Jack,
There are "sliding ring" antennas, however those have a contact point with a base coil. By making a balum or choke with feed line, you can change the tuning in some antennas. However since the coax is not part of the radiating antenna, any affect had in the VHF range would be minimal.
My Morad VHF . antenna has a whip which can be adjusted--same with the Shakespeare base loaded whips. Most of the fiberglass encased antennas cannot be tuned once assembled. Be great if sliding a cylinder of metal up and down, would tune those--which arrive with a high SWR....
There are "sliding ring" antennas, however those have a contact point with a base coil. By making a balum or choke with feed line, you can change the tuning in some antennas. However since the coax is not part of the radiating antenna, any affect had in the VHF range would be minimal.
My Morad VHF . antenna has a whip which can be adjusted--same with the Shakespeare base loaded whips. Most of the fiberglass encased antennas cannot be tuned once assembled. Be great if sliding a cylinder of metal up and down, would tune those--which arrive with a high SWR....