Trap Dipole for 40 and 20 meters

Created Tuesday 01 October 2024

I'd like a good performing antenna for 40 and 20 meters primarily. The center fed resonant dipole appeals to me for its simplicity. Easy to feed. No antenna tuner required. When I was a young ham I built a trap dipole for 80 and 40 meters. I used it for 80, 40 , and 15 meters. At my current location I don't have room for an 80 meter dipole so I'm planning the antenna for 40 and 20 meters. If it works out well, I expect I'll install additional traps for 15 and 10 meters.

I've started by cutting a 40 meter dipole and getting it up in the air.

This shows the backyard layout. The left circle shows an end insulator and bungee cords to take up the wind motion in the tree. I expect that the bungee cords will degrade within a year, so they are a short term solution. The center circle shows the center insulator and balun at the feed point. The right circle shows the placement of the other end insulator. I've reached an age where I don't climb trees and don't get up on roofs. so this installation was accomplished without much climbing.

One end of the antenna support is about 30' up in a poplar tree. That's what I could manage working with 25' of PVC to get a cord over a branch at 30'. But this winter when leaves are gone from that tree I hope to be able to use airborne missiles and get the end point up higher. Right now the tree is so dense, I don't think firing a projectile over the top would yield a good result. I estimate that the total height of the tree to be about 80 feet.

This shows the potential of that tree!

Currently the antenna is performing OK on 40 meters. From a technical perspective it is too close to the ground. Most recommend that a dipole be 1/2 wavelength above the ground. So for 40 meters that's about 60 feet. N3FJP has an excellent article on optimizing dipole antennas, where he shows the optimal height is 0.6 wavelengths. This implies that the trap dipole is less than ideal, since only one band can be at "optimal height." For my real estate constraints, I will just be getting things up as high as possible, without a tower and without roof climbing.

For the 20m traps, I'll be using the clever coaxial cable traps described by K7MEM in his detailed article.