Catching a picture from the air
A television signal travels as a radio wave, and every television needs something to capture it. The devices that do so, television antennas, are easy to overlook. Yet the shape of the aerials on a roof tells a good deal about how television spread, which channels were available and how far a home was from the transmitter.
Early television frequencies are at much higher frequencies than the AM radio signals of the same period. The very high frequency (VHF) channels used in the 1930s to 1950s behave mostly like line-of-sight signals: they travel in straight lines, are blocked by hills and large buildings, and fade with distance. A good antenna, placed high and pointed the right way, matters a great deal. This is why antennas ended up on roofs and masts.
The dipole and the rabbit ears
The simplest television antenna is a dipole: two metal rods in a line, with the receiving cable connected at the middle. For a given channel, the best length is related to the wavelength, roughly half of it. Because television channels span a range of frequencies, a telescoping pair of rods can be adjusted to suit the channel.
That is the principle of the “rabbit ears” that sat on top of so many sets. They were a pair of adjustable rods on a base, often with a loop for ultra-high-frequency (UHF) channels. They were handy for people who lived close to the transmitter in strong signal areas. In weaker areas, moving the rods, turning the set and standing near it to improve the picture became a familiar domestic ritual. Some people added aluminum foil to the tips, with uncertain effect, as a matter of folk practice.
The roof aerial
Where signals were weak or came from a particular direction, households installed outdoor antennas. The design most associated with this role is the Yagi-Uda antenna, which the Japanese engineers Hidetsugu Yagi and Shintaro Uda described in the 1920s. It consists of a driven element plus a reflector behind it and several directors in front, mounted along a boom. The extra elements focus the reception in one direction, increasing the signal from the transmitter and reducing signals from other directions. Many rooftop television antennas are Yagi variants.
A roof antenna has several practical considerations.
- Direction: It must point toward the transmitter, and rotors, driven by small electric motors, were sold that let a household turn the antenna from indoors to receive stations in different directions.
- Height: Higher mounting usually gives a stronger, less obstructed signal.
- Lead-in cable: Early installations used flat twin-lead cable, which was inexpensive but picked up interference and was affected by rain and ice. Coaxial cable became the better option and later the standard.
- Ghosting: Signals bouncing off buildings or hills arrive slightly late, and on analog sets they produced faint duplicate images, called ghosts, next to the main picture.
When the UHF channels were added in the United States in the early 1950s, many sets needed a separate tuner and a different antenna design for the new channels. UHF signals are more affected by obstacles, so UHF reception was often poorer than VHF reception.
Shared antennas and cable
Hills and tall buildings left some towns with poor reception. In the late 1940s, entrepreneurs in places in the United States such as Pennsylvania and Oregon, according to most accounts, built large antennas on high ground and ran cables to subscribers’ homes. These community antenna television systems, often shortened to CATV, were the origin of cable television. At first they only delivered signals from distant stations. Cable systems later replaced much of their copper with optical fibre. Over time operators began carrying extra channels, which turned cable into a rival to broadcast television.
In apartment buildings, a similar idea appeared in the form of a single shared rooftop antenna that fed every flat through a distribution network, and this is still common in many countries.
Satellite dishes
Satellites added a different kind of antenna. A dish uses a curved reflector to concentrate a weak signal on a small receiver at the focus. In the 1970s and 1980s, homeowners in some places, especially rural areas without cable, installed large dishes, a few meters across, to receive satellite programming intended for cable operators. As satellites became more powerful and digital compression improved, much smaller dishes, often less than a meter across, became practical for direct-to-home service in the 1990s. The signals come from communications satellites in geostationary orbit, so the dish is fixed and points to the same part of the sky.
Digital transition and the revival of the aerial
The change to digital broadcasting meant that old antennas often continued to work, because the digital channels used the same bands. But some viewers needed to re-aim or replace their aerials. The behavior of digital signals is different from analog ones: the picture does not become gradually snowy, but remains perfect until the signal falls below a threshold and then freezes or vanishes. This put a premium on a good antenna and its position.
In the 2010s, as streaming services and rising subscription prices led some people to drop their cable or satellite packages, the antenna enjoyed a modest revival. A flat indoor antenna for local channels became a common accessory. The device that began as a pair of rods on top of the set has therefore survived from the earliest television age to the present, joined by other ways of getting a picture.