Took some readings on my HF antennas this morning. Here they are.
Solarcon A99 CB vertical. Left at its 27 MHz settings. Works OK with a tuner from 18 - 30 MHz. Don't know why there is a 2nd resonant frequency around 20 MHz but they all seem to be like that. A normal half wave would have the SWR go high below 25 MHz.
14 MHz half wave made using 10 metres of wire sloping down from the same pole the A99 is on, to the back yard. This is matched to coax with a 16:1 transformer. I use this one for below 18 MHz. It works a bit on 3.5 and 7 MHz too, with a tuner.