The 15m antenna is an simple 1/2 λ dipol with a "current balun" of Guanella type. It is wond on an Amidon T200-2 toroid. It has 16 turns of 0.75mm isolated wire. I have not done measurements on the balun itself, but it is obvious that it working as the RFI level in the shack is much lower that it was without the balun.
The VSWR for 15m is 1.7:1 (Click on the picture to get an enlarged version) at 21.074 MHz which is the frequency that I uses the most. :) I could be re-tuned as seen on the Smith chart but for me this is good enough. The blue circles in the Smith chart represent a VSWR of 2:1, the inner cirle and 3:1, the outer circle.
At 18.100 MHz it is not much worse with the balun, here the VSWR is 1.9:1 (Click on the picture to get an enlarged version). This is actually not so bad in a case like this. Without the balun the VSWR was much worse, 3:1.
After updating the wall feed through and changing my antenna for 2m / 70cm, which was used as a support for the 15m antenna, the VSWR changed a lot. Now it is what can be expected for a dipol, but unfortunately this is giving me much higher VSWR for both bands!
After making the antenna 30 cm shorter at each end to tune it to the 21 MHz only it now look like this. Of course it can be further shortened to get the dip a little bit higher up in the low end of the 21 MHz band. That will be an exercise for a sun day later this year!
As seen in the chart, the VSWR is just about 1:1.5. By a slight adjustment the dip can be moved into the band but the VSWR will probably not be much better.