by Bob Hill KG5WRY
In a previous article I wrote about a low cost quick deploy portable antenna (in the February 2023 Chawed Rag). I described building the telescoping 16.5 foot vertical antenna and the ground spike mount. The antenna works great on bands 20, 17, 15, 12 and 10 meters. This is great for daytime radio, but I wanted to play radio at night which means 40 meters. What I needed was a base loading coil to tune the antenna into the 40 meter region.
Taking a guess, I choose to load a 10 foot vertical antenna for 7.2 MHz, with a 2 inch (outer diameter of 1.75 inch PVC) diameter coil. The website “66pacific.com” has useful calculators for Hams. I used the “Coil-Shortened Vertical Antenna Calculator”to find the inductance needed to load the antenna. The calculated value came to around 10 uH of inductance.
Next I used the “Coil Inductance Calculator” from 66pacific to determine the number of turns of wire to achieve 10 uH inductance. I used a length of 1 inch for the the coil form and 2 inches for the diameter of the form. I walked in the number of turns needed until I got close to 10 uH of inductance. This turned out to be 14 turns of wire.
For the construction of the coil, I used 1-3/4 “ ID PVC and 12 gauge magnet wire. Each end of the coil was capped with a PVC cap. Each cap has a 3/8”x24 bolt for connection. To find the center of the PVC cap, I printed out a center-finder tool (see photo). The coil was wound on the PVC pipe and the ends captured between stainless steel washers (see picture). A 3/8” x 24 coupler is added to one end of the coil to accept the antenna. The SWR plot of the antenna is good enough to get the antenna on the air. With this coil, I am now ready to run “Late Shift” POTA.