Antenna Lessons Part Three - Antenna Rigging

My home is smack in the middle of a forest, a mix of conifer (Douglas Fir, Western Red Cedar, Western Hemlock) and deciduous (Red Alder, Cottonwood, and a variety of smaller species).

I have one VHF/UHF antenna on the roof of our garage but in general all the other antennas are suspended from trees in some way.

General Pattern

After some failed experiments I’ve settled on the following pattern for using trees to support an antenna or rigging for antennas.

First, there’s a rope that goes from the bottom of the tree, up over the top of the tree (or as close to the top as I can arrange), and then back down to the bottom, where the two ends of the rope are tied off to form a closed loop.

Along this closed loop, there’s a pulley on a thimble knotted into the rope using a loop formed by an alpine butterfly knot. This pulley is raised as high as practicable by hauling on the rope loop and then fixed in place using a Nite-Ize Figure 9 carabiner or a Nite-Ize CamJam which is clipped to a stainless steel eye-bolt screwed into the trunk of the tree.

Through this pulley is a second loop of rope which reaches the ground plus a bunch of excess to allow flexibility in the future (namely, raising the pulley higher). This loop (the ‘halliard’ or ‘halyard’) has a thimble set in an alpine butterfly knot, and this is what the antenna or other support is attached to. Once the antenna is raised using the halliard, the halliard is fixed in place using a Figure 9 or Camjam and another eyebolt set into the tree trunk, or by suspending a weight on the halliard to counterbalance the antenna and keep it aloft.

All this is a lot of rope and a lot of hardware when compared to simply putting a rope over a tree, attaching the antenna, and then dragging the rope back over the tree to lift the antenna. But it makes the entire system easy to maintain and makes it simple to raise and lower the antenna for inspection and maintenance.

How to get a rope over a tree

I’ve used a bunch of different methods to get a rope over a tree:

using a slingshot to throw a weight over the tree, with the weight pulling a line as it goes over
using an air cannon to shoot a tennis ball with added weight over the tree, with the tennis ball pulling a line as it goes over
using a drone to drag a line attached to a tennis ball over the tree, and then drop the tennis ball to the ground on the other side.

In each case, the initial line over the tree is braided slippery fishing line - lightweight, slippery so it does not bind up on foliage, and plenty strong enough to pull nylon twine over. Once the fishing line is over, I use that to pull a length of brightly colored nylon twine over. The brightly colored twine is easy to see, so you can assess whether you’re happy with exactly how the line is situated. Then, finally, I use the twine to haul 3/16” black Dacron rope over. All this seems like a lot of extra work but I’ve found over time that making sure the rope ends up in a place you like is the key to having it not slide off the tree later during a windstorm. And in any case its not as much work as it sounds once you’ve done it a few times and got some skills.

Slingshot Method

I know a fair number of hams use this method. It has the advantage of being pretty cheap; all you need is a slingshot, something to feed out fishing line at great speed (like a spinning reel), fishing line, and a lead weight. I liked this method until one day, I was trying to get a line over a tree in the middle of the forest, which forces you to fire from up close to the tree you want. The weight went up, but I’d miscalculated the angle (it’s astonishingly easy to misjudge angles when you’re looking nearly straight up). When the weight came down, it landed about three feet from where I was standing.

And it occurred to me that since the velocity at impact is probably pretty close to the velocity at launch, if the weight hit me, it would be like someone firing a one ouch lead weight at me with the slingshot, point blank range.

I decided not to use the slingshot after that.

Air Cannon method

My next step was to build and use an air cannon out of PVC pipe. The projectiles are tennis balls with a wire loop threaded through them, and weight added by putting pennies inside through a slit in the ball. There’s a spool for the fishing line that is coaxial with the barrel which pays out the line as the weight flies, and makes it easy to wind the line back up to drag twine back over.

This method works well, and I still use it from time to time if I’ll be putting a line over a tree that is not close to other trees.

Drone Method

I have a DJI Mavic drone, and a clever drop hook mechanism that is activated by the running lights on the drone to release.

So the drill is basically:

hook the weighted tennis ball and line to the drone
pilot the drone up and over the tree, watching both the camera view (to avoid hitting the tree) and the line trailing behind/below the drone (to see how it gets placed over the tree).
Once you have the line placed the way you want it, turn the camera to face directly down. You’ll see the tennis ball hanging below the drone.
Position the drone exactly over the spot you want the ball to pass through the tree canopy, and 1.5-2x as high as the tree height, so the ball is moving pretty fast when it hits the foliage and it doesn’t have to pull line over the tree to reach the ground.
release the ball, and watch the camera view so you know exactly where the ball entered the canopy.
land the drone safely
Go find the tennis ball, take the tennis ball off the line, hook nylon twine to the line, and then drag the twine over the tree using the fishing line.

Assuming I’ve remembered to charge drone batteries, this is easily the fastest, simplest, and most reliable method. Even on windy days, you can do several test drops in a clear area to get some feeling for windage and then adjust your aim a bit when doing the real drop.

Using this method, it’s possible to put a line over a single tree that is in the midst of a bunch of other trees, something that is a lot of work with a slingshot or aircannon.

Tree dynamics

Beyond the ability to lower and raise antennas with ease, part of the reason I like using pulleys as I’ve described is that it makes the system of rigging flexible in the sense that it can shift to accomodate the trees moving in the wind.

Consider an antenna suspended between the pulleys on halliards attached to two trees at considerable height. The wind blows, and the trees wave back and forth, and because the pulleys are pretty far up the tree, they move back and forth as well. Sometimes the trees will move back and forth in unison, the distance between them will be more or less constant. And sometimes they will move in opposition, and the distance between the pulleys will first shrink, then grow, and repeat the cycle. The one thing we really don’t want to do is have the trees move apart more than the antenna has slack, but because of the suspended weights, if the trees move apart, the weights rise, and the antenna is fine.

I’ve actually watched my main antenna during a big windstorm, and watched the end weights rise and fall to keep constant tension on the antenna as the trees move.

If the antenna is only suspended at two points (say, an end fed antenna fixed at both ends) you’d only need the weight arrangement at one end. If the antenna is suspended at more than two points, you’re going to want weights at all but one of the points.

Fixing a line in place

Once you have the antenna situated the way you want, you’re going to want to fix lines in place, because you can’t just stand there holding the line forever.

What I do is set eye bolts into the trunk of the relevant tree, use a Figure 9 carabiner or a CamJam to grab the line, and attach the carabiner or CamJam to the eyebolt.

Excess line is then neatly folded up and dressed and then put between the carabiner and the tree trunk, to keep it off the ground. It’s absolutely vital to arrange that the excess rope does not end up on the ground. The ground is a very hostile environment for rope - a damp source of mold, moss, fungi, and little animals who might decide they want Dacron rope for breakfast. On top of all that, rope on the ground will rapidly become entangled with the undergrowth as it grows, and once the rope is looped through the roots of a (say) vine maple, you’re either cutting the rope to get it free, or you’re digging up the root ball of the vine maple. Don’t ask me how I know this.

When putting eyebolts into trees, remember that trees grow, and if you set the eyebolt with the eye flush with the bark, the tree will grow to entirely encapsulate the eye, and you will be very sad next time you try to do something with that eyebolt. So I set the eyebolts a good 1/2”-1” proud of the bark, and I go around about once a year to back the eyebolts out a bit as the tree grows.