Pneumatic Antenna Launcher

HF wire antennas, particularly those for frequencies below 14 MHz, seem to work so much better when you get them high above the ground. This isn't always possible if you don't have tall towers but some of us have a few “organic” towers available: Trees! Each year they get a little taller. What a bonus! Getting your antenna up there is another thing entirely. What are you going to do? Hire someone to climb them? No, that isn't an option that most want to even consider.

Some just toss a small weight with a line attached and settle with what altitude they can achieve. Usually not very high. A practiced fisherman can use a casting pole with a spinning reel to chuck a line pretty high but accuracy is hard to achieve.

A sling shot is better. It's small, light, and cheap. With the help of a small spinning reel you can sometimes get it up to 70 feet. Accuracy is vastly improved but you can only shoot just so far and the fact that you are dragging a line behind your projectile keeps the range pretty short.

A better solution is the pneumatic launcher. It began as a “spud gun”, a simple cannon for making lots of noise and flinging tubers to spectacular distances. These were often powered by propane (usually from aerosol cans) and ignited with a spark. Dangerous? Of course. Useful? No. Entertaining though. So if we tie a line to the projectile then we can justify the existence of this toy. Better yet, ditch the explosive and craft a superior projectile. Make it cheaply from common PVC pipe, make it so that it can be broken down for easy transport and storage, and design it to be powered by air and you have an ideal gadget to put your antenna up in any tree.

An inexpensive bicycle tire hand pump supplies an easy source of power. The kind with a pressure gauge will allow a well calibrated propulsion charge.

It will probably only cost about $25 to build the launcher. If you build several then you can spend even less per unit since you can better utilize the construction material.

For each launcher you will need one each of the following (schedule 40 PVC pipe):

30” length of 2” diameter for the air storage tank.

24” length of 114 “ diameter for the barrel.

114“ threaded ball valve.

2” cap for the end of the storage tank

Reducer to be glued to the 2” tank with the other end threaded for the ball valve.

114 “ adapter for gluing to the barrel and allowing it to screw into the threaded ball valve.

3” length of 34“ pipe with two end caps for each projectile.

Additionally you will need some thread-seal tape, some good PVC glue, a thread-mounted Schrader valve, a spinning reel, and two hose clamps to mount it to the barrel. You had better add a hacksaw to the list if you don't have a way to cut the pipe and a drill of some sort to drill the necessary holes. Drill a hole in the side of the 2” end cap for the Schrader valve. Use a drill bit just smaller than the valve thread diameter so that you can just thread it into the material and make a tight secure mounting. I also add a flat nut on the inside to make it even more secure. Be sure to place it down near the end of the cap so that it will clear the pipe end yet with enough clearance to allow for the nut to be placed on the inside. I have made some launchers with this valve up on the side of the pipe but the cap provides a much stronger mounting place. More convenient for filling it with air, too.

Glue the cap to one end of the 30” storage tank. Use your best technique since you want to have an even coating of PVC cement with no air leaks. There is going to be a lot of stress on this area. Now glue the reducer to the other end. Same here, make the best bond possible. Set the now completed tank aside to let it set up completely. I recommend waiting 24 hours for a complete cure.

Apply a couple of layers of thread-seal tape to the threaded end of the tank's reducer and screw on the ball valve for a snug fit. Close the valve and pump in about 5 psi air pressure and immerse in some water to look for air leaks. A few tiny bubbles from the ball valve is acceptable, since you are not going to be storing air in the tank for more than a few minutes, but anything else should be suspect. You do not want any weak points in the tank when you pressurize it. If you see a problem then scrap that tank and make another. The valves can be salvaged so it isn't going to cost much for the learning experience. The tank is the most critical part of the project and well it is worth while to get it done correctly.

Glue the remaining adapter to the 114 ” diameter barrel. No need to tape the threads on this end as you will be frequently removing the barrel from the valve. No need to screw it in tightly to the valve, there will only be pressure while firing, just screw it on for a comfortable fit so that it can be removed without tools. You do not want it wobbly and you do not want it flying apart when you “pop” the big valve.

Mount a spinning reel on the output end of the barrel using two hose clamps. Just about any inexpensive reel will work but ensure that it will hold enough line and work smoothly. Do not clamp the mounting so tightly that it distorts the barrel but put it on solidly enough that it does not wiggle and will hold steadily while reeling in the line with a load on it.

The projectiles are made with 3” sections of 3/4 ” pipe. It doesn't need to be schedule 40 so use what you have. Before you glue the caps on, drill two small holes in the end of one of them. Place these holes about 3/8” apart. Open up a wire paper clip and feed it through those holes forming a small loop on the outside upon which you will clip your line. On the inside twist the ends firmly together to secure it. Remember that you won't be able to access this after the thing is all glued together. On the other hand, no need to be neat since no one will ever see it. Glue the caps on. No need to be fancy. Do not put anything inside, there is sufficient weight to get the job done. Check to see if it slides down the barrel smoothly. Sometimes the caps have a few protrusions from the molds they use making them and they may impede free movement through the barrel. If there are, a few strokes with a file should smooth them right off. While you are at it you might want to make several projectiles just in case you lose one.

Load the reel with enough line to get the job done. Remember that you not only have to have enough to get over the top of the tree but also enough to go back down as well. I would suggest 200 yards of 6 pound test at least. You will place a clip and swivel on the end. The clip makes for easy connection and the swivel keeps the line from tangling. You will shoot the projectile up and over the limb that you want to use, unclip the projectile and clip on to the heavier line that you are going to pull back up.

You are ready for a test. Assemble the launcher, drop the projectile down the barrel with the clip going in first. The reason for this is to not have the projectile switch ends when it starts dragging line, thus wobbling a bit and perhaps changing course. Close the ball valve, connect your pump and put in just a tiny bit of air, like 5 psi or so. Careful now, it's loaded, so treat it like it was a gun (it is!) and keep it pointed skyward. Don't go banging it around. Keep away from the “trigger” lever. Wear eye protection since you can never tell if something may blow apart. With 5 pounds per square inch you now have around 100 pounds of force trying to get out. Have anyone nearby move way back. Aim it straight up in a clear area. Do Not Forget To Open The Bail On The Reel! With the bottom of the tank held against the ground quickly rotate the trigger lever as rapidly as possible to the full open position. You will hear a satisfying “PONK!” and the projectile will make its first voyage skyward.

Make several low pressure test shots before using the system to put a line over a tree. You need to get a feel of how much pressure to use and how the wind affects the flying projectile and line. I seldom use more than 45 pounds and have not tried 65 psi. I'm not sure that the reel will hold enough line for that much altitude! Keep in mind that 50 psi represents nearly half a ton of collected force against the inner surface of your tank. Treat it accordingly.

The best tactic is to get in close to the target tree and make a high angle shot so that the projectile comes back down near the trunk on the far side of the tree, avoiding the heavier foliage and the problem of having lots of line draping over the outside of the tree. Shooting another 50 feet above a 100 foot tall tree lets it get good momentum before it plows in. The white plastic projectile is easy to spot as it dangles down through the branches. Just pay out line and move it up and down if necessary until it gets within easy reach. Clip on the interim pull line (nylon cord works well) and reel it in. Then tie on your cord that will ultimately hold your antenna and pull that through. When you have that one in hand then you are ready to attach insulator and antenna wire. Pull it up and tie it down.

This is the simplest form that I can come up with. Some constructors have used more than one storage tank, folding the tank back close to the barrel, using 34 “ electrically operated valves, and different tank/barrel ratios. Feel free to experiment. If you do, let us know how it works.

de ND6T

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