Module 2 : Transmission Lines. Lecture 16 : Impedance Matching using Transmission Line continues. Objectives

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1 Objectives In this course you will learn about the following Principle of double stub matching techniques and its step by step procedure. Limitation of double stub matching technique. Use of triple stubs to overcome limitations of the double stub matching technique.

2 Double-Stub Matching Technique To overcome the drawbacks of the single-stub matching technique, the double-stub matching technique is employed. The technique uses two stubs with fixed locations. As the load changes only the lengths of the stubs are adjusted to achieve matching. Let us assume that a normalized admittance is to be matched using the double stub matching technique. The first stub is located at a convenient distance from the load say (Figure). The second stub is located at a distance of from the first stub.

3 Double-Stub Matching Technique (contd.) We can note the steps involved in the double stub matching as follows: (a) Mark the admittance on the Smith chart (Point A). (b) (c) (d) (e) Move on constant VSWR circle passing through A by a distance to reach. Move along the constant-conductance (constant- ) circle to reach ( a point on the rotated circle). Note that a stub at B will change only the reactive part and therefore we move on a circle which keeps the real part of same while going from to. Transform admittance at to by moving a distance of on a constant VSWR circle passing through. The point must be lying to the circle. Let the transformed admittance at point be. Add a stub to give susceptance at location C so as to move the point to which is the matched point. (f) (g) To calculate the length of the first stub we note that this stub must provide a susceptance which is the difference between the susceptances at and. That is, the stub susceptance is equal to. Mark the susceptance on the chart to get point. Distance from to in the anticlockwise direction gives the length of the first stub. The second stub should have a susceptance of. To get the length of the second stub the procedure is same as that used in the single stub matching. That is, mark on the Smith chart to get point. Measure distance in anti-clockwise direction to give. Limitation The whole matching process relies on the fact that by moving along a constant conductance circle one can go from point to. ( lies on the rotated circle). If this step is not realizable then the whole matching process is unrealizable.

4 If point lies in the hatched region, moving along constant-g circle can never bring a point on rotated g=1 circle. Hence that admittance cannot be matched by the Double Stub method.

5 Triple-Stub Matching Inorder to overcome the drawback of double-stub mathching technique one can use three stubs seperated by distance, instead of the two stubs (see Figure). One still uses only two adjacent stubs (B, C or C, D) at a time for matching purpose. For those loads which have outside the forbidden region we use stub B and C as used earlier and stub D is disconnected. The disconnection is not physical and is achieved by simply adjusting the length of the stub to -stub appears open at the junction point D.. The If the admittance at B happens to lie in the forbidden region, disconnect stub B by adjusting its length to, and use stubs C and D for double-stub matching. Disconnection of stub B effectively increases by and the admittance now corresponds to location C. So if the admittance at B is in the forbidden region, the admittance at C would definitely be out of it. A three-stub matching technique therefore is the final destination in the impedance matching, as it can match all impedance/admittance without changing the stub locations.

6 Recap In this course you have learnt the following Principle of double stub matching techniques and its step by step procedure. Limitation of double stub matching technique. Use of triple stubs to overcome limitations of the double stub matching technique.