What happens to SWR bandwidth when one or more loading coils are used to resonate an electrically short antenna?
Why An electrically short antenna has a lot of capacitive reactance and a small radiation resistance; adding inductive loading cancels the reactance but the resulting circuit has a high ratio of reactance to resistance, meaning high Q. Bandwidth is roughly the resonant frequency divided by Q, so raising Q narrows the range over which SWR stays low. That is why a loaded 40 m mobile whip may cover only tens of kilohertz while a full-size dipole covers hundreds.
Watch out The idea that coil placement leaves bandwidth unchanged is wrong: moving the coil up the radiator can improve efficiency and slightly change the Q, but no placement escapes the fundamental high-Q, narrow-bandwidth behavior of a short antenna.
Short antenna plus loading coil equals high Q equals narrow SWR bandwidth. Small antenna, small bandwidth.