An axial-mode turnstile antenna for 136-137 MHz to receive data from
weather satellites
, consisting of a pair of driven crossed dipoles above a pair passive crossed dipoles serving as a reflector.
High gain axial mode
Yagi
turnstile array used to communicate with weather satellites on 136-137 MHz at
Redu
, Belgium. Each of the 6 components of the array consists of two 9-element
Yagi antennas
mounted on the same axis at right angles and fed in quadrature to radiate a narrow beam of circularly polarized radio waves
A
turnstile antenna
, or crossed-dipole antenna,
[1]
is a
radio antenna
consisting of a set of two identical
dipole antennas
mounted at right angles to each other and fed in
phase quadrature
; the two currents applied to the dipoles are 90° out of phase.
[2]
[3]
The name reflects the notion the antenna looks like a
turnstile
when mounted horizontally. The antenna can be used in two possible modes. In
normal mode
the antenna radiates
horizontally polarized
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
antennas. Axial mode turnstiles are widely used for satellite ground station antennas in the
VHF
and
UHF
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
[
edit
]
The turnstile antenna was invented by George Brown in 1935
[2]
and described in scholarship in 1936.
[4]
The patent history reveals the popularity of the turnstile antenna over the years.
[5]
Characteristics
[
edit
]
- (left)
First turnstile antenna, a normal-mode array built by station
W8XH
, Buffalo, NY, in 1936 that broadcast on 41 MHz
- (center)
Normal-mode turnstile
(lower antenna)
on the
Empire State Building
in New York City for NBC's experimental 46.5 MHz TV station W2XBS in 1939. The cigar-shaped elements gave the antenna a wider bandwidth of 30 MHz necessary for TV transmission.
- (right)
Specialized type of normal mode turnstile antenna used for television broadcasting, called a superturnstile or
batwing antenna
The antenna can be used in two different modes:
normal mode
and
axial mode
.
[
citation needed
]
Normal mode
[
edit
]
In directions perpendicular to its axis the antenna radiates
linearly polarized
radio waves (
horizontally polarized
when the antenna's axis is vertical). This is called
normal mode
. The
radiation pattern
, 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.
[3]
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
.
[3]
Axial mode
[
edit
]
Array of 4 axial-mode turnstiles for portable military
satellite communication
terminal
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.
[1]
The waves in that direction are reflected back 180° and the reflection reverses the polarization sense, so the reflected waves reinforce the forward radiation.
[1]
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
.
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,
[6]
since circular polarization is used in
satellite communication
.
[
citation needed
]
This is because with circularly polarized waves the relative orientation of the antenna elements does not affect the gain.
Feeding the antenna
[
edit
]
For the antenna to function, the two dipoles must be fed with currents of equal magnitude in
phase quadrature
, meaning the phase of the sine waves must be 90° apart.
[3]
This is done with feed-line techniques or by adding reactance in series with the dipoles.
[3]
Quadrature feed
[
edit
]
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.
[3]
Modified dipole dimensions
[
edit
]
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.
[3]
[6]
This method of changing the physical dimensions of the antenna element to yield quadrature currents is known as
turnstile feeding
.
[1]
Applications
[
edit
]
Stacked arrays
[
edit
]
Brown's original patent described stacking multiple turnstile antennas vertically to make a high gain horizontally polarized omnidirectional antenna for radio broadcasting.
[3]
[2]
These were used for some of the first
FM broadcasting
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
[
edit
]
A later innovation involved changing the shape of the dipole elements, from simple rods to broader shapes, to increase the
bandwidth
of the antenna.
[7]
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
[8]
The
batwing
shape of each element produces an antenna with wide impedance bandwidth.
[1]
Up to eight batwing antennas are usually stacked vertically and fed in phase to make a high gain omnidirectional antenna for TV broadcasting.
[3]
[1]
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.
[1]
Spacecraft antennas
[
edit
]
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.
[6]
External links
[
edit
]
Construction plans for ATA(Asymmetric Turnstile Antenna
Bibliography
[
edit
]
- John Daniel Kraus (1988).
Antennas
(Subsequent ed.). McGraw-Hill College. p. 892.
ISBN
978-0-070-35422-7
.
$ 16-7:
Turnstile Antenna
, pp. 726-729.
References
[
edit
]
- ^
a
b
c
d
e
f
g
Milligan, Thomas (2005). "5 - Dipoles, Slots and Loops".
Modern Antenna Design
(2nd ed.). Hoboken, New Jersey: John Wiley & Sons, Inc. pp.
231
?237.
ISBN
978-0-471-45776-3
.
- ^
a
b
c
Brown, George.
"US Patent 2086976"
.
Antenna system
. Retrieved
14 January
2014
.
filed: September 20, 1935; granted: July 13, 1937
- ^
a
b
c
d
e
f
g
h
i
Kraus, John (1988). "16: Antennas for Special Applications: Feeding Applications".
Antennas
(2nd ed.). McGraw-Hill, Inc. pp.
726
?729.
ISBN
0-07-035422-7
.
- ^
Brown, George (April 1936). "The Turnstile Antenna".
Electronics
.
- ^
"Patents about Turnstile Antennas"
.
- ^
a
b
c
Martin, John (1952).
"[Missile] Antenna"
. Retrieved
15 January
2014
.
- ^
Masters, Robert (1945).
"[Batwing] Antenna"
. Retrieved
15 January
2014
.
- ^
Whitaker, Jerry (1996). "Antennas for Specific Applications". In Jerry Whitaker (ed.).
The Electronics Handbook
. CRC Press, Inc. p. 1341.
ISBN
0-8493-8345-5
.
The turnstile is the earliest and most popular resonant antenna for VHF broadcasting.