I have a question regarding dipole length, SWR and Smith charts.

I'm using a 40/80 trap dipole and wish to lengthen the 80M portion to
lower the SWR in the CW part of the band. I'm reading a SWR about
1.2:1 at 3.8MHz, 4.8:1 at 3.5MHz, which I wish to drop a little to
ease up on the auto-tuner in my radio. I plan on adding a couple of
feet and remeasure and I'll probably be close enough for rock and
roll.

Now, I use this antenna on other bands - I can load it on 20 (SWR
3), 15 (SWR 3) and 10 (SWR 3.2). I'd like to know what the impact
of adding another 2 ft to each side will do on these bands.

So - looking at the Smith chart, I can draw my circles for each band
and I can measure the physical length of each side of the dipole. How
do I relate length to SWR on the chart? And, would this be useful to
first-order? The whole function of a dipole is to radiate, so is this
a very lossy system, or would this be close enough to lossless to get
a good look?

Thanks for your thoughts

Mark/n0lf