Here's a bit of technology talk just for information and fun:
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
My title started out as "Transfer of Power", which would probably be the correct objective for transmitter-antenna, but I am reminded of another impedance situation that may describe something slightly different.
Your presentation reminded me of a microphone matching "white-paper" I keep handy: "The Connection of Neumann Condenser Microphones to the Inputs of American Type Amplifiers".
Quoting briefly from the opening paragraph will make things clear:
1.) The Impedance Situation: The preamplifiers found in Neumann condenser microphones are designed to operate solely as voltage amplifiers with unterminated output. When set for a source impedance of 50 or 200 ohms they should look into a load of a magnitude at least five times this value, or a minimum of 250 ohms or 1000 ohms. Non-linear distortion will increase rapidly for high sound pressure levels if this is not strictly observed. This same problem concerns ALL condenser microphones no matter whose manufacture, but many firms ignore it in order to make the condenser microphones more acceptable to the general market.
By observing this rule of thumb I am able to achieve very life-like quality from microphones, even non-condenser microphones.
Is this quite different from the transmitter-antenna situation?
This is the reverse of what I described where efficient power transfer is desired in a transmitter. Here, if the load on the microphone (preamp) is too low then it distorts. They have traded efficiency for linearity. The signal amplitude will be reduced due to the high source Z but the priority is linear operation.
Neil
All of this is only applicable if your working with the notion to achieve the mythical 90 + percent touted lately, and that seems to only be achieved with odd ball impedance loads and not the industry standard.
20 ohms difference is not much, but apparently it is to class E designs where slight load changes throw the curve to the wayside and ruin that high efficiency.
None of that happens in class D and lower designs, at least not as radically as the class E.
I seriously doubt the industry standard is going to change anytime soon for the sporadic E.
RFB