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AM Tuning Networks
In January 2012 I looked at the venerable T network for impedance matching in AM antenna systems. Now I'll discuss the L and Pi networks, and look at their use in antenna systems.
The L network is the simplest of the impedance transformation networks, and can be constructed with two reactive elements. Since it is the simplest of the transformation networks it essentially is the building block for the more complex T, Pi, and other types of networks. The simplicity in construction, however, also limits the usefulness of this network, and its ability to make transformations.
Because capacitors and inductors are the flavors of reactive elements available, and it takes two elements to create the L network, there are two different general topologies available. One has the shunt leg on the load side of the network, while the other is the mirror image with the shunt on the generator side. Four different arrangements are possible within these two topologies depending on the combination of components utilized. Thus, eight different combinations would be possible for matching various impedances, although the four cases utilizing both a capacitor and an inductor are used are much more common. The shunt leg of the network will be located on the side with the greater impedance.
Not a simple transformation
It should be noted that an L network cannot be used to transform between two impedances where the resistance values are identical. Intuitively this should make sense as such a transformation can easily be accomplished through the use of a series inductor or capacitor depending on the direction you need to move the reactance. Inductors in series will add positive reactance, while a capacitor will take things the other direction adding negative reactance. If control of phase over such a transformation is required, then a T, Pi, or some other flavor of network must be utilized.
The Q of the network, which relates to the bandwidth, is determined by the ratio of the impedances to be transformed. This quantity, which cannot be independently selected by the network designer, is low for small transformations, and grows with the range of the desired transformation. So shifting high impedances down to standard transmission line values may result in higher than desired reflected power, and by extension VSWR, within the spectrum of a particular signal.
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