18Reading an antenna: towers, farms and the public records behind them

The short version: Tall transmitting towers tend to gather in clusters called antenna farms, usually on high ground and often outside the city center. In the US, anyone can look up registered towers for free in the FCC's Antenna Structure Registration (ASR) database, and look up wireless licenses in the Universal Licensing System (ULS); broadcast stations have their own Media Bureau search tools, and regulators in Canada, the UK and Australia run similar public registers. The records cover tall structures and licensed transmitters, not every small antenna, and the shape of a structure tells you something about what it does.

What is an antenna farm?

What you see: Several tall towers standing close together on a hill or beside a highway, miles from the city center.

An antenna farm is a place where many transmitting or receiving antennas, towers or dishes are clustered, usually on high ground.³²⁵ Stations go there for height, to share costs, for backup (one tower can cover for another), and because high ground has fewer obstructions.³²⁵ Wikipedia's examples include Dedham/Needham outside Boston, Mount Wilson above Los Angeles and Sutro Tower in San Francisco.³²⁵

The term has an odd legal history in the US. The FCC's rules contain a section titled "Designated Antenna Farm Areas," 47 CFR 17.9, but it has been a placeholder for decades: the FCC has never designated or defined an antenna farm.³²⁸ Its environmental rules do refer to de facto areas "in which similar antenna towers are clustered."³²⁸ So the antenna farms you see grew by habit and economics, not by federal map.

Why are there so many towers in Needham?

What you see: The tall towers on the Needham/Newton line near Route 128 that most Boston TV antennas point toward.

Needham is Boston's TV antenna farm. WBZ-TV's Cedar Street tower went up in 1957, about 1,200 ft (366 m) tall at first, replacing a tower destroyed in 1954; by 2009 it was described as a 1,295 ft (395 m) guyed tower, with WCVB a tenant since 1972.³⁰³ Nearby stand the Cabot Street candelabra and the "FM-128" community tower, the former WHDH-TV channel 5 tower, where WCRB 102.5 moved.³⁰³ American Tower is landlord at both the Cedar Street and Cabot Street sites.³⁰²⁵⁷

The farm works as a set. After the TV repack, WGBH and WFXT made Cabot Street their main site, and the candelabra became the full-power backup for WBZ-TV, WCVB, WSBK and WGBX.⁵⁷ That is the redundancy argument for antenna farms in practice: if one antenna needs work, a station moves to its neighbor and stays on the air.

How do I tell a guyed tower from a candelabra?

What you see: One very tall, thin tower held up by sloping cables, and another whose top splits into several arms.

A guyed tower is a slender mast held upright by steel cables (guy wires) anchored in the ground around it. It uses little steel for its height, but the cables need a wide footprint of land. Cedar Street in Needham is described as a guyed tower.³⁰³

A candelabra tower is a self-supporting tower whose top divides into arms, each holding a separate antenna, like a candle holder. It lets several TV stations each have an antenna near the top instead of stacking them one above another. The Cabot Street tower in Needham is a stainless-steel candelabra that went up at the end of the 1960s.⁵⁷ Even on a candelabra, height still matters: WGBH-TV's antenna sits at the very top of the stack there.⁵⁷

FCC license records also label each structure with a type code. In the FCC's instructions for Form 601 Schedule M (2008, revised 2010), TOWER means "a free standing or guyed structure used for communications purposes," MAST means a self-support structure, POLE means a pole used only to mount an antenna, and BANT means a building with an antenna on top.³²⁹ Towers in an array are coded by count and position, so 3TA2 is the second tower in a three-tower array.³²⁹

Why do AM stations use several tall towers?

What you see: Two or more plain towers standing in a line in a marsh or field, with no visible antennas on them.

On AM, the tower is the antenna. The whole steel structure radiates, which is why AM towers are so tall and look bare. Its height is tied to the wavelength: WBZ's two towers in Hull are each a half wavelength tall at the station's old frequency of 990 kHz and stand a quarter wavelength apart.³⁰⁵

Several towers fed together form a directional array. The signals add up in some directions and cancel in others. In Hull, the two towers make a heart-shaped ("cardioid") pattern that cuts the signal over the Atlantic and sends it toward metro Boston, at 50 kW day and night.³⁰⁵ So a row of identical AM towers usually means a station is shaping its signal, often to protect another station on the same frequency or to avoid wasting power over water or empty land.

What are the flat panels on a rooftop mast?

What you see: A mast on top of a skyscraper with rectangular panels stacked up its sides.

On a downtown FM mast, those panels are often broadcast antennas, with each stacked unit called a bay. As of 2004, the Prudential Tower's mast held a six-bay panel "master" antenna that carried four FM stations at once (100.7, 104.1, 106.7 and 107.9), plus a separate ERI panel antenna for three more (92.9, 96.9 and 105.7).³⁰⁴ A master antenna is one antenna shared by several stations, whose transmitters are combined before the signal goes up the mast.

Not every panel on a roof is FM. Cell carriers also use panel antennas, and the two can look similar from the street. How carriers build their networks is in cellular, and the sister guide Street Code covers small cell equipment on poles.

The FCC describes the "sector" antennas cell carriers commonly use in urban and suburban areas as rectangular panels "about 1 by 4 feet in size," usually arranged in three groups of three; some base stations instead use pole-like antennas 10 to 15 ft (3 to 4.6 m) long.²⁸³ Point-to-point microwave antennas are "usually circular dish or rectangular in shape" and send a directed beam to a receiving antenna.²⁸³ The FCC says cell towers are typically 50 to 200 ft (15 to 61 m) high.³³⁰

What carries cell signals into a tunnel?

What you see: A thick cable running along a subway tunnel wall, and small antennas on station ceilings.

Inside tunnels and big buildings, carriers use a distributed antenna system (DAS): many small antennas fed from a central point. In the MBTA's system, carrier signals are converted to light, carried over fiber, then turned back into radio.⁷³ The 2012 build-out used 71 remote amplifier sites, 354 antennas and 41,585 ft (12.7 km) of radiating cable, also called leaky feeder: coaxial cable with slots that let signal leak out along its whole length.³⁰⁷ A company that owns a network like this and lets several carriers share it is called a neutral host.

Which towers are in the FCC's public database?

What you see: A tall tower on a hill or near an airport, and a question about whether it is on record.

The FCC's Antenna Structure Registration (ASR) applies to structures more than "200 feet above ground level" (61 m), and to shorter ones that may interfere with the flight path of a nearby airport.³³¹ Exemptions are listed in 47 CFR 17.7(e).³³¹ Each registered structure gets an Antenna Structure Registration Number (ASRN).

The flip side: a tower under 200 ft that is not near an airport generally does not need to be registered, so many shorter cell towers and most rooftop and pole-mounted antennas will not appear in ASR. For those, the license records in ULS are the better route.

The FCC no longer mails hard-copy registrations; the public searches online.³³²

How do I look up a tower near me?

What you see: A tower you pass every day, and a question about who owns it.

1. Open ASR Registration Search. The FCC's ASR page offers a public Registration Search and an Application Search.³³²
2. Search by location. You can search by coordinates (NAD83) with a radius, by city and state, or by a height range. If you already know it, you can also use the registration number, the FAA study number, or the owner's name or FCC Registration Number (FRN).³³³
3. Read the record. It names the owner and gives the structure's details. The owner's Taxpayer Identification Number is never shown.³³³
4. Check for broadcast stations. The Media Bureau's "ASRN within radius" tool takes a latitude, longitude and radius, and the AM, FM and TV Query results link to registered structures within 0.5 km (about a third of a mile) of a station.³³⁴

How do I find out who holds a radio license?

What you see: A tower or rooftop antenna that is not tall enough to be registered, or a radio system you hear on a scanner.

The Universal Licensing System (ULS) is the FCC's database of wireless licenses. Its License Search offers a basic search (call sign, FRN or name), an advanced search, a geographic search by radius, market-based searches, site-based searches (including by ASR number and by frequency), buildout searches and service-specific searches.³³⁵ Results can be mapped or downloaded.³³⁵

Broadcast stations are handled separately, in the Media Bureau's AM, FM and TV Query tools, which link back to ASR records.³³⁴

A license tells you who is authorized to transmit, on which frequencies and from where. It does not tell you what anyone is saying, and listening rules are a separate matter, covered by jurisdiction in scanners.

In Boston

To find the Needham towers, run an ASR search on coordinates of about 42.310 N, 71.237 W with a small radius. WBZ-TV's Cedar Street site is at about 42°18′37″N 71°14′12″W.³⁰² The records give official heights, which matter here because the trade and reference sources do not always agree: for WBZ's AM towers in Hull, Wikipedia gives 520 ft (158 m) and Fybush gives 500 ft (152 m).³⁰⁶³⁰⁵ What each of these sites broadcasts is in Boston.

Elsewhere in the world

The antenna farm pattern is common, though the standard examples cited here are all American: Mount Wilson and Sutro Tower in California, alongside Needham.³²⁵ Outside the US, national regulators such as Canada's ISED, the UK's Ofcom and Australia's ACMA handle radio licensing, and each runs a public lookup.

Canada's ISED offers Spectrum Management System searches for radio licenses by license number, call sign, geographic area and frequency, plus a broadcasting certificate search.³³⁶ In the UK, Ofcom's Wireless Telegraphy Act Register shows who is licensed in which frequencies and areas, with power and antenna details by site, but it does not show mobile base station or TV transmitter locations.³³⁷ Australia's Register of Radiocommunications Licences lists current licenses, licensees and sites, and can be searched by site, location, frequency and licensee.³³⁸ In Japan, the Ministry of Internal Affairs and Communications (MIC) links a Radio Station Information Search from its regional bureau pages, hosted on its Radio Use Portal.³³⁹

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