I built a center-ish loaded coil-shortened vertical. The lower radiator is 3 meters, then the coil, then 5M more radiator, like this (Early prototype with shorter radiators and no covers for the coil and motor):
The coil is a rolling inductor. It can tune the antena from 80 to 40M. The ground plane is a metal roof, 10x6 meters. The antenna ground is soldered to the roof.
The problem I have is that SWR goes high the lower the frequency. On 40M, SWR is 1.1:1, on 60M it's 1.4:1, and on 80M it's 2.5:1.
Measured with a $30 NanoVNA, it tells me the impedance is near "50 ohms 2.5uH" at the lower band. On 40M it's near 50 ohms, but no significant L or C.
What does this mean? Should I compensate the 2.5uH with a capacitor placed across the feedpoint? Is my ground plane too small for 80M?