When trimming a mobile antenna for the lowest reflected power I use a trick learned many years ago. At some point additional trimming will cause the reflected power to start to increase. The whip is beginning to be too short at this point. When unsure if you have reached the ideal length, I temporarily make the whip a little bit longer by adding a small piece of wire to the top of the whip where the little ball is attached. A short piece of solder works great. Wrap a couple of turns around the whip and extend the solder past the ball on the end about a quarter of an inch. Measure the reflected power/swr. If it improves or stays about the same stop any additional trimming. You are about as good as you can get it. If adding a small amount of solder causes the swr/reflected to go back up then you are on the right track. Continue trimming a small amount at a time. If removing a sixteenth of an inch from the whip fails to make any improvement then stop. Probably as good as you can get it.
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Antenna tuning
