Friday, March 22, 2019

Low Band DX - 2 - Vertical to Inverted L

Last weekend I decided to upgrade the vertical to an inverted L, specifically a 1/4 wave on 80m.

Measuring the 43 ft vertical purchased from ZeroFive, it has 14 x 32" sections and the base section is 36" for a total of 484" or 40.3 ft - where is the other 3 feet? No idea! Running some rough calculations I added a 22 ft horizontal wire to the top, and after a fight with the maple tree snagging the wire a few times I got it up.

I was a little surprised at the changes, SWR:
  • 80m, 3.5 MHz is near 1:1 and rises towards the top end of the band.
  • 60m, SWR high, remote ATU fails to find a match.
  • 40m through 10m SWR high, remote ATU matches it in no problem.
Since I have no real idea as to what load the antenna is presenting to the ATU, its hard to know much more, to solve that I have a RigExpert AA-55 antenna analyzer ordered.

With that, I decided to bang my head on antenna modeling to help get a better understanding on what to expect from this inverted L I have created. Being brand new to modeling software, I found 4NEC2 "too hard". I took at a look at MMANA-GAL, which includes an inverted L file. After a quick edit of the geometry to match what I have, the results were interesting. Granted I might have missed an important detail with this, it could easily be totally wrong.

Starting with 80m and working our way up, blue is horizontal, red is vertical polarization:

80m, what I expected, the small horizontal component is from the horizontal wire at the top. An anecdotal observation is the small horizontal component has made receiving local signals via NVIS stronger.

40m, calculating the length on 40 turns out to be a 3/8 wave, so that works.

For reference, here is the contents of the MMANA-GAL file:

Inverted L: v40 ft + h22 ft
0.0, 0.0, 0.0, 0.0, 0.0, 12.28, 0.015, -1
0.0, 0.0, 12.28, 6.7, 0.0, 12.28, 0.001, -1
1, 0
w1b, 0.0, 1.0
2, 0
w1b, 0, 0.0, 400.0, 0.0
w1b, 1, 10.0, 0.0
400, 40, 2.0, 2
2, 0.0, 4, 50.0, 120, 60, 0.0
Feeding direct coax 50 Ohm.
Bandwidth (SWR<2) 210 kHz.
Load 2 ia a ground loss.