Microwave Passive Components: Types & Functions

The part of the electromagnetic spectrum referred to as the microwave region loosely includes 1-300 GHz for practical purposes. These microwave bands are especially advantageous for transmission services such as satellite transmission, telephone transmission, broadcasting of commercial radio & television, and oil platform transmission. Some of the key advantages of microwaves are that the energy can be focused into narrow beams and large signal bandwidths are possible.

Special components and devices are required for operation at microwave frequencies. Microwave components can be classified as passive and active components. In this article, we focus on passive components i.e. those that are required to couple or direct microwave energy such as directional coupler, waveguide junctions, probes, circulators, isolators, and so on.

Directional Coupler

The directional coupler is a device which, when installed in a waveguide, will respond to a wave travelling in one direction but will be unaffected by a wave travelling in the opposite direction. Directional couplers are four-port devices and the power ratios between the ports are defined by the coupling factor, directivity, and insertion loss.

Let consider the four-port directional coupler in Figure 1.0 below.

In reference to Figure 1.0 above, the power entering port 1 gives an output at ports 2 and 3 but no output at port 4. This directional coupler can be inserted in two ways: forward and reverse.

The coupling factor is given by:

C_f = Power in Port 3/Power in Port 1 (dB)

The directivity is given by:

D = Power in Port 4/Power in Port 3 (dB)

Directivity is defined as the ratio of power appearing at port 4 when the coupler is in the forward direction to the power appearing at port 3 when the coupler is in the reverse direction and port 4 is terminated in its characteristic impedance (Z₀).

The insertion loss is the amount of by which the signal in the main guide is attenuated and may be given by:

I = Power in Port 4/Power in Port 1 (dB)             

Waveguide Junctions

Waveguide junctions are needed to reroute power in a similar way to junction devices in optical fiber applications. They are also used to mix power from multiple sources. However, anything that alters the geometry of a waveguide will have an effect on the electric and magnetic fields and may change the characteristics impedance of the guide.

One of the most common waveguide junctions is the T-junction shown in Figure 1.1 below. A signal applied at port 1 appears at port 2 and 3, both these signals being out of phase.

Cavity Resonators

Cavity resonators are used to control frequency in microwave transmitters, receivers and test equipment. To have a basic understanding how this special type of resonator works, consider the common tuned circuit illustrated below in Figure 1.2(a). Increasing the resonant frequency implies that C and L must be reduced until two small plates and a single-turn inductance comprise the LC circuit as shown in Figure 1.2(b). To reduce the inductance further, several, one-turn inductors are placed around the edge of the capacitor plates as shown in Figure 1.2(c). Continuing the process, it can be envisaged that the two small capacitor plates are completely enclosed by several inductor bands until basically a box (Figure 1.2 (d)) is formed referred to as a cavity resonator.

Cavity resonators thus have a natural frequency which depends on the dimensions of the box. At some established frequency, there will a high-amplitude electric field between the plates of the capacitor and a high current up and down the sides of the box. At some instant the energy is stored in the electric field, while at the next instant it is transferred to the magnetic field.

Probes

In order to extract energy from or deliver energy to a cavity, special techniques of coupling are required. Such methods include shooting electrons through the resonator at a point where the electric field is high, or introducing a small conducting loop into the area at point where the magnetic field is strong. A probe or loop usually is connected to a coaxial cable with the aid of an appropriate matching device.

In the technique shown in Figure 1.3 above, it is necessary to ensure the transit time of electrons passing through holes is small compared to the duration of a resonator cycle.

In the technique illustrated in Figure 1.4, shows a small probe placed in the central area of the cavity resonator. The probe is the extended inner connector of a coaxial cable and should be placed to coincide with the electric field vector.

Isolators

An isolator is a device that has the ability to pass a signal in only one direction while the other direction exhibits high attenuation. A typical isolator type is the ferrite isolator. Isolator working is based on Faraday rotation.

In reference to Figure 1.5, the transmission is in the forward direction; when the electric field lines of the microwave are at right angles to the attenuation vanes there is a little attenuation however this will increase as the angle decreases from 90° to 0°.

Circulators  

The circulator basically allows the same microwave antenna to be coupled to the receiver and transmitter

A circulator allows the separation of signals and involves the operation of ferrites. The often used type is the three-port circulator:

The device consists of three isolators in which power entering 1 is received at 2 only, while power entering 2 is received at 3 only, and power entering at 3 is received only at 1 only.

Circulators can have more than three ports and operate without using ferrites, but generally a maximum of four ports are used with a ferrite construction for greater efficiency.

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