![]() ![]() Figure 3.23.1 shows a practical implementation of this idea implemented in microstrip. Section 3.16 explains how a stub can replace a discrete reactance. Whatever the reason, a possible solution is to replace the discrete reactance with a transmission line “stub” – that is, a transmission line which has been open- or short-circuited. In many problems, the required discrete reactance is not practical because it is not a standard value, or because of non-ideal behavior at the desired frequency (see Section 3.21 for more about this), or because one might simply wish to avoid the cost and logistical issues associated with an additional component. The resulting transmission function of the network has a passband ripple like the Chebyshev filter, but the ripples never reach 0 dB insertion loss at any point in the passband, as they would do for the standard filter.In Section 3.22, we considered impedance matching schemes consisting of a transmission line combined with a reactance which is placed either in series or in parallel with the transmission line. For instance, the matching network may be designed as a Chebyshev filter but is optimised for impedance matching instead of passband transmission. The resulting structure is filter-like, and filter design techniques are applied. Several stubs may be used spaced along the main transmission line for wideband matching. Ī single stub will only achieve a perfect match at one specific frequency. Matching stubs can be made adjustable so that matching can be corrected on test. This is not the same as the actual impedance of the load since the reactive part of the load impedance will be subject to impedance transformer action and the resistive part. The stub is made capacitive or inductive according to whether the main line presents an inductive or capacitive impedance, respectively. The length of the stub is chosen so that it exactly cancels the reactive part of the presented impedance. This distance is chosen so that at that point, the resistive part of the load impedance is made equal to the resistive part of the characteristic impedance by impedance transformer action of the length of the main line. The stub is positioned a distance from the load. Stubs can match a load impedance to the transmission line characteristic impedance. In a stripline circuit, a stub may be placed just before an output connector to compensate for minor mismatches due to the device's output load or the connector itself. ![]() Z s c = j Z 0 tan ( β ℓ ) is approached. The input impedance of a lossless, short circuited line is, Stub circuits can be designed using a Smith chart, a graphical tool which can determine what length line to use to obtain a desired reactance. ![]() Stubs can be constructed with any type of transmission line: parallel conductor line (where they are called Lecher lines), coaxial cable, stripline, waveguide, and dielectric waveguide. Stubs are commonly used in antenna impedance matching circuits, frequency selective filters, and resonant circuits for UHF electronic oscillators and RF amplifiers. They are often used to replace discrete capacitors and inductors, because at UHF and microwave frequencies lumped components perform poorly due to parasitic reactance. Therefore, stubs are most commonly used in UHF or microwave circuits in which the wavelengths are short enough that the stub is conveniently small. Their reactive properties are determined by their physical length in relation to the wavelength of the radio waves. The behaviour of stubs is due to standing waves along their length. Stubs may thus function as capacitors, inductors and resonant circuits at radio frequencies. Neglecting transmission line losses, the input impedance of the stub is purely reactive either capacitive or inductive, depending on the electrical length of the stub, and on whether it is open or short circuit. The free end of the stub is either left open-circuit, or short-circuited (as is always the case for waveguides). In microwave and radio-frequency engineering, a stub or resonant stub is a length of transmission line or waveguide that is connected at one end only. 10 kW FM broadcast transmitter from 1947 showing quarter-wave resonant stub plate tank circuit ![]()
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