David, You are correct, this *is* a common point of misunderstanding. You referred to the driving "source impedance" in your statement, and I believe that is the source of the misunderstanding.
What you say is true about the driving generator. Amplifiers are designed for some efficiency into some load - in our case, usually 50 ohms, and whatever the amplifier designer must do to make that happen is up to the designer. However, to obtain maximum power transfer *into* the antenna, the antenna and feedline must have an impedance that matches the output impedance of the amplifier - we all strive to match our antennas to a 50 ohm load (or an SWR of 1:1 based on a 50 ohm system). We typically adjust the parameters of an antenna system to have an input impedance of 50 +j0 ohms in order to obtain the maximum power into the antenna (because that the the load the amplifier needs to see for proper in-spec operation). To accomplish that, we might have a tuner of some nature between the transmitter and the input of the feedline. If the antenna feedline has an impedance of (for example) 120 +j30. then the matching network will have an input impedance of 50 +j0 and an output impedance of 120 - j30 -- that is as far as it goes. I stand on my statement (but will not extend it to the internals of amplifier design) - it is only related to feedline and antenna matching. Consider that if one builds a matched antenna system at some frequency as I described above and terminates it at the transmitter end with a 50 ohm pure resistive dummy load. Now split the feedline at any point and measure the impedance of both the open ends created by the split - you will measure an R+jX impedance in one direction and an R-jX impedance in the other direction - that is a conjugate match. If it is something other than that, there will be significant loss in the antenna system. Yes, if we put a theoretical (equivalent circuit) generator on that 50 ohm feedpoint, that theoretical generator must have an internal resistance of 50 +j0 ohms to achieve maximum power transfer between that theoretical generator and the antenna feed - and the power dissipated by that theoretical resistance on the theoretical generator will be the same as that delivered to the load. However, real amplifiers are not usually built the same way we create equivalent circuits. We do know that amplifier efficiencies can be much higher than 50%. When we replace the real amplifier with an equivalent circuit, that will contain a driving generator of zero loss and a series resistance of 50 ohms, but we never analyze the internals of an equivalent circuit, it is only the *representation* of the driving source which makes the system being analyzed behave as though the real generator (amplifier) were driving it. 73, Don W3FPR > > Although I agree with most of the preceding, the above statement hints > at a common misunderstanding. In general, you do not want to feed an > antenna from a source impedance that is the the complex conjugate of its > impedance. Doing so guarantees that you cannot exceed 50% efficiency. > > Whilst such matching will give you the highest power for any given open > circuit voltage, it will not give you the highest output power for a > given input power. > > When one says is a transmitter is matched to 50 ohms, one actually means > that its output network presents the optimum load to the finals when > feeding a 50 ohm load. > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
