#### Ferrite Circulators

Antenna
Transmitter
Antenna

Figure 1: Ferrite circulator: principle of operation

Antenna
Transmitter
Antenna

Figure 1: Ferrite circulator: principle of operation

#### Ferrite Circulators

A ferrite circulator is a passive, non-reciprocal, three-terminal device that uses magnetized ferrite materials so that a high-frequency signal can exit only through the terminal that directly follows the input.

A permanent magnet is used to create a static magnetic bias in the ferrite material. This gives the electrical signal a different propagation speed depending on the direction of propagation. When a high-frequency signal is fed into port 1, its energy initially splits into two equal parts that propagate in opposite directions. Due to the different propagation speeds, both signal halves are in phase opposition at port 3, i.e., they cancel each other out. At port 2, both signal halves are in phase, so they add up to the complete signal again.

An ideal, lossless ferrite circulator has the scattering matrix:

(1)

Due to the symmetrical design of the ferrite circulator, it is always possible to determine a defined path direction by the choice of the port. If an antenna is connected at a port, the transmitted energy is always directed to the antenna, while the echo signals always take the path to the receiver.

Figure 2: S-parameters on a three-port ferrite circulator.

Figure 2: S-parameters on a three-port ferrite circulator.

The isolation of a circulator is the insertion loss from what is the output port to the input port, i.e., in the reverse direction. Referring to the circulator in Figure 1, if port 1 is the input port, there are two output ports, and so there are two isolations equal to the return loss from port 3 to port 2, and from port 2 to port 1. The smaller value is the isolation quoted if only one value is given. If this would be an ideal circulator and port 2 is perfectly matched, then the isolation would be infinite. If port 2 is not perfectly matched, then there will be finite isolation from port 3 to port 1. However, the most common source of limited isolation is when the circulator is not perfectly matched. In practice, values for isolation between -20 and -30 dB are achieved. Ferrite circulators are often used as duplexers in radar sets.

Ferrite circulators can also be used as protection against reflections and are called isolators. In this case, a sufficiently dimensioned termination (dummy load) is connected to port number 3, which converts the reflected energy from port number 2 into heat while the desired signal flow direction from port 1 to port 2 is given unobstructed passage.

Figure 3 shows a disassembled ferrite circulator for low power. It consists of two solid metal blocks, which have only a load-bearing function. The conductor track in the center is made of copper. The dimensions are chosen so that the surface from the metal blocks forms an impedance of 50 Ω. The two ferrite plates are shaped like a small tablet and enclosed in a ceramic disc. Mechanically, the whole construction is held only by the HF sockets (here in N standard). In addition, everything is glued together so that any mechanical vibrations do not affect the function.

Figure 4 shows a ferrite circulator of the height finder PRW-13. The permanent magnetic field is generated here with an electromagnet.

##### Picture gallery ferrite circulator

Figure 2: Ferrite circulator for 1030 MHz with SMA connectors

Figure 3: Explosive diagram of a ferrite circulator

Figure 4: Ferrite circulator in the height finder PRW-13