Microwaves are used in radar, radio transmission, cooking and other applications that have become essential in our modern society. Microwaves are electromagnetic waves generally defined as lying within the frequency range of 100 MHz (3 m wavelength) to 300 GHz (1 mm wavelength).[i] Above 30 GHz, because wavelengths are measured in mm, it is the convention to call them mmWaves. The optical frequency regime begins above 300 GHz with infrared. The radio frequency range extends down from the microwave region to as low as 3 KHz (100,000 m wavelength). It is important to understand that these are all electromagnetic waves that obey the same classical laws.
Because the dimensions of common discrete electrical components (e.g. carbon resistors, mica capacitors, wire-wound inductors) and the wires connecting them become large relative to a wavelength at microwave frequencies and above, they are no longer suitable for the construction of microwave circuits. Distributed “printed” circuit components, waveguide and specialized active components (i.e. amplifiers) with microscopic internal dimensions are used instead, until wavelengths become so small that optical techniques, such as the use of lenses and mirrors, are employed.
Skin Effect at Microwave Frequencies
Another reason common lower frequency discrete components and connecting wires cannot be used at microwave frequencies is a phenomenon called skin effect, which is the concept that high frequency energy travels only on the outside skin of a conductor and does not penetrate into it for any great distance. The concept of skin effect can best be understood by the following example. If you tie a string to a ball and then twirl the ball around your head at a slow speed, you will see that the ball stays fairly close to your body as you spin it around. As you spin it faster, it stretches further away from your head and body and at a high enough speed it stretches straight out. The force that causes that to happen is centrifugal force.
Relating the speed of the ball to frequency (slow speed is low frequency, high speed is high frequency), as the frequency gets higher, an increasing centrifugal force arises. The force is inductance that is set up in the transmission line simply because a current is flowing. This force, which we refer to as a microwave centrifugal force, keeps the energy from penetrating the surface of the transmission line and makes it follow a path along the skin of the line rather than down into the entire cross-sectional area, as in low-frequency circuits; thus, a skin effect determines the properties of microwave signals.
Corresponding to the skin effect is a term called skin depth. This is how far the microwave energy actually penetrates into a conductor, and is dependent upon the material being used and on the frequency of operation. For example, the skin depth in copper at 10 GHz is 0.000025 in.; for aluminum at 10 GHz, it is 0.000031 in.; for silver, it is 0.000023 in.; and for gold, it is 0.000019 in. Thus, it can be seen that the energy truly does travel along the outer edge of the metal. This is emphasized even more when considering that for a microwave circuit board with copper cladding, the thickness of the copper is typically 0.0014 in.
Since transmission lines do not allow high frequency energy to penetrate very far into a conductor, it makes no sense to have round (radial) wire leads on components for microwave applications. The energy would travel only on the skin of the lead and be very inefficient. That is why there are ribbon leads or no leads with only solder termination points on most microwave components. It also is why there are relatively few physical components on a microwave circuit board. They are there, but they are distributed over a large, thin area with values equivalent to discrete devices used at lower frequencies; hence, the term distributed element components. This is what prompts many people to look at a microwave circuit and ask, “Where are all the parts?” Microwave circuits require special design and fabrication techniques.