After a long amateur radio break, I rediscovered this great hobby and especially SOTA about two years ago.
I tried and evaluated different HF portable antenna designs that fulfilled my personal needs, which finally brought me to experiments with resonant end-fed halve-wave antennas.
During these experiments, I came up with a EFHW design that I had not seen before: a radiator containing one bypassable loading coil for more bands to cover. The result is a 20m long EFHW antenna that is resonant on the 60-, 40-, 30-, 20-, 17-, 15- and 10m-band, depending if the coil is short-cut or in use.
Further, I experimented with small but efficient couplers, adding more bands to the antenna, and much more. All the measurements were taken during my SOTA activations.
Please have a look at the resulting A Portable 7-Band End-Fed Half-Wave (EFHW) Antenna document, consisting of more than 50 pages that I promised to several hams for so long.
Update: Linked to version 1.2 that contains more information about the transformer winding and several small fixes.
I hope that some contained ideas will be a useful source of inspiration for your own antenna projects.