Trans-oceanic flights go far beyond the VHF radio horizon to land so routinely use HF radio. It's not a backup, it's the norm. Example: https://www.iaa.ie/air-

Trans-oceanic flights go far beyond the VHF radio horizon to land so routinely use HF radio. It's not a backup, it's the norm. Example: https://www.iaa.ie/air-traffic-management/north-atlantic-communications https://www.iaa.ie/air-traffic-management/north-atlantic-com... "Huge" is probably not that big vs VLF. Maybe on the order of ~50m width (~3MHz / ~100m wavelength for ~50m half-wave antennas). VLF is more like wires across entire valleys - kilometres.