Loading coils

When I set out to build a widget, whether it’s an antenna, farm implement, hot rod, or whatever, my process is usually the same: study several different widgets, note the things I like most about each widget, then combine all those things into my own version of the widget. A few of the things I wanted for my own coil was: a quick and easy way to change and tune through the bands; allow me to tune from 80 meters and up using a 102” whip; be able to operate on digital modes at full duty cycle using 100 watts (not that I do this, but for me it was a “requirement” for the design), lightweight but strong; easy to build with mostly hardware store items; and be inexpensive.

What I ended up with has been dubbed the BNC Coil. It checks all my boxes, and whether it’s the current SFI numbers or not, I have set my own personal records for number of POTA contacts while using this coil, and its little brother, the BNC V.2 Coil.

I use loading coils on my vertical antennas to allow me to run a shorter antenna. For example, a 40 meter 1/4 wave vertical antenna is about 33' tall. For some portable operations, that might or might not be feasible. Adding my V.2 loading coil will lower the resonant frequency of the antenna (adding the coil is adds induction in series to the radiator), thereby allowing a shorter overall height. By center loading my vertical with a 4' mast under the V.2 coil (scroll to the bottom of this page for more info on why I center load my vertical), and a 9.5' tall stainless steel collapsible whip, my portable vertical is about 14.5' tall overall. This combo will allow me to tune down to 60 meters, and I can bypass the coil for 15 meters and up and tune those bands by simply shortening the whip to the appropriate 1/4 wavelength height.

The three coils pictured are:

Top: The BNC V.1 coil, 80m

Middle: The BNC V.2 coil, 60m

Bottom: A 40m experimental that has been shelved

The V.1 coil was my first go at making my own coil. The length was chosen simply based on the length of the wire traps used to wind the coil. With this coil, bottom loaded with a 102" whip, it will tune down to 75 meters.

Since I hardly ever use 80 meters, I decided to make it shorter. Thus the V.2 was born. Much shorter and lighter, it allowed me to operate on 40 meters and up with the 9.5' whip. It is my go-to antenna when operating portable. If you would like to build your own, you can do so for about $25. To download the plans, click HERE. The PDF contains all the info and part numbers you need to build either the V.1 or V.2 coil.

In the experimental coil, I had a brainstorm to machine grooves directly into a piece of material instead of using the wire traps. I ended up with a really cool looking oil, but with a weight that was far too heavy for what I wanted. So it sits on the bench just looking cool. :)

Top to bottom: BNC V.1, BNC V.2, and experimental that is now on the shelf.

A close up of the bottom of the coil to show the Tram quick disconnect, and where the jumper wire is attached. The jumper wire attaches to the coil wire with a small alligator clip (seen below), and effectively bypasses any inductance possible between the bottom of the coil, and where the clip is attached. Only the part of the coil between the clip and the top of the coil will add inductance to

I prefer using these small alligator clips as opposed to the Pomona-style mini test clips. The very small hook on the test clips are fragile and easily bent, and the plastic housing isn't very tough either. After some testing, I will mark the spots for the different bands on the coil with colored fingernail polish. I use RED for when using the 78" whip, and BLUE for the 102" whip.

Until May I had been using a piece of 3/8" x 36" aluminum round bar for an extension. But a trip to South Dakota and an activation during some heavy mind made me rethink this setup. With the antenna on the hitch mount, a big gust of wind come through and bent the extension and folded the antenna system over 90°! So a stronger solution was in order. I ended up making a 3' extension (for the hit

As a lot of us use some form of a loading coil, and some center load them. The question usually comes up about what advantage is there to center loading the coil on a ground mounted physically short vertical. After reading through several antenna books on cap hats, I ran across this little blurb in an ARRL Handbook:

A group of short vertical radiators is presented in Fig 20.44. Illustrations A and B are for top and center loading. A capacitance hat is shown in each example. The hat should be as large as practical to increase the radiation resistance of the antenna and improve the bandwidth. The wire in the loading coil is chosen for the largest gauge consistent with ease of winding and coil-form size. The larger wire diameters will reduce the resistive (I2R) losses in the system. The coil-form material should have a medium or high dielectric constant. Phenolic or fiberglass tubing is entirely adequate.

A base-loaded vertical is shown at C. The primary limitation is that the high current portion of the vertical exists in the coil rather than the driven element.

With center loading, the portion of the antenna below the coil carries high current, and in the top loaded version the entire vertical element carries high current.

Since the high-current part of the antenna is responsible for most of the radiating, base loading is the least effective of the three methods.

The goal is the highest possible radiation resistance, which means the longest base length possible under the loading coil of a short antenna. Most losses occur in the loading coil. A large top hat will allow one to reduce the reactance of the loading coil which will reduce losses.

The way it's loaded has nothing to do with radiation angle; it's all about current flow and minimizing ground loss.

Have you noticed a number of people melting a certain popular bottom loaded portable vertical antenna when running high duty cycle digital modes at even moderate power settings? Bottom loading the antenna is the reason!! Center load the antenna, move the coil up, thereby moving the high current point up, more evenly distributing it across the system. This greatly reduces losses and HEAT, as well as making the antenna far more efficient.

I have also played with capacity hats a bit, but not enough to report on it's effectiveness in my portable operations yet. But when I do have some more info, I will let you know my thoughts.

Loading coils

center loading versus bottom loading