We can express the physical length of the line in meters. However, in high-frequency electronics (microwave engineering), we usually convert this length to the fraction of a wavelength of a signal that is traveling on the line.
Typically, but not always, is a fraction, for example, , or ; although it can be any The lenght of the line is then written as
If the physical length of the line is , we say: This line is quarter-wavelength long at 1GHz, meaning one-quarter of the wavelength fits on the line. We could also say that the line is 7.5cm long, as wavelength is at GHz.
When we say quarter-wavelength long, we refer to the lines physical length at a specific frequency.
The phase shift between input and output signal on a transmission line is . is called the phase constant. It represents the spatial frequency of the signal. is the phase in degrees or radians (related is a time delay in seconds). can be, for example , , . is a function of frequency, because is a function of frequency. If , we say: The line is 90 degrees long at 1GHz, meaning the output signal at 1GHz will be shifted for with respect to the input signal. When we say , we refer to the lines electrical length, representing the number of degrees that the line introduces between the input and the output signal.