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Output Balun vs. Input Balun with Floating L-Network
This page describes the difference between placing a balun at the input of a L-Network tuner and placing it at the output.
When a balun is located at the output between the tuner and a balanced line one side of the balun transformer is connected to ground at the tuner and the other is connected to one side of the balanced line. The full RF voltage is then applied to the the balun transformer. For a given balun design, if that voltage is high enough then the balun transformer will overheat. (See lower drawing, below.)
When the balun is located at the input between the balanced feedline and an unbalanced network, one side of the balun transformer is still connected to ground and the other is still connected to one side of the balanced feedline so the voltage applied to the balun is exactly the same. (See upper drawing, below.)
Therefore there is no advantage to floating the L-network with a balun at the input, and doing so greatly complicates an antenna tuner and increases cost.
Additional information can be found by reading information from the following links: