Stub:- Stub is the piece of short circuited transmission line which is used to tune out the reactance of the load when connected across the transmission line as close as possible.
Single stub: Stub is the piece of short circuited TL which is used to tune out the reactance of the load when connected across the TL as close as possible.
1. The most important feature of single stub matching is that the stub should be located as near to the load as possible. 2. The characteristic admittance of the stub so connected in shunt should be same as that of the main line. 3. The main element of this transmission line is a short circuited section of line whose open end is connected to the main line at a particular distance from the load end. 4. Where the input conductance at that point is equal to the characteristic conductance of the line, and the stub length is adjusted to provide a susceptance equal in value but opposite in sign, to the input susceptance of the main line at that point. 5. So the total susceptance of the main line at that point is zero. 6. The combination of stub and the line will thus present a conductance which is equal to the characteristic impedance of the line, i.e. the main length of the HF transmission line will be matched.
Double stub: The disadvantages of single stub matching are overcome by using double stub matching as shown in fig.
Here, two short circuited stubs at two fixed point usually λ/4 apart are utilized. Their positions are fixed but lengths are independently adjustable. The double stub matching provides wide range of impedance matching.