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Personally, I don't think an SWR of 1.4 will give you much problem, but you might consider using NanoVNASaver (free application) with your VNA to sweep the frequency from, say, 30 to 60 MHz and save the result as a Touchstone file (.s1p format).? It is VERY easily done, and then you can use Zplots by AC6LA (also free) to read that Touchstone file to plot almost any parameter you want as a function of frequency ... including resistance and reactance.? It might help your investigation.

Keep in mind that the reactance of a resistor doesn't change with frequency, whereas the reactance of the driven element of the antenna will.? The resistor will only be accurate at (in your case) 37.8 MHz.?

Also, it takes very little difference in the length of a 1/4 wave stub to make a difference in its transformation.? You obviously know what you're doing so I'm sure that you are aware that it is a hyperbolic function in form ... and it will transform BOTH resistance and reactance.? You could possibly be off just a little in the physical length or in the estimate of 0.63 for velocity factor.? I just did a bit of playing around with TLW, the free transmission line application, and at 50 MHz a difference of only 0.5 inches in length of RG-6 coax changes the reactance of the input of a quarter wave transmission line terminated in 28 ohms by 10 ohms.? A difference in velocity factor between 0.63 and 0.64 would also be about 0.5 inches for a quarter wavelength at 50 MHz.

Once you build the antenna, you could probably bring the resonance almost exactly to the frequency you want by using some sliding sections of dielectric (like nylon or HDPE).? It has a loading effect that is variable depending upon where on the driven element it would be placed ... more effect at the ends where the E-field is greater and almost no effect at the center.

Just some thoughts ...

Dave?? AB7E