Introduction to Microwave Technology (RF Academy)

Microwaves are used in radar, radio transmission, cooking, and other applications that have become essential to modern society. Microwaves are electromagnetic waves that are generally defined as being located in the frequency range of 100MHz (3m wavelength) to 300GHz (1mm wavelength). Above 30GHz, it is customary to call them millimeter waves because wavelengths are measured in millimeters. The optical frequency range begins in infrared above 300 GHz. The RF range extends down from the microwave region down to 3KHz (100,000m wavelength). It is important to understand that these are electromagnetic waves that follow the same classical laws.

Since common discrete electrical components (such as carbon resistors, mica capacitors, wirewound inductors) and the wires connecting them become larger in size relative to microwave frequencies and wavelengths above, they are no longer suitable for the construction of microwave circuits. Instead of using distributed "printed" circuit elements with microscopic internal dimensions, waveguides, and dedicated active elements (i.e., amplifiers), until wavelengths become so small that optical techniques, such as lenses and mirrors, are employed.

Another reason why common low-frequency discrete components and connecting wires cannot be used at microwave frequencies is a phenomenon known as the skin effect, where high-frequency energy only travels on the outer skin of a conductor and does not penetrate great distances. The concept of the skin effect is best understood by the following examples. If you tie a rope to a ball and then spin the ball around your head at a slow pace, you will find that the ball is very close to your body when you spin it. When you spin faster, it extends farther from your head and body and directly at a high enough speed. The force that causes this to happen is centrifugal force.

The speed of the ball is related to the frequency (slow is low frequency, high speed is high frequency), and as the frequency increases, the centrifugal force will increase. This force is an inductance that builds up in the transmission line due to the flow of current. This force, which we call microwave centrifugal force, prevents energy from penetrating the surface of the transmission line and causes it to follow the path along the skin of the line and not down into the entire cross-sectional area, such as a low-frequency circuit; Therefore, the skin effect determines the characteristics of the microwave signal.

Corresponding to the skin effect is a term called skin depth. This is the distance at which microwave energy actually penetrates into the conductor, depending on the material used and the operating frequency. For example, copper has a skin depth of 0.000025 inches at 10 GHz; 0.000031 inches for aluminum at 10 GHz; 0.000023 inches for silver; and 0.000019 inches for gold. Thus, it can be seen that the energy does propagate along the outer edge of the metal. This is even more emphasized, considering microwave circuit boards with copper cladding, which is typically 0.0014 inches thick.

Since transmission lines do not allow high-frequency energy to penetrate deep conductors, it does not make sense to use circular (radial) leads on components for microwave applications. The energy only travels on the lead skin, which is very inefficient. This is why on most microwave components there are ribbon leads or no leads with only solder termination points. This is also the reason why there are relatively few physical components on microwave circuit boards. They are there, but they are distributed over a large, thin area whose values are equivalent to discrete devices used at lower frequencies; Hence the term distributed element component. This is what motivates many people to look at microwave circuits and ask, "Where are all the parts?" ". Microwave circuits require special design and manufacturing techniques.

A common term for microwaves

After defining microwaves, here are some basic terms related to microwave and wireless technologies, namely jargon and buzzwords in the microwave field. The list will be expanded and further refined as subsequent topics are introduced.