Turnstile antenna

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A turnstile antenna for 136-137 MHz LEO weather satellites comprising a single turnstile crossed dipole above a passive set of dipoles serving as a reflector.

A turnstile antenna is a radio antenna with a set of two dipole antennas aligned at right angles to each other with currents of equal magnitude and in phase quadrature.[1] The name reflects the notion the antenna looks like a turnstile when mounted horizontally.[2] The turnstile antenna is often referred to as crossed dipoles.[3]

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]

Turnstile Antenna Characteristics[edit]

The antenna can be used in two different modes: Normal Mode and Axial Mode.[citation needed]

Normal Mode[edit]

Normal mode, is the original configuration of the turnstile antenna where the orthogonal set of dipoles are each parallel and above the ground.[citation needed] Several sets of the crossed dipoles can be arranged one above the other where each is fed a portion of the RF power.[2] This configuration radiates omnidirectional, horizontally-polarized radio waves in all azimuth directions.[2] The turnstile antenna's azimuth gain pattern would be perfectly circular if each dipole was infinitely short.[1] Practical dipoles yield a pattern departing from perfectly circular by +/- 5 percent.[1]

The full name of the antenna used this way is the George Brown turnstile antenna.[1]

Axial Mode[edit]

When used in axial mode each dipole orients perpendicular to the line of communication. The antenna radiates circularly-polarized (CP) radio waves along this axis.[1] A standalone turnstile antenna generates opposite circular polarized signals in each axial direction; For example, right hand in one direction and left hand the other.[3] For this reason, the turnstile crossed dipole pair is often arranged over a conductive reflecting surface.[3] This reflects one axial emission whereby the reflection changes the polarity of the CP adding to the opposite CP emission and reinforcing a stronger CP signal.[3] The conductive reflective surface is sometimes replaced with a set of dipoles arranged like the reflecting elements in a Yagi-Uda antenna.[citation needed]

The axial mode turnstile antenna is often used for satellite communication because, being circularly polarized, the polarization of the signal doesn't rotate when the satellite rotates.[citation needed]

Another use of the axial mode arrangement is with missiles.[6]

Quadrature Currents[edit]

The fundamental requirement for the turnstile to function is ensuring each dipole's currents are of equal magnitude and in phase quadrature.[1] This is done with feed-line techniques or by adding reactance in series with the dipoles.[1]

Quadrature Feed[edit]

A popular method of feeding the two dipoles in a turnstile antenna involves splitting the RF signal with a two way splitter then delaying one by 90 degrees additional electrical length. Each connects to a dipole resulting in the proper quadrature currents.[1]

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.[1][6] This method of changing the physical dimensions of the antenna element to yield quadrature currents is known as turnstile feeding.[3]

Popular Turnstile Configurations[edit]

Horizontal Omni Arrays[edit]

The original purpose of the George Brown turnstile antennas was providing a horizontally polarized signal in all directions from a transmitting location.[2] To this end, the original patent describes arranging multiple copies of the basic turnstile cross dipole pair, one above the other and feeding all with RF power.[1][2]

Batwing Arrays[edit]

Main article: Batwing antenna

A later innovation involved changing the crossed dipole to a set of orthogonal large area antenna elements.[7] The batwing turnstile is popular for VHF broadcasting.[8] The batwing shape of each element produces an antenna with wide impedance bandwidth.[3] This antenna is most often arranged into vertical arrays similar to the George Brown turnstile array and is well known as the superturnstile antenna.[1][3] The wide bandwidth is beneficial to VHF lowband high bandwidth transmissions such as broadcast television.[3] compared to a lowband VHF version.

Satellite and Missile/Rocket Antenna[edit]

The benefits of circular polarization between moving objects justifies considering the axial mode of the turnstile antenna.[citation needed]

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]

References[edit]

  1. ^ a b c d e f g h i j k Kraus, John (1988). "16: Antennas for Special Applications: Feeding Applications". Antennas (2nd ed.). McGraw-Hill, Inc. pp. 726–729. ISBN 0-07-035422-7. 
  2. ^ a b c d e f Brown, George (1935). "Antenna System". Retrieved 14 January 2014. 
  3. ^ a b c d e f g h 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. 
  4. ^ Brown, George (April 1936). "The Turnstile Antenna". Electronics. 
  5. ^ "Patents about Turnstile Antennas". 
  6. ^ a b c Martin, John (1952). "[Missile] Antenna". Retrieved 15 January 2014. 
  7. ^ Masters, Robert (1945). "[Batwing] Antenna". Retrieved 15 January 2014. 
  8. ^ Whitaker, Jerry (1996). "Antennas for Specific Applications". In Jerry Whitaker. 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."