True, the reasons for an ATU are not all associated with feedline loss.
One thing that is often overlooked in these discussions is that older tube type transmitters had an adjustable output network which had a sizable range for matching antennas. You would 'dip the plate' and adjust the 'loading' until you could feed the antenna efficiently. With that, one did not have to pay much attention to SWR or the feedpoint impedance. The PA output network did the job of matching the plate load resistance to whatever was connected to the network output (the antenna).
Today's transmitters need to see a low SWR because they were designed to operate into a 50 ohm load. The adjustable elements of the transmitter output have been moved from the transmitter into the ATU because there is nothing to adjust in the transmitter other than the band and VFO.
If all your antennas present a 50 ohm load on each band - and across the span of those bands, no ATU is needed at all, but that requires many tuned antennas. Not everyone has that luxury.
73, Don W3FPR On 4/27/2016 2:52 PM, Ken K6MR wrote:
Somewhere in the past hams have become obsessed with SWR. With a 2:1 SWR on a feedline with a matched loss of 3.0 dB (which is a bit extreme) there is an additional 0.35 dB of loss. It’s just not big enough to worry about. Of course now that we all have solid state finals 2:1 can be a problem. But a tuner in the shack is a completely reasonable and efficient way to deal with it.
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