I've had better luck following this video's advice:
https://www.youtube.com/watch?v=z4dhRt7tmd4
I don't have an antenna analyzer, but I know that on a dipole like that shortening a dipole by the same percentage of length will have the same effect on impedance match/SWR on any dipole of any frequency.
One additional trick: sometimes a ground loop develops between cord and or power supply and or signal grounds in the system which will cause hum. On my main rack all the BT modulators (and only the BT modulators) are powered by a single 120V isolating transformer that lets the ground prong of the BTs float. I could have possibly fixed it by isolating all my signal sources but 12 of them (including a PC) would have required a bigger isolation trans than I could fit and I may have had to come up with isolation for the CATV line which would have been more work.