A turnstile antenna, or crossed-dipole antenna,
is a
radio antenna consisting of a set of two identical
dipole antenna
In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole w ...
s mounted at right angles to each other and fed in
phase quadrature
In physics and mathematics, the phase of a periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a scale that it v ...
; the two currents applied to the dipoles are 90° out of phase.
[ filed: September 20, 1935; granted: July 13, 1937] The name reflects the notion the antenna looks like a
turnstile
A turnstile (also called a turnpike, gateline, baffle gate, automated gate, turn gate in some regions) is a form of gate which allows one person to pass at a time. A turnstile can be configured to enforce one-way human traffic. In addition, a ...
when mounted horizontally. The antenna can be used in two possible modes. In ''normal mode'' the antenna radiates
horizontally polarized
Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the ...
radio waves perpendicular to its axis. In ''axial mode'' the antenna radiates
circularly polarized radiation along its axis.
Specialized normal mode turnstile antennas called ''superturnstile'' or ''
batwing antennas'' are used as
television broadcasting
A television network or television broadcaster is a telecommunications network for distribution of television program content, where a central operation provides programming to many television stations or pay television providers. Until the mid- ...
antennas. Axial mode turnstiles are widely used for satellite ground station antennas in the
VHF
Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter.
Frequencies immediately below VHF ...
and
UHF
Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...
bands, as circular polarization is often used for satellite communication since it is not sensitive to the orientation of the satellite antenna in space.
History
The turnstile antenna was invented by George Brown in 1935
and described in scholarship in 1936.
The patent history reveals the popularity of the turnstile antenna over the years.
Characteristics
The antenna can be used in two different modes: ''normal mode'' and ''axial mode''.
Normal mode
In directions perpendicular to its axis the antenna radiates
linearly polarized
In electrodynamics, linear polarization or plane polarization of electromagnetic radiation is a confinement of the electric field vector or magnetic field vector to a given plane along the direction of propagation. The term ''linear polarizati ...
radio waves (
horizontally polarized
Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the ...
when the antenna's axis is vertical). This is called ''normal mode''. The
radiation pattern
In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: ...
, a superposition of the two dipole patterns, is close to
omnidirectional but actually "cloverleaf shaped", with four small maxima off the ends of the elements. The pattern departs from omnidirectional by only ±5 percent.
The radiation in these horizontal directions is often increased by vertically stacking multiple turnstile antennas (called "bays") fed in phase. This increases the gain by strengthening the radiation in the desired horizontal directions but causes partial cancellation of the radiation in vertical directions, reducing power wasted radiated into the sky or down toward the earth. These stacked normal mode turnstile antennas are used at VHF and UHF frequencies for FM and television broadcasting.
Since the first turnstiles invented by Brown operated in this mode, the normal mode turnstile is occasionally called the ''George Brown turnstile antenna''.
Axial mode
Off the ends of the antenna's axis, perpendicular to the plane of the elements, the antenna radiates
circularly-polarized (CP) radio waves. This is called ''axial mode''. The radiation off one end is righthand-circularly-polarized and the other end is lefthand-circularly-polarized. Which end produces which polarization is determined by the phase of the feed connections. Since in a directional antenna only a single beam is wanted, in a simple axial-mode antenna a flat conducting surface such as a metal screen reflector is added, a quarter-wavelength behind the crossed elements.
The waves in that direction are reflected back 180° and the reflection reverses the polarization sense, so the reflected waves reinforce the forward radiation.
For example, if the radio waves radiated forward are right-circularly-polarized, the waves radiated backwards will be left-circularly-polarized. The flat reflector reverses the polarization sense so the reflected waves are right-circularly-polarized. By locating the reflector λ/4 behind the elements the direct and reflected waves are in phase and add. Addition of the reflector increases the axial radiation by a factor of 2 (3 dB).
Another common way to increase the axial mode radiation is to replace each dipole with a
Yagi array Yagi may refer to:
Places
*Yagi, Kyoto, in Japan
*Yagi (Kashihara), in Nara Prefecture, Japan
*Yagi-nishiguchi Station, in Kashihara, Nara, Japan
*Kami-Yagi Station, a JR-West Kabe Line station located in 3-chōme, Yagi, Asaminami-ku, Hiroshima, Hi ...
.
In a circularly polarized antenna, it is important that the direction of polarization of the transmitting and receiving antennas be the same, since a right-circularly-polarized antenna will suffer a severe loss of gain receiving left-circularly-polarized radio waves, and vice versa.
Axial mode turnstile antennas are often used for satellite and missile antennas,
since circular polarization is used in
satellite communication. This is because with circularly polarized waves the relative orientation of the antenna elements does not affect the gain.
Feeding the antenna
For the antenna to function, the two dipoles must be fed with currents of equal magnitude in
phase quadrature
In physics and mathematics, the phase of a periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a scale that it v ...
, meaning the phase of the sine waves must be 90° apart.
This is done with feed-line techniques or by adding reactance in series with the dipoles.
Quadrature feed
A popular method of feeding the two dipoles in a turnstile antenna is to split the RF signal from the transmission line into two equal signals with a two way splitter, then delay one by 90 degrees additional electrical length. Each phase is applied to one of the dipoles.
Modified dipole dimensions
By modifying the length and shape of the dipoles, the combined terminal impedance presented to a single feed-point can achieve pure resistance and yield quadrature currents in each dipole.
This method of changing the physical dimensions of the antenna element to yield quadrature currents is known as ''turnstile feeding''.
Applications
Stacked arrays
Brown's original patent described stacking multiple turnstile antennas vertically to make a high gain horizontally polarized omnidirectional antenna for radio broadcasting.
These were used for some of the first
FM broadcasting
FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is c ...
antennas in the 1930s. However most modern FM broadcast antennas use circular polarization so the signal strength will not vary with the orientation of the receiver's antenna.
Batwing or superturnstile array
A later innovation involved changing the shape of the dipole elements, from simple rods to broader shapes, to increase the
bandwidth of the antenna.
Early TV broadcast antennas used "cigar shaped" elements, shown in image of 1939 RCA Empire State Building antenna above. A common shape today is the ''batwing'' or ''superturnstile'' antenna, used for television broadcasting in the VHF or UHF bands
The ''batwing'' shape of each element produces an antenna with wide impedance bandwidth.
Up to eight batwing antennas are usually stacked vertically and fed in phase to make a high gain omnidirectional antenna for TV broadcasting.
The wide bandwidth was needed at the low VHF analog TV broadcast band, as the 6 MHz TV channel bandwidth was about 10% of the frequency.
Spacecraft antennas
Circular polarization was used for spacecraft (satellite and missile) communication, since circular polarization is not sensitive to the relative orientation of the antennas, and the space vehicle's antenna could have any orientation with respect to the ground antenna. High gain Yagi turnstile antennas were often used for the ground station.
The US
Nike missile program made use of the axial mode for telemetry and used the ''modified dipole'' technique to force the quadrature currents.
External links
Construction plansRadiation of Turnstile Antennas Above a Conducting Ground Plane
Bibliography
* $ 16-7: ''Turnstile Antenna'', pp. 726-729.
References
{{Antenna Types
Radio frequency antenna types
Antennas (radio)
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