Screwdriver style antenna for portable use - 100uH loading coil

General description

I wanted to try making a screwdriver antenna for portable stationary use.  I have no prior experience with screwdriver antennae other than what I have read.   This represents a first try, not a polished product.  I have included source files (Fusion 360 & STEP).

The model is basically a movable loading coil that uses a motor & screw to move the coil past electrical pickups to change the inductance presented by the coil. The motor is controlled with a handheld momentary DPDT switch.  

The coil in this model provides a potential for 100 uH of inductance, bottom to top.  Changing the coil position in relation to the contact ring can change this across the full range of potential values, 0 to 100 uH.

The majority of the model structure is printed.  I used ASA for heat and UV stability.  The hardware used includes:

•  300rpm 12v gearmotor (with encoders) 
• momentary DPDT switch
• M3 screws (both cap head and countersunk varieties)
• M5 pan head screws
• 400mm M6 allthread 
• M6 nut
• chassis mount SO239 female UHF connector
• crimp on ring terminals fitting 3/8" and 5mm
• 3/8-24 thread extension
• washers fitting 3/8"
• 68+ feet of 16 awg tinned copper buss wire (stainless could also be used)
• 6+ feet of silicon coated 16awg (stranded)
• Keystone 209 battery terminals for contact ring

I also added a clamp-on tripod assembly for the base.  Legs are easily added/removed for portability.

Assembly

Construction mainly consists of the coil form,  cover and tripod components.  The coil form is printed in three pieces - upper, lower and lower extension.  the lower coil form & extension have fins for stability during movement.   The cover consists of upper, lower and lower extension components.  

Coil components are joined using CA glue on the tabs.  Cover components are assembled with M3 screws.  The M6 nut fits in a recess in the lower coil form, then held in place by heating the recess edge & folding tabs in over the nut.  

The top of the coil is topped with a cap and whip mount.  First attach cap & whip mount with M3 screws, then attach the assembled cap to the coil with countersunk M3 screws.  The whip mount is threaded for 3/8-24.  Crimp a 3/8 ring terminal to the terminal end of the coil wire after wrapping once on the M5 screw. Sandwich the ring terminal between a washer and 3/8-24 thread extension screwed into the whip mount.

The inner cover components have grooves on the inner walls that guide the fins on the coil form.  They keep the coil from rotating when the motor turns the screw and help stabilize it when the coil is fully raised.

The signal pickup is from the M5 screw on the side of the UHF box attached to the center conductor of the SO239.  A wire runs from there to the contact ring mounted at the top of the inner cover.  Keystone 209 terminals provide contact to the coil.  A ring of 16 awg tinned buss wire is at the top of the ring, joining the contacts.  A second wire runs from the contact ring back down to the bottom starting point of the coil, attaching via a ring terminal at the M5 screw anchoring the coil start.  These wires run in the space between coil & inner cover.  The signal feed wire is not attached at the UHF box component until everything else is assembled so as not to require excess length.

Attach ground wires to the SO239 mounting screws via ring terminals.  These will be used with radials or ground sheet.

The motor assembly is clamped into place in the base using M3 screws.  The openings in the base are sufficient for passage of the motor control wires and signal wire.

Final assembly should be done with the coil elevated sufficiently on the screw to facilitate access to the coupler joining motor shaft and M6 screw during final attachment to the motor shaft. 

Screw the M6 rod into the coupler before threading into the M6 nut embedded in the coil.  When attaching the coupler to the motor shaft, be sure to tighten an M3 screw to secure the coupler on the shaft.  A drop of CA glue as threadlock may be prudent.  An alternative to the printed coupler would be a metal 4mm to 6mm shaft coupler like these : https://www.amazon.com/uxcell-Coupling-L20xD12-Coupler-Connector/dp/B07P9693LZ

Inner cover segments and base are all assembled using M3 screws.

Non standard parts & sourcing

The base clamp assembly and motor/screw coupler were sized for use with this 300 rpm 12v motor:

https://www.amazon.com/dp/B07GNGCNK1

The 3/8-24 thread extension on the top was this:

https://www.amazon.com/dp/B08W56Y1WQ

The 400mm M6 allthread used was this:

https://www.amazon.com/dp/B0BLGQ4V35

The hand switch used was this:

https://www.amazon.com/dp/B011JW7IZ2

The coil windings are anchored top & bottom with pan head M5 screws.

See attached pictures for assembly clues.

Fusion 360 source and STEP files are included as well as STL files. 

Movement test mounted on an early tripod version (later replaced with a more stabile clamp-on version)