This spring I constructed a 20/40m fan dipole and hung it up in the my backyard. I cut the lengths of each element according to the standard formula (234 / design frequency = each dipole side length in feet), and left some extra on each side for tuning. SWR was under 2:1 for both 20m and 40m, but there was definitely room to improve. Today I was able to spend some time taking measurements and tuning the antenna.
I used the excellent SWR Plotter program by K9DUR that automatically transmits across a selected band and records the SWR measured by my Flex 5000a radio.
After the initial measurements showed that the 40m element of the fan dipole was tuned too low, I folded back about 6” on either side.
I ran the SWR Plotter test again and it showed a nice improvement on 40m (chart via LibreOffice):
Unexpectedly there was a small improvement on 20m as well—maybe just from jostling things around?
There’s still some work to do to get the SWR down for 40m, but it looks like I’m heading in the right direction.