While going through my stash of old parts, I came across a coil assembly I built over 20 years ago. Originally, it was part of a homebrew antenna inspired by the MFJ-1622 Apartment Antenna; but I used it over the years with a variety of whip antennas while “stationary-mobile.” As I moved on to other antennas for my portable operations, the coil was relegated to the junk box and forgotten—until now.
I also had an old MFJ-1956 12-foot telescopic whip that I haven’t used in years. It was stashed away in the basement waiting to become part of a new antenna project. Having just installed a ⅜-24 antenna mount in the bed of my pickup truck, I thought the coil and whip might work well with it.
The coil is a beast. It’s made from 1.5-inch PVC pipes and wound with bare copper wire—16 awg, I think. I used four strips of nylon grommet edging material to keep the turns evenly spaced. (I don’t remember where I got the grommet material, but it’s similar to this Panduit product.) The coil is about 5.4 inches long and 2 inches in diameter with 40 turns. Using an online calculator, I figured the coil is approximately 25.4 μH.
The coil assembly weighs in at a hefty 14.1 ounces, and the overall length is 10.5 inches. The whip is 24 inches collapsed and weighs 9.6 ounces, so it wouldn’t be my choice for a backpacking antenna. However, on my truck, it should do fine.
Using another online calculator, I reckoned the coil should be more than enough to resonate the 12-foot whip on the 40M band and possibly the 60M band. Since the whip, coil, and mounting bracket all use ⅜-24 hardware; it was just a matter of slapping it all on the truck to see what how it would perform.
To test it out, I made a trip to Valley Forge National Historical Park (K-0761, KFF-0761) this morning. With a minor geomagnetic storm underway, the forecasted band conditions looked pretty dismal. I also got an early start, since we were expecting some severe storms around mid-day. So, I wasn’t expecting much, in the way of contacts.
I mounted the antenna on the back of the truck and broke out the antenna analyzer. It took a bit of fiddling to find resonance on 40M. The lowest SWR was around 3.8:1. That’s not great, but my little Elecraft T1 tuner handled it with no difficulties. On the air, I was getting some decent spots from the Reverse Beacon Network, and I made about 8 contacts before moving to 30M.
The SWR on 30M was down to about 2:1. Again, the T1 made sure my TR-35’s finals stayed happy. I made one park-to-park contact on 30M before moving up to 20M
The SWR on 20M was about 1.3:1 across the band. I made another seven contacts here, before stopping to do some experimenting with the antenna.
I checked 17M and measured an SWR of about 1.2:1 across the band. I didn’t try to make contacts on 17M. Instead, I went back to 40M to pick up a few more contacts before shutting down.
I intended to bring a Sharpie® pen along to mark the coil for each band to speed up band changes. Of course, I forgot to bring one along. Oh well, I’ll do that next time. For 15M and above, I’ll need to bypass the entire coil and shorten the length of the whip accordingly.
The static crashes were getting louder, and I heard thunder off in the distance. So, I called it quits. Just as I shut the rig off, the heavy rains started. I quickly took down the antenna and packed up to leave.
Despite the lousy band conditions and heavy QRN, I ended up with 18 contacts. I had three park-to-park contacts today. And, as it turns out, this was my 20th POTA activation at Valley Forge, earning me a “Repeat Offender” award for this park.
It looks like this mash-up of antenna parts works pretty well. I’ll give it another shot in a few days. Hopefully, the weather and band conditions will be better.
72, Craig WB3GCK
WB3GCK QRP Amateur Radio 