load the tuned circuit

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I'm well aware that power matching is a nonsense for applications such as mains and batteries.  However a transmitter needs to work into a matched load to prevent reflections. If the the load is accurately matched to the feeder then ultimately the tx match to the feeder doesn't matter too much.  Perhaps the gap in my knowledge is about the practicalities of tx to feeder to aerial matching.

This is a cut and paste from a discussion on a group that is for broadcaster.

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Here's a bit of technology talk just for information and fun:

We hear many references to "transmitter output impedance" with the "standard" being 50 ohms. It would seem, then, that the best load to present to the transmitter is 50 ohms. But all is not as it appears with output impedance.

The specification that a transmitter has a "50 ohm antenna output" refers to the transmitter being designed to drive this 50 ohm load, but this should not be the output impedance.

The maximum power theorem states that maximum power transfer from a source to a load occurs when the load impedance is the complex conjugate of the source impedance. Complex conjugate means that the reactance in the load is equal and opposite to the reactance in the source and that the source and load resistances are equal. This is essentially what is done when a base loading coil is used with an antenna. The inductive reactance of the coil is equal and opposite to the capacitive reactance of the antenna, in other words it is at resonance.

When the source and load impedances are conjugates the efficiency of power transfer is 50% with half the power being lost as heat in the source. Thus, it is not efficient to have the same source and load resistance so what does this mean? It means that for efficient power transfer the source resistance needs to be much lower than the load resistance (with the reactances equal and opposite). For example, a transmitter designed to drive a 50 ohm load could have an output resistance of typically 5 ohms or less. This would result in 9% of the power being lost in the transmitter for a transfer efficiency of 91%.

The proper load is important for the operation of the transmitter and when it is specified for use with a 50 ohm load this translates into the load for which the final amplifier and filters will work properly and is generally not the actual output impedance.

Neil


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