Most of that is accurate, but this part is misleading:
"Antennas are tuned with wire cutters or a hacksaw so the feed impedance is the desired value, ideally 50 ohm resistive or at least a match for the feeder concerned."
I think we're getting a bit off track by lumping "tuning" and "matching" together as if they are the same effects. They are not. At least in my world, tuning refers to compensating for reactance at the feedpoint by some means that nets it out to zero. In that respect, doing so with a network at the other end of the feedline accomplishes the exact same thing as using wire cutters or a hacksaw on the antenna itself. They are not different. As you say, the current and voltage distributions are not the same as if you cut the antenna to length, but the lack of feedpoint reactance is.
Matching the feedpoint impedance to the feedline, or compensating the effects of the mismatch at the transmitter end, is an entirely different matter and there is no requirement that "tuning" per se accomplishes a match at either end ... only that it bring the reactance at the antenna to zero. We do, of course, also want our "antenna tuner" to give us the proper match, and in common practice that's what it does. In that regard, calling it an "antenna tuner" only tells part of the story and "antenna coupler" might be a more rigorous term ... more rigorous even than "antenna matching unit."
73, Dave AB7E On 2/20/2018 6:16 PM, Alan B via Elecraft wrote:
This all depends on what is meant by antenna tuning. When teaching newbies the wrong phrase can cause problems that are not seen till later. Antennas are tuned with wire cutters or a hacksaw so the feed impedance is the desired value, ideally 50 ohm resistive or at least a match for the feeder concerned. Of course that is not always practical so an antenna matching unit brings the antenna or antenna plus feeder input impedance to the value wanted by the transmitter. Too often I have seen students believe the ATU alters the current and voltage distribution on the antenna so it looks exactly like the distribution on a dipole of the correct length for the frequency concerned. Granted the currents and voltages might change as the ATU is adjusted but that does not make the antenna radiate more efficiently. The reflection at the feeder/antenna junction is unchanged. In training we use the term antenna matching unit, AMU, to avoid best we can the students starting off with the wrong impression. Amongst ourselves we can get away with slack terminology, we all know what is meant; in front of trainees it is a different story. 73 Alan G0HIQ ______________________________________________________________ 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 Message delivered to [email protected]
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