HF Vertical Antenna Freestanding Loading Coil by Chetmystery

Last crawled date: 3 years, 1 month ago

Free-standing manually adjusted loading coil form for amateur radio use on the HF bands.
This will give a maximum inductance of about 19 microhenries and is designed to interface with standard 3/8"-24 antenna stud hardware, mounts, an whips.
You'll need to print the coil form, two of the end caps, one of the rings for the shorting point, and two of the small set screws used to set the ring on the coil. I recommend printing the end caps with 100% infill to make them solid, otherwise when you add the mounting hardware to them you'll compress and weaken the plastic, leading to a broken coil down the line. Also print the thumb screws with full infill, just to make them more durable.
In addition to these printed parts, you'll need the following:

For mounting the whip on top of the coil, one 3/4" long 3/8"-24 bolt with washers, lock washer and a long coupling nut. (these parts can all be bout as a set used for mounting CB antennas such as this (remove the plastic insulator ring parts): https://www.amazon.com/dp/B000X380KQ/


For mounting the bottom of the coil to a standard stud antenna mount, a second set of the parts above except consider going with a slightly longer 3/8"-24 bolt and use a standard lower-profile nut.


1mm diameter stainless steel, uncoated wire for wrapping the coil. You can find small spools of this at most hardware stores in the aisle that sells picture wall-hanging hardware.


A small #8-32 screw or bolt with washers and a matching "acorn nut". These parts act as the shorting tap on the ring and the acorn nut's rounded tip will be what touches and shorts to the coil inside that ring.


About 10 inches of flexible coated wire as the shorting lead


A handful of ring terminals for the coil wire and shorting wire that fit the 3/8" and #8-32 bolts.

Super glue to attach the caps to the coil.
To build, begin by drilling 3 small holes at each end of the grove molded in the coil form for the coil wire, this is to pass the wire in and out and in on each end for strain relief. Wind the coil tightly by hand and leave a few inches of wire on each end after the strain relief holes, the ends of the wire should be on the inside of the coil. Crimp on a ring terminal on each end that fits the 3/8" stub bolts, and attach the bolts to the end caps, tightening down the hardware well. once the parts are all secure to the caps, install them to the coil body and secure with super glue. At this point, the coil should be wound and each end of the coil wire secured to the studs on the inside of the caps that you just glued on. Attach the shorting lead to the stub bolt on the bottom of the coil and the #8-32 bolt on the shorting collar. Slip the collar ring over the coil and use the two printed thumb screws in the printed threaded hols on the ring to lock it in place on the coil. Loosen these thumb screws to move the shorting ring up and down.
Now you can add any whip element to the top of the coil and screw the bottom to any stud antenna mount. Add a ground and/or radials and attach your coax and tune the coil to your desired frequency. With a 10-foot whip on top, there should be enough loading in the coil to tune down to 7MHz and up.