On that truck is the AS-3684/VRC (I found the files), similar to the '1729? The matcher in there is a mystery, I do not dare disassemble the 3684. There are different size threads on the bottom sections, and the top sections are also slightly different lengths. So one is forced to put the correct bottom section on the base, but could accidentally put the wrong top section on.
Here is the SWR curve, perhaps it will be useful.
Also the curve if the AT-677/VRC which is for high band VHF and looks electrically like the same dipole design.
Some decades ago I bought a cracked base /tuning unit that looked like that. It had been full of water due to the crack. The threads on top of the spring are a coaxial connector to the antenna whip. Below the spring in the plastic cylinder above the mounting flange was a ferrite rod with a number of turns of coax around it to form the decoupling choke. It was mounted in foam.
In the bottom part of the unit was a solinoid operated rotary switch to select the frequency range. I assume it selected different matching networks, I cant remember what the matching networks consisted of.
Most of the unit is long gone, but the spring lives on in the base of my mobile whip!
Ian VK3KRI
