www.radartutorial.eu www.radartutorial.eu Radar Basics

Stubs

current
voltage
resistance

Figure 1: Current and voltage characteristics on transmission lines>

current
voltage
resistance

Figure 1: Current and voltage characteristics on transmission lines

Figure 2: Stubs as spacers

two-wire cable

Figure 2: Stubs as spacers

Stubs

In high-frequency technology, cable sections are usually used for tuning purposes that are matched in length. Such line sections can be connected parallel to the line and are then referred to as stubs or resonant stubs. The length adjustment is an adaptation to the wavelength to be transmitted.

These line sections are deliberately mismatched: that is, they are not terminated with a resistor the size of the line impedance, but are either short-circuited or left open.

A short circuit means that at this point the resistance is close to or equal to zero, so the voltage measurable at this point is also equal to zero, but a maximum current flows. With an open line, no current can flow, but the voltage has its maximum value at this point. With a high-frequency wave on this line, the wave is completely reflected at such mismatched points. Interference between the incoming and outgoing waves results in a standing wave. Along the line, this maximum (or in the case of a short circuit: minimum) voltage can now be repeatedly measured at a distance of half the wavelength from this mismatch.

If the distance is only a quarter of the wavelength, the ratio is reversed. The maximum voltage at the mismatch (open wire) is zero at this point, the current is maximum. If the mismatch is a short circuit, then the current is zero at this distance, but the voltage is maximum.

According to Ohm's law R = U / I, a resistance can be calculated from both values. If the voltage is zero, then the resistance is also zero; if the current is zero, then we have a division by zero: the resistance goes to infinity. For example, the lengthwise tuned line has now reversed the short circuit into its opposite: transformed it into an open line.

Applications