Building Antenna Coils Outside The Box
I built this 80 meter 4" coil years ago when I first started building mobile HF antennas, and I was experimenting with a number of different building practices and materials.
This coil is a interesting build, as I used Delrin for the core and Lexan for the end caps, with the coliform built from a heavy cardboard mailing tube that I coated with fiberglass resin inside and out, making it impervious to the weather.
The coliform was wound with 12 gauge insulated house wire that worked out well, with the insulation on the wire spacing the winds on the coliform uniformly.
The Hustler quick-disconnect allowed for rapid coil changes for the different band coils that were part of my antenna system at the time.
The red finish on the coil is actually 3M electrical tape. It wasn't required as the insulation on the wire kept the coil from de-tuning in wet weather, but it matched the color of my mobile at the time. Who said hams aren't vain!
At one time there were many companies that sold loading coils, but unfortunately that is no longer true.
Air-wound coils with large diameter wire are especially good for using as the loading inductor in your center loaded HF antenna project.
If your attempting to build your first loading coil for a center loaded mobile antenna, as a starting point you may want to build a coil with the inductance values that I have listed here.....
17 meters - 4uH
20 meters - 9uH
40 meters - 40uH
80 meters - 150uH
Coilforms measuring approximately 3" diameter would be appropriate.
Loading coils you build with insulated 14 or 12 gauge house wire are easily adjusted through the spacing of the wire when resonating your mobile antenna initially, and provide excellent circuit Q.
If you wish you can space the winds of wire on the coilform with string or cord of various diameters wound between the winds of wire.
There are various materials that you can use that will make good coliforms, and I already mentioned cardboard mailing tubes coated with fiberglass resin, but less messy and works well is white PVC found in various diameters at your favorite home building supply outlet.
Once your loading coil is pruned to resonance you will want to coat it with resin, or even do as I did when I built this coil, and cover it with 3m electrical tape to hold the winds in place.
You can expect to experience some loss when covering the wire wound on the coliform, but it will be minimal on a high quality built coil.
So remember that there a lot of ways to design and build antennas, and there is no end to materials around that can be used for building antennas if you think outside of the box.