trapped 40m/80m nvis dipole
Local and Regional Comms Made Easy
I am an active member of our ARES group, and I am the EC for my county EOC. One of the first HF antennas I put up at my new house, other than my small transmitting loop antenna, was an 80m dipole installed on my privacy fence. Now, when I say on the fence, I mean that the wire was held to the fence with thumbtacks. Improvise, adapt, and overcome right!? It isn't crazy if it works!
The initial antenna used a BNC to binding post adapter for the feed point, with the wire about 4' off the ground. As to be expected being so close to the ground, the antenna ended up being about 112' feet overall in length when trimmed for a frequency of 3.810 MHz (our weekly statewide ARES net frequency), as opposed to around 125' or so for a typical dipole on that same frequency.
The antenna worked amazingly well, so well that I also wanted to use it on 40m. So I added separate legs for 40m and tuned them for 7.195 MHz, the frequency for our monthly FEMA AuxComm Net.
I knew I could do better than thumbtacks though. So I set out to rebuild it with a couple things in mind; a better feed point, and to use traps to multiband the antenna. I just happen to have an LDG 1:1 in a parts box, so I already had the feed point taken care of. I mounted that at 5' AGL, and used real screws. No thumbtacks.
I went back and forth between coaxial traps, and toroid traps. I decided on the coaxial traps because as always, it's something I had not made before, so now was the time. I tried out several different online trap calculators until I stumbled on K7MEM's amazing website, and his most excellent trap calculator at https://k7mem.com/Ant_Trap_Antenna.html . Since I had plenty of extra chunks of RG8X, I used that. The next coaxial traps I make will be out of RG58, and smaller diameter coax.
I built the first trap according to the specs from the calculator, but made it juuuust a touch long. Then I could set the number of turns to the exact spot I wanted to tune the traps for, which was to be just below the 40m band. Once I had the parameters figured out, I built a matching pair of traps. Both traps ended up tuning at 7.062 MHz, perfect!
New 18ga wire was used. I first tuned wires for 40m, then added the traps. Once I did that I ended up needing to retune the 40m legs by shortening them a touch. Once tuned for 40m with the traps installed, I started adding wire for 80m. A bit more pruning, and the antenna was perfectly tuned for my two desired frequencies of 3.810 MHz and 7.195 MHz. I was able to reap the rewards of the new antenna on the following Sunday morning statewide ARES net buy getting a ton of great signal reports, better than the original antenna. Yeah, yeah, yeah, I know. Conditions, propagation, etc. The end result is I now have a great NVIS antenna permanently installed that gets me on the bands and frequencies I need most for ARES work, plus local and regional nets. And it's not uncommon to work stations all across the country on this antenna too.
