George Allison, K1IG, writes on the PART of Westford mailing list:
The December 2020 issue of CQ magazine has an article by PART member Bob Glorioso, W1IS, and Bob Rose, KC1DSQ, titled “A Simple Dual-Band Upgrade for Your 40-Meter Dipole.” The article describes an easy modification to a 40-meter dipole antenna to allow it to resonate on 15 meters as well, and it has some good (and easily understood) antenna theory.
The two Bobs (W1IS and KC1DSQ) will be the speakers at the January 19, 2021 PART meeting, giving a presentation titled “My Dipole Has Gain?” Be sure to be there!