What we mean by a microwave rule of thumb may be an inaccurate but remarkable relationship between one or more design parameters and performance, or it may simply be an easy way to remember something that other smaller people often confuse. Obviously, you need to exercise some discretion when applying these rules, the precise results can vary greatly depending on the influences that you have not taken into account, such as the moon phase. We will continue to compile the microwave rules of thumb on this page, in no particular order, and we do not guarantee that we will reorganize the rules in the future. These rules are also scattered across the website in appropriate locations. We try to compare this section with other parts of the Microwaves101 encyclopedia so that you can learn more about each topic that interests you. The following section explains the effects of “long” transmission lines, especially if the length of the line corresponds to certain fractions or multiples of the signal wavelength. Altium Designer`s powerful PCB design and analysis® tools can help you analyze all aspects of your transmission line, including adjusting impedance and determining the critical length of the transmission line. These tools are based on a unified, rule-driven design engine that connects to a set of simulation tools. You also have access to a comprehensive set of manufacturing planning and documentation features on a single platform. The following graph shows the size of the input impedance up to 2 wavelengths.
Here we can clearly see that the oscillation of the input impedance decreases in the direction of the characteristic impedance of the transmission line. If a “long” line is considered to have at least 1/4 wavelength, you can see why all the interconnections in the circuits discussed so far have been assumed to be “short”. The propagation speed is usually the speed of light when calculating the wavelength of the signal outside or in a vacuum, but it is lower if the transmission line has a speed factor of less than 1. The rules of thumb of microwaves were passed on from old microwave ovens to new recruits in the last century. This rule can be extended to enterprise combiners by examining each Wilkinson separately. However, when it comes to high-frequency systems, the length of the transmission line is anything but insignificant. Consider a 100 MHz radio signal: its wavelength is only 9.8202 feet, even at the full speed of light propagation (186,000 miles / s). It can be seen that in the event of a deviation, the degree of permissible non-compliance depends on the permissible level variation, which can be tolerated in the input impedance away from the load impedance. In the graph showing the actual part of the input impedance, we see that an input impedance change of about 10% occurs when the line length is about 1/10 of the wavelength. If your tolerable deviation in the input impedance is 10%, your critical length is about 1/10 of the wavelength.
Longer wavelengths tend to break around small objects. This is also done with waveguides. When the waveguide is “small”, the energy around it breaks. A simple formula for calculating wavelength is this: A rule of thumb can`t really be true or false. It does not belong to science, but to engineering or practice. So it may or may not be useful. The basic idea on an intuitive level is that a short wave does not affect the signal much, since the reflection returns to the source before the source phase has changed much (remember that for a sine wave, the phase progresses linearly at a constant rate over time). The big problems caused by reflections have to do with the creation of minima and maxima in space on the waveguide and the self-suppression associated with it.
But even if we use the coaxial cable speed factor of the last example (0.66), the distance is still 2046 miles for a very long time! The distance we calculate for a given frequency is called the wavelength of the signal. An important point in the design of transmission lines is the need to adjust the impedance, which led to the definition of the critical length of the transmission line. The definition of the critical length of a transmission line depends on who you are asking. For analog signals, the critical length is sometimes given as one-sixth or one-eighth of the signal`s wavelength. A leading manufacturer, not named here, will tell you that the critical length is exactly equal to the quarter wavelength, and the impedance of the transmission line does not matter at this length. A prominent designer stated that the critical length is 1/20 of the wavelength of the signal.