UPDATE: Added two variations on the coil for someone who wants to use it for a screwdriver antenna. It should measure out to be about 50uh. It has a plain interior because I believe that the center should contain supporting structure such as a piece of ABS plumbing pipe, or a wooden dowel epoxied in place. The bending load on something this long will be considerable, so better interior support will definitely be required.
NOTE: Updated. Removed Glue Together assembly instructions. If you downloaded the files before Midnight on Monday May 29th, you may want to re-download the files to get the newset version. Please read the instructions below for the latest assembly instructions.
This is a loading coil for the fold-up PRC Antenna pictured. This antenna is very lightweight, and when horizontal, it only droops about 4 inches. It is 113.5 inches long (2.88M). The AT-271 "Fishing Pole" antenna was used on the PRC-25 and PRC-77 Squad Radios and also on the PRC-104 HF radio. A number of commercial radios also use the AT-271. See http://www.prc68.com/I/AT271.shtml for more details.
The antenna, with all 7 sections extended, operates at 25Mhz.
This Loading Coil at 22uh should bring the operating frequency into the 40M band (7Mhz). I designed it to be 25uh total, so you can easily fine tune it by shorting out a few coils with a clip to one end.
I printed this in PLA at 0.100mm layer thickness so that it would print the overhanging spiral grooves without needing any support. When printing at 0.200, i found that support was required to keep the extrusion from drooping in the overhanging grooves.
If this will be exposed to the elements for extended periods, please print with a UV resistant filament. PLA will be ok for occasional outdoor use, but will not handle the elements well.
A UV resistant ABS is the preferred filament for strength and it's ability to withstand long periods of exposure to sunlight and weather.
I found that Princess Auto sells a nice 16 Gauge galvanized fencing wire that works nicely with this coil form. Wind it on the coil as tightly as possible with about 3" (60mm) of wire on the inside of the WireForm to attach to the end contacts.
Place the insulator into the Wireform. Insert the end of the wire into the hole on the side of the insulator, (do this for both ends of the insulator before screwing the threaded brass 3/8-24 rod or tube into the insulator) then screw the ends of the threaded rod into the insulator. Carefully tighten down on the wire, then snip off the excess wire. Place the end caps over the threaded rod and press them into the WireForm until fully seated. Optionally, you can put a 55mm diameter x 2.5mm O-ring on the cap to provide a seal. Or, you can put a bead of RTV Silicone around the cap and let it cure before assembling everything. Secure the end caps with a Low Profile 3/8-24 nut. The low profile nut will sit in the hexagon shaped recess in the end cap. Rotate the end cap to tighten. Lubricate with silicone grease to assist assembly and provide a better weather seal. Be careful not to over tighten them and damage the 3D printed end caps or insulator. 3D printed parts can be fragile and excess pressure may break the end caps, or pull the insulator apart. Cut the end of the threaded rod to be 1/2 inch (12mm) beyond the end of the nut. This will provide enough room on each end for the next section.
On one end, use another low profile 3/8-24 nut to secure a 6" length of stranded wire with an alligator clip at the end. (silicone wire is best for flexibility) Then put a long 3/8-24 nut on one end to act as the Female connector for the fold up antenna. I use hanger bolt joiners (available at Home Depot). They're a little long at about 1 1/4" long, but they work suitably.
You can get Ultra Flexible Silicone insulated stranded wire at Hobby King
I'm a proud member of the Barrie Wireless Amateur eXperimenters (WAX) group, where we socialize and build lots of Amateur Radio oriented projects.
WAX Group Website
WAX Group Blog