Turnstile antenna
A crossed-dipole antenna with two modes of operation.
A turnstile antenna, also known as a crossed-dipole antenna, consists of two identical dipole antennas mounted at right angles and fed in phase quadrature, meaning the currents are 90° out of phase. Its name comes from its resemblance to a turnstile when mounted horizontally. The antenna can operate in two modes: normal mode, which radiates horizontally polarized waves perpendicular to its axis, and axial mode, which radiates circularly polarized waves along its axis. It is notable for its use in television broadcasting (as the superturnstile or batwing antenna) and in satellite ground station antennas.
- Inventor
- George Brown
- Year invented
- 1935
- Year described in scholarship
- 1936
- Modes
- normal mode, axial mode
- Typical frequency bands
- VHF, UHF
Lore & Background
The turnstile antenna was invented by George Brown in 1935 and described in scholarship in 1936. The patent history reveals its popularity over the years. In normal mode, the antenna radiates horizontally polarized radio waves perpendicular to its axis, with a radiation pattern that is close to omnidirectional but actually cloverleaf-shaped, departing from omnidirectional by only ±5 percent. Stacking multiple turnstile antennas vertically (called bays) fed in phase increases gain in horizontal directions and reduces wasted radiation vertically. These stacked normal mode antennas are used for FM and television broadcasting. The normal mode turnstile is occasionally called the George Brown turnstile antenna.
In axial mode, the antenna radiates circularly polarized waves off the ends of its axis, with one end producing right-hand circular polarization and the other left-hand circular polarization, determined by the feed connections. A flat conducting reflector placed a quarter-wavelength behind the elements reinforces forward radiation by reflecting backward waves with reversed polarization sense, increasing axial radiation by 3 dB. Axial mode turnstiles are widely used for satellite and missile antennas because circular polarization is not sensitive to the orientation of the antenna elements. Feeding the antenna requires equal-magnitude currents 90° apart, achieved via feed-line techniques (splitting the signal and delaying one path) or by modifying the dipole dimensions to yield quadrature currents, a method known as turnstile feeding.
Reader's Guide
The turnstile antenna's significance lies in its versatility and widespread adoption in broadcasting and satellite communications. In normal mode, it provides a nearly omnidirectional horizontally polarized pattern, making it suitable for FM and television broadcasting when stacked vertically. The batwing or superturnstile variant, with broadened dipole elements, offers wide impedance bandwidth essential for TV channels. In axial mode, its circular polarization is critical for satellite and missile communication, as it eliminates gain loss due to antenna orientation. The antenna's design allows for simple feed techniques, and its legacy includes use in early FM broadcasting, the US Nike missile program for telemetry, and modern satellite ground stations. The ability to switch between linear and circular polarization by changing the mode of operation further underscores its adaptability.
Did You Know?
- A flat reflector placed a quarter-wavelength behind the elements increases axial radiation by 3 dB.
- The batwing or superturnstile antenna is a specialized normal mode turnstile used for television broadcasting.
Frequently Asked Questions
What exactly is a turnstile antenna and why is it called that?
A turnstile antenna is a crossed-dipole design made of two identical dipole elements set at right angles to each other and fed with a 90-degree phase offset between them. The name comes from the visual resemblance to a rotating turnstile gate when the assembly is mounted horizontally.
Who is credited with inventing the turnstile antenna and when?
George Brown is recognized as the inventor, with the design dating to 1935. It entered formal academic literature the following year, in 1936, when it was described in published scholarship.
What are the two operating modes of a turnstile antenna?
In normal mode it radiates horizontally polarized waves in a plane perpendicular to its axis, while in axial mode it produces circularly polarized radiation directed along that same axis. This dual-mode capability is what makes the geometry so versatile for different reception and transmission tasks.
Where do you typically see turnstile antennas in real-world applications?
They are widely used in television broadcasting, where the larger variant is often called a superturnstile or batwing antenna. They also appear in satellite ground stations, where their axial-mode circular polarization is especially valuable for linking with orbiting spacecraft.
Which frequency bands is a turnstile antenna suited for?
The design is most commonly deployed in the VHF and UHF bands. Its compact crossed-dipole geometry makes it practical at those mid-range frequencies without requiring the very large structures needed for lower bands.
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