Loading coil for vertical antenna with sliding contact

I built this coil to expand the coverage of an elevated vertical antenna.  I used a "bridge" with a sliding contact for tuning.  The bridge component is embossed with letters to aid in documenting placement of the contact for a given whip & frequency.

The coil was done with 16awg tinned copper, using a bit less than 40'. 

The sliding contact is a keystone 5231 battery contact.  

M10 heat inserts were used on both ends. An M10 grub (allen) screw was later threaded in at the bottom.  The M10 allen screw was like these:       https://www.amazon.com/dp/B0D8TCWYSS

M4 heat inserts were installed for the coil anchor screws & slider bridge screws. 

The coil ends were crimped to M10 (3/8") ring terminals. 

If you want to use this design with a whip using 3/8-24 threads you would need to use an adapter for a M10x1.5 thread.

The contact slider has two parts, the body and the cap.   The battery contact fits in the slot in the body and is secured with an M2.5 cap head screw.   The body and cap are assembled with an M3 cap head screw.  A wire is soldered to the battery contact and the bottom of the coil near the ring terminal.

The inductance calculator says it should be in the ballpark of 70 microhenries.  66 turns of a 1.4mm conductor on a 50mm form over 135mm span.  Some minor ripples in the wire are seen because it was recycled from an earlier test build.

It is extremely important that the wire remains under tension while wrapping.  I suggest inserting the "anchor" screws before starting and securing the wire to one at the beginng and the other at the end of the wrap process.  Decent grip strength is needed to be successful, you really don't want to let it slip!

I printed in ASA for heat and UV stability.  I recommend a minimum wall/top/bottom thickness of 3mm for anchoring the heat inserts and overall strength.    I used a 0.6mm nozzle so my wall count was lower than someone using 0.4mm.   For me, a 0.3mm layer provided adequate definition for successful construction.  A 0.2mm layer would probably give even better definition for the winding track. 

Fusion 360 source and .STEP files were provided along with the .STL files.

An initial test can be seen here:     https://www.worldwidedx.com/threads/tested-a-portable-elevated-v-today.271117/post-871544

The antenna was based on this pole top base: