It sounds like your antenna system is working as well as can be expected, there's certainly nothing wrong with a 1.3:1 or 1.5:1 SWR.
Try this just for grins. Measure that antenna from tip to the base of that ball-mount. For instance, 102" of whip, about 4" of spring, and about 4" of ball-mount. That means your antenna is 102+4+4 = 110" long. Right? So, depending on how much inductance that coil provides, and how/where it's connected, you've at the very least got into the ballpark for tuning it given how it's mounted.
If the SWR changes much (10%+/- some) then the antenna isn't necessarily out of tune, the accuracy of your meter and any calibration that's done also factors into it. It's still ballpark'ish.
When adding an amplifier to the line up then the output impedance of that amplifier has to be figured into it. The most common reason for any significant changes in SWR in such a case isn't the antenna, but the amplifier. Sorry, a no tune amplifier's output impedance is seldom 50-52 ohms. So, until you can say from measuring that output circuit's impedance, that circuit would be the first 'suspect'.
An antenna system's SWR of less than 1.5:1 shouldn't be a problem for a properly designed and constructed amplifier.
One very simple way of eliminating one of the variables in it is to run that amplifier into a 50 ohm dummy load. What does the SWR tell you in such a case? It should tell you two things. How well the amplifier's output circuit is tuned, and if the antenna system is as close as it could/should be. See how that would work?
- 'Doc
Don't expect 'numbers' except by pure accident.
