|Can you see it? I can't either. The black wire in the middle|
ascends into the tree, but the traps are virtually invisible
The 160m Inverted-L went up nearly a year ago. Six months ago, I added a trap for 80m, making it an 160m / 80m Inverted-L. At that time, it seemed like a good idea to add a 40m trap. It's taken me a few months to do this.
I used CocoaNEC 2.0 to model the trapped inverted-L. With multiple traps, there's lots of interaction in the antenna segment lengths. More than you'd think, since the trap resonant frequencies are well outside the operating frequencies of the antenna. This means that the trap impedance, while high, doesn't completely cut off the flow of current in the rest of the antenna. This means the 160m segment length affects 40 and 80m and vice versa.
My model showed the 40m segment would be about 32.5 feet, the segment between traps would be about 3.6 feet and the rest of the antenna would be about 69 feet. The problem with this model is that it uses NEC 2 -- which assumes a perfect ground, so the real antenna different. My approach was to insert the 40m trap at about 34 feet, and then slowly trim to a resonance in the band. Then I'd trim the segment between traps, and finally the 160m segment.
|Comparison of 40m (left) and 80m (right) traps. Note the|
turns on the 40m trap are a bit loose
Trap went in at 34' and after three trims it was a 29' 7". A couple of trips and the 80m segment was 7' 4" long. Everything looked good.
Note that the between traps segment is considerably longer than the model. Could very well be the traps I built are not exactly the same as what I modeled, in addition to the lossy ground effects. I think the modeling work is useful, though, because it has gotten me in the ballpark.
|Another view. 40m trap uses 100 pF, 80m trap uses 200 pF.|
I'm not sure if maybe the turns on the trap may have loosened when I raised it last, or if the 160m trim caused the shift. I ran out of daylight to test it. However, the antenna still works pretty well. I've been running this antenna on all bands through the KAT3 antenna tuner, and it is pretty effective on the designed bands, as well as on 30, 17, 12, 10 and 6m. It works ok on 20 and 15m, but I always feel I'm competing with tribanders and other beams there.
Hasn't been a whole lot of activity on 80m this fall -- I think part of it is because everyone is enjoying the conditions on 10 and 12m. Hope to see more activity as the nights lengthen, so I can work some more DXCC entities there. I also want to try this puppy in the ARRL 160m contest.
This antenna really worked well during SS CW. I used it on all bands through a tuner. Although, for domestic contests, the 80/40m dipole works much better.