long, spread radially from the base and bonded to a ground screen. A monopole works against its image in the earth, and earth is a mediocre conductor — the radials are there to be a better one. Coverage depends enormously on soil conductivity, which is why AM stations in the wet Midwest travel and desert stations don't.
Strain insulatorsBroken into every guy at intervals, so no guy section is resonant near the operating frequency. An unbroken guy would re-radiate and wreck the pattern.
Austin ring transformerThe tower lights need mains power, but the tower is at RF potential. A donut-shaped air-gapped transformer couples 60 Hz across the insulator without providing an RF path to ground.
IsocouplerThe same problem for coaxial cable, when an FM or cellular tenant is mounted on a live AM tower. An isocoupler passes the tenant's RF while blocking the AM frequency.
Phasor and sample systemDirectional AM stations use two to six towers. A phasor cabinet splits the transmitter's power among them with precisely controlled amplitude and phase, and sampling loops on each tower feed an antenna monitor that verifies those ratios continuously. This is the one part of a broadcast site that genuinely has to be tuned by hand, and the reason many AM stations change pattern and power at sunset.
Base current meter and lighting chokeA thermocouple ammeter in the tower's base leg, read as the primary evidence the antenna is working, plus RF chokes keeping the lighting circuit out of the RF path.

The rest of the site

Everything else inside the fence

Standby generator and fuel

Diesel or propane, sized for the transmitter plus HVAC — 40–60 kW for a full-power FM. An automatic transfer switch starts it within seconds of a utility failure, and it exercises itself weekly whether anyone is watching or not.

Electrical service and switchgear

Three-phase service, distribution panel, and a surge protective device at the entrance. A 100 kW-ERP FM site typically draws 30–40 kW continuously, which makes electricity the largest operating cost after staff.

UPS

Batteries for the control, monitoring and STL equipment only — nobody carries 19 kW of RF on a UPS. The point is that the site can still be reached by the engineer during the seconds before the generator picks up.

HVAC and ventilation

Sized to move the transmitter's waste heat, with an outside-air economiser in cold climates and a high-temperature alarm wired into the remote control.

Monitoring and telemetry

Modulation monitor, spectrum analyser, off-air receiver, tower light monitor. Loss of a top beacon has to be reported to the FAA within 30 minutes and repaired promptly — this is one of the most common enforcement issues at a tower site.

RF exposure controls

The fence, the signage, and posted access procedures exist because the fields near an energised antenna can exceed exposure limits. Climbing crews get the transmitter reduced or switched off before they go up.

Coverage

Why height beats power

Two numbers determine an FM station's licensed coverage, and only one of them is bought from the electricity company.

ERP = TPO × line efficiency × antenna gain = 19.0 kW × 0.881 × 6.03 = 100.9 kW line loss 0.55 dB; antenna gain 7.8 dBd HAAT = radiation centre AMSL − average terrain elevation averaged from 3 to 16 km out, along at least 8 evenly spaced radials Radio horizon d ≈ 4.12 (√h_t + √h_r) km, metres; the 4/3-earth approximation h_t = 600 m, h_r = 2 m → d ≈ 4.12(24.5 + 1.4) ≈ 107 km

Doubling transmitter power costs double the electricity forever and buys 3 dB. Doubling the effective height above terrain costs a one-time construction bill and buys roughly 6 dB — plus a horizon that is further away, which no amount of power can substitute for. VHF does not usefully bend around hills. This is why broadcasters fight over mountaintops.

The FCC caps the two together, so a station cannot trade one for the other without limit. The class table is really a list of permitted (ERP, HAAT) pairs:

Class	Max ERP	Max HAAT	Protected contour	Typical role
A	6 kW	100 m	60 dBµV/m	Small-town and suburban stations
B1	25 kW	100 m	57 dBµV/m	Northeast and West Coast zones
B	50 kW	150 m	54 dBµV/m	Major markets in the dense zones
C3	25 kW	100 m	60 dBµV/m	Smaller markets elsewhere
C2	50 kW	150 m	60 dBµV/m	Mid-size markets
C1	100 kW	299 m	60 dBµV/m	Large regional coverage
C0	100 kW	450 m	60 dBµV/m	Mountaintop and tall-mast stations
C	100 kW	600 m	60 dBµV/m	The biggest signals in the country

The separate principal community contour of 70 dBµV/m must cover the city of licence — a station may not simply point its coverage at the suburbs it would rather have.

Reference

One site, by the numbers

A plausible full-power Class C FM installation, from the microphone to the anchor bolts.

Item	Figure	Note
Carrier frequency	98.7 MHz	200 kHz channel spacing; odd tenths in the US
Peak deviation	±75 kHz	100% modulation, plus 10% for subcarriers
Occupied bandwidth	≈180 kHz	Carson's rule
Pre-emphasis	75 µs	50 µs outside the Americas
Transmitter power output	19.0 kW	Solid-state LDMOS, ~72% efficient
Site AC demand	≈35 kW	Transmitter, HVAC, lights, ancillaries
Transmission line	3⅛ in	Rigid coax, 50 Ω, nitrogen at 5 psi
Line length / loss	300 m / 0.55 dB	≈2.3 kW dissipated as heat in the copper
Antenna	6 bays, CP	Side-mounted, 1λ spacing, radomes and heaters
Antenna gain	7.8 dBd	≈6.0× power gain per plane
Beam tilt	1.5°	With null fill to remove ground-level dead zones
Effective radiated power	100 kW	Per plane, horizontal and vertical
Tower height	300 m	Guyed triangular lattice, 4 guy levels
Guy anchor radius	≈200 m	Roughly 2/3 of tower height, three radials
HAAT	599 m	Tower on a ridge; class limit is 600 m
Radio horizon	≈107 km	To a 2 m receiving antenna
VSWR alarm / trip	1.4 / 2.0	Foldback, then shutdown
Structural standard	TIA-222-I	2023 revision; Rev H still widely cited

The shortest answer

A radio station is a chain of impedance transformations. Air pressure becomes a voltage at the microphone, a voltage becomes a frequency deviation at the exciter, a small RF voltage becomes a large one at the amplifier, a guided wave in 50 ohms of copper becomes a free wave in 377 ohms of empty space at the antenna — and the tower's contribution to all of this is to hold the last transformation up where the horizon is far away.