What is impedance matching?
This page covers impedance matching circuits,methods and devices.Impedance matching circuits are L network,pi network,split capacitor network,transmatch circuit etc.Impedance matching devices include coaxial cable balun transformer,matching stubs,quarter wavelength transformer,series matching section etc.
Let us understand impedance matching with the example of antenna used for radio frequency transmission. The common setup here is RF transmitter is connected with transmission line and transmission line is connected with RF antenna. In order to have maximum power transfer using this setup, output impedance of transmitter should match with transmission line impedance and transmission line impedance should match with the antenna feed impedance. In order to achieve impedance matching various circuits and methods are used.
As antenna impedance consists of both resistive and reactive components and hence matching network should have both of these elements in order to provide matching. In order to match resistive source with the complex load impedance matching network with complex conjugate of complex load impedance is needed. For example, if load impedance is R+j*X then matching network should have impedance of R-j*X and vice versa.
Impedance matching circuits
In this section, we will understand various impedance matching circuits such as L network, Pi network, split capacitor network, different transmatch circuits etc.
The L network is one of the most commonly used antenna matching network. Different L sections exist such as inverted L section and reverse L section networks.
The Pi network is used to match high source impedance to the low load impedance. These circuits are commonly used in vacuum tube RF power amplifiers which requires to match with low value antenna impedances.
Split Capacitor Network:
This network type is used to transform source impedance which is less than load impedance.
This circuit is combination of split capacitor network and output tuning capacitor. This circuit is used for coaxial to coaxial impedance matching.
Impedance matching devices
In this section we will understand various impedance matching devices such as coaxial cable balun transformer, matching stubs, quarter wavelength transformer, series matching section etc.
Coaxial Cable Balun Transformer:
The balun is a transformer which matches an unbalanced resistive source impedance with a balanced load. For example, source impedance can be coaxial cable and load can be dipole antenna.
The typical circuit is shown in the figure. With this balun type, impedance transform of ratio 4:1 can be achieved. The length of balun section of this coaxial cable is expressed as follows,
L(feet) = 492*V/F(MHz), where V is velocity factor of the coaxial cable.
The shorted stub can be constructed which can produce reactance of any value. This can act as impedance matching device which cancels reactive part of complex impedance.
Example: If we have impedance of say Z=R+j*25 then we need stub with reactance value of -j25 Ohm to match it.
Quarter wavelength transformer:
The transformer as shown can be connected between transmission line and antenna load. This transformer is also known as Q-section. This transformer is capable of matching feedline impedance of Zs with antenna feed impedance of Zr. In order to have impedance matching, transmission line impedance of value Zo should be of value equal to (Zs*Zr)0.5.
There are many disadvantages of quarter wavelength section as mentioned below.
• It should be located at the feedpoint of antenna.
• It must be of quarter wavelength long.
• Impedance value should be specified.
Series matching section:
This series matching section as shown in the figure overcomes the drawbacks mentioned above of the quarter wave transformer.
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