Pneumatic Antenna Launching, Maui style
Here at NH7O the terrain is very steeply sloping, with no flat places, and completely forested. In order to get any long wires stretched out for 80m and 160m, it is necessary to cross very deep, tree filled, gulchs. Probably the only way to get a wire across these spans is to use a pneumatic launcher. There are endless variations of these now on the internet. I started by looking at the version made by WD6CMU, and adding a longer barrel. The span that needed to be covered was over 100m, so a little more ooomph is needed than a typical field day style launcher. The version I came up with is shown below:
To get a feel for how these things work, I used GGDT, the gas gun design tool from Hall's Consulting. This really helps to show what is needed for best perfomance. In my case, it is necessary to have as low a muzzle velocity as possible, so as to reduce any potential problems with paying out the fishing line. Too fast of a velocity tends to break things easily. A slow velocity necessitates a heavy projectile, and a very long barrel. To pay out the line I at first layed fising line into a large bucket. This did not work so well, as a few snags resulted. But the line still made it over the gulch successfully. My design results are below:
Here is the terrain that had to be covered. It is hard to see, but the land drops away in front of the launcher at a steep angle. The trees in the bottom of the gulch are quite tall, which makes it look flat. The projectile sailed over all that and ended up on the far ridge.
I found that this was more than adequate to easily put the projectile where it needed to go. It was also very accurate. The antenna that resulted, an 80m sloping vee beam array, is difficult to show in it's entirety. Looking Southeast, one #14 copperweld wire and the open feeder is shown below. The red circle indicates where the far end is. The other wires, not visible, go to the left on the same ridge, and to the right sloping steeply downhill. It is difficult to get a feeling for the terrain from the picture, as it so covered with trees. It is almost impossible to just walk through this gulch, due to the steepness. It takes about 15 minutes of walking a trail to get to the points on the far ridge line.
The last wire to go up was to the southwest. I had to lift the launcher up the tower to get over the trees that were close by. This time I just wrapped the fishing line around the outside of a plasic bucket, and it worked quite well, with no tangles. Fire in the hole!
Here is the resulting wire. Again, it is hard to tell that there is a very deep gulch hidden by all the trees, but I did again easily clear the whole of it on the first try. Looking along the catenary makes it seem more droopy than it appears when looking from the side. This is #12 hard drawn solid copper, and is very heavy to pull up compared to the #14 copperweld.
How does it work? There are now 4 wires starting at 20m high on the tower end. The antenna is not very high above the average terrain, even though it is more than 35m above ground in the middles. The initial result is that it is very much better to the south and east (6db or more) than my previous 80m ground plane with 6 elevated radials, such that it has now been taken down. The antenna is fed with open wire feeders and a tuner at the bottom, transitioning to coax to the shack. I can select any pair of wires for directivity to the Northeast or Southeast or Southwest, using relays at the top of the tower. I would have thought the low wires would be poor on 160m, but they have often proved to be better than my shunt fed tower. It has been fascinating to observe the 80m and 160m long path opening into EU after my sunset, around the equinox months. The path is skewed. When really open, signals manage to avoid the South pole resulting in some real 599 reports. My signal is reported to come from the Southwest in EU, with signals coming in best here from the Southeast. See the website of K9LA, and his article on 160m skew path propagation, for details on how this can happen.
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