HF Transceiver - Antenna Coupling
Just speaking as an end user with previous comms experience ...
The aircraft VHF transceiver has only to work over a fairly small spread of frequency of 118 - 136 MHz or wavelengths of 2.5m - 2.2m. So the VHF antenna can be made to match the centre frequency of around 127MHz for the transceiver in impedance etc, and the there will not much be much of a change in the characteristics of the antenna as the wavelength change will only be small at around less than 10% either side of that centre frequency. I've not measured a VHF antenna length but it looks like it's around a 1/4 wavelength - around 50cm ? - which is (or was) a common antenna length.
But the HF transceiver operates over a much wider frequency band of around 3 - 20ish MHz -which equates to 100m - 15m in wavelength terms - a very significant spread. So a tuning device is required to match the characteristics of the transceiver to the antenna for efficient use every time as otherwise the transceiver will see a large variation in antenna impedance and will be very inefficient.