Editing Circulator (section)

====Stripline junction circulators====
[[File:Rotating Modes in Junction Circulator.jpg|thumb|400px|Rotating modes in a junction circulator.]]

A stripline junction circulator contains a resonator, which is located at the central junction of the [[stripline]]s.  This resonator may have any shape that has three-fold [[Rotational symmetry]], such as a disk, hexagon, or triangle.  An RF/microwave signal entering a circulator port is connected via a [[stripline]] to the resonator, where energy is coupled into two counter-rotating circular [[Mode (electromagnetism)|modes]] formed by the [[elliptically polarized]] waves.  These circular modes have different [[phase velocities]] which can cause them to combine constructively or destructively at a given port.  This produces an [[anti-node]] at one port (port 2 if the signal is incident upon port 1) and a [[Node (physics)|node]] or null at another port (port 3 if the microwave energy is coupled from port 1 to port 2 and not reflected back into port 2).

If losses are neglected for simplification, the counter-rotating modes must differ in phase by an integer multiple of <math>2\pi</math> for signal propagation from port 1 to port 2 (or from port 2 to port 3, or from port 3 to port 1):<ref name="Soohoo 85">{{Cite book|title=Microwave Magnetics|first=Ronald F.|last=Soohoo|date= 1985|publisher=Harper & Row|isbn=0-06-046367-8}}</ref>

:<math>2\Gamma_-l - \Gamma_+l = 2m\pi</math>

and similarly, for the remaining port (port 3 if signal propagation is from port 1 to port 2) to be nulled,

:<math>-\Gamma_-l + 2\Gamma_+l = (2n - 1)\pi</math>

where <math>l</math> is the path length between adjacent ports and <math>m</math> and <math>n</math> are integers.  Solving the two preceding equations simultaneously, for proper circulation the necessary conditions are

:<math>\Gamma_-l = \frac{4m + 2n - 1}{3}\pi</math>

and

:<math>\Gamma_+l = \frac{2m + 4n - 2}{3}\pi</math>

Each of the two counter-rotating modes has its own resonant frequency.<ref name="Fay & Comstock" />  The two resonant frequencies are known as the split frequencies.  The circulator operating frequency is set between the two split frequencies.

These circulator types operate based on [[Faraday effect|faraday rotation]]. Wave cancellation occurs when waves propagate with and against the circulator's direction of circulation.  An incident wave arriving at any port is split equally into two waves. They propagate in each direction around the circulator with different phase velocities. When they arrive at the output port they have different phase relationships and thus combine accordingly. This combination of waves propagating at different phase velocities is how junction circulators fundamentally operate.

The geometry of a [[stripline]] junction circulator comprises two ferrite disks or triangles separated by a stripline center conductor and sandwiched between two parallel ground planes.<ref name="Helszajn Stripline Circulator">{{Cite book|title=The Stripline Circulator: Theory and Practice|first=Joseph|last=Helszajn|date=2008|publisher=John Wiley & Sons|isbn=978-0-470-25878-1}}</ref>  A stripline circulator is essentially a stripline center conductor sandwich on ferrite, between ground planes. That is, there is one ferrite disk above the stripline circuit and one ferrite disk below the stripline circuit.  Stripline circulators do not have to be constructed with disk- or triangle-shaped ferrites; the ferrites can have almost any shape that has three-way symmetry.  This is also true of the resonator (the center junction portion of the center conductor)- it can be any shape that has three-way symmetry, although there are electrical considerations.<ref name="Microwave Circulator Design" />

The ferrites are magnetized through their thicknesses, i.e., the static magnetic bias field is perpendicular to the plane of the device and the direction of signal propagation is transverse to the direction of the static magnetic field. Both ferrites are in the same static ad RF magnetic fields.  The two ferrites can be thought of as one continuous ferrite with an embedded stripline center conductor.  For practical manufacturing reasons, the center conductor is not generally embedded in ferrite, so two discrete ferrites are used.  The static magnetic bias field is typically provided by permanent magnets that are located external to the circulator ground planes.  Magnetic shielding incorporated into the circulator design prevents detuning or partial demagnetization of the circulator in the presence of external magnetic fields or ferrous materials, and protects nearby devices from the effects of the circulator's static magnetic field.

<gallery class="center" mode="packed" caption="Internal Construction of Stripline Junction Circulators">
Stripline Junction Circulator.jpg|Internal construction of a [[stripline]] junction circulator having triangular ferrites and an irregular triangle-shaped resonator.
Circulator Disk Ferrite Suspended Stripline with Bullets.jpg|Internal construction of [[stripline]] junction circulator having disk ferrites and a disk-shaped resonator.
BR Circulator Disk F & D Air Stripline with Bullets.jpg|Internal construction of a [[stripline]] junction circulator having disk ferrites and a triangle-shaped resonator.
</gallery>