40 meter loading coil for the portable vertical

To make the portable vertical antenna more versatile, I built a loading coil for the 40 meter band. My objective was to build a coil which would load the antenna at near its full extension. Additionally I wanted that extension to be somewhat less than full so that minor adjustments to resonant frequency could be made with changes to the antenna length.

Conceptual drawing of the loading coil.

For a starting point, I used these online calculators to estimate the configuration of the coil:

Coil Shortened Vertical Antenna Calculator For 40 meters, with 16.7' radiator: 8.3 microhenries is needed.

Coil Inductance Calculator If I interpretted it correcty, this indicated I'd need 42 turns on a 1 inch pvc 8 inches long.

With the hardware I selected, the system tuned to 7.1 Mhz with 22 turns of 14 guage insulated wire, tightly wound on a piece of 1-inch PVC. I chose to tightly wind the coils because it would look good, and while expermenting to get the right inductance, other variables were eliminated by maintaining a tight winding pattern.

This is the final point with 22 turns. I'll tidy it up a bit before calling it done.

The desired result.

Mechanical Details

I created threaded inserts on each end of the PVC pipe from aluminum rod. The coil attachments are terminal rings. Since they are non-permanent, further experimentation can be done.

These inserts were epoxied into each end of the PVC. The bottom one has a trough bolt that serves as a stud for attaching to the tripod base coax junction. The top one is epoxied into the PVC as-is, and serves as a socket for the whip antenna.

The old Atlas lathe makes axial boring and tapping a breeze.

Comments