Compressed Air Antenna Launcher

Building a Compressed-Air
Antenna Launcher
(a.k.a. spud gun)

Corey, KC5YCO fires one off with the K5LXP Antenna Launcher During Field Day 2003

Background

Building the launcher started when I needed to get a wire over the crowns of some 50-60ft trees in my yard. The EZ hang sling shot I tried was a joke, I could almost throw a weight by hand as far and as accurately as it could. Leaves and branches easily deflected the small line weight. I looked around at various stores for an inexpensive bow and arrow set. Thanks to liability I'm sure, there is no such thing as a child's inexpensive bow and arrow set, only expensive hunting models for adults. I didn't want to spend $150 or more on a nice bow just to put an antenna up once in a while. Then I remembered a QRP buddy of mine having an "antenna launcher" he used while in the field to put up his portable wire antennas. All it amounted to was a compressed air spud gun and a fishing reel. A plastic plug takes the place of the spud which takes the fishing line with it when launched. So I duplicated the setup for the sum total of less than $20, the most expensive part being the 24V sprinkler valve from Lowe's. Using this launcher literally gives you point and shoot antenna placement capability. Not only can I get a line over a tree, I can even pick the branch I want. Adjusting the pressure determines your range. At full pressure I can easily run all the line off the fishing spool, more than 60 yards, at a height than can go over any tree I've encountered. Might seem like a lot of trouble to go through to get a line over a tree, but it was quick, simple and cheap to build. Along with a 12 volt tire inflator and gel cell to pressurize it, it's portable and quick to use. It was a real time saver during Field Day putting antennas up. When you're not putting up antennas it'll launch a spud well over 100 yards, adding to the between-QSO fun.

What you'll need

Shopping List:

2ea 36" pieces of 1-1/2" schedule 40 PVC pipe
1ea 30" piece of 1" schedule 40 PVC pipe
1ea 8" piece of 1-1/4" schedule 40 PVC pipe
2ea F-F 1-1/2" pipe couplers
2ea 1-1/2" to 1" NPT adapters
2ea 1.5" long 1" NPT nipples
1ea 1-1/4" PVC Tee
1ea 1-1/4" PVC Cap
1ea 1-1/2" PVC Cap
2ea 1" PVC round end plugs (*not* caps, *not* octagon flange)
PVC Glue and Primer
1ea Orbit brand 1" Anti-Siphon valve, Model 57224
1ea Spin Casting Reel
1ea Schrader (tire) valve (Pep Boys)
1ea SPST Switch (suitable for trigger)
2ea Hose Clamps
Teflon Tape
Tie Wraps
Zip Cord
Crimp Connectors
2ea 9V Alkaline Batteries
2ea 9V Battery Clips
1ea 1/4" screw eye

Sounds like a lot of pieces but you probably already have some of this stuff around the house, and PVC pipe and fittings are cheap.

Assembly Hints
(Photos are clickable for a larger view)

Here are some basic instructions to get you going. If you can think of a better way to do it, by all means feel free to strike out on your own, there really aren't many rules for these things. There are many plans for compressed air guns on the internet if you need additional inspiration (some people have *way* too much fun with these things...).

Drill a hole for the schrader valve in the 1-1/2" cap. Glue it on one of the 36" pipe sections.

Put the F-F fittings on the 36" pipes. Glue the 1-1/2" to NPT adapters into the other end of the F-F fittings.

Let this dry overnight. Really. If you can smell the PVC cement, it's not dry yet.

Put teflon tape on the 1" NPT nipples, and thread into the ends of the pipes. Thread these into the valve, and tighten. I tightened them as hard as I could by hand. The one with the schrader valve goes to the side of the valve that has the solenoid, this is the air tank. The other side is the barrel. There are flow direction arrows molded into the valve, the arrows should point towards the barrel. You may want to sharpen the end of the barrel if you expect to use this for spuds or other organic projectiles.


The photo above shows a how you would sharpen the end of the barrel. (It will cut the potato to size as you shove it in the barrel.)	This top view shows the 1" PVC rib between the tank and barrel.

Now, the two pipes will be pretty unstable just screwed into the valve, so I used some 1" PVC to serve as a rib between the two. It makes the entire assemby sturdy and parallel, as well as provides a place to run the valve wiring.

Using a hole saw I modifed the 1-1/4" PVC tee so it fit between the barrel and the tank. The 8" long 1-1/4" PVC pipe sticks out the bottom and serves as a trigger grip. The trigger switch and 9V batteries go in here. The 1-1/4" cap at the end is friction fit only, so it can be removed to change batteries. (A set should last several years, the duty cycle is pretty low, HI.)

The batteries and switch are all wired in series. Connect the zip cord, run it through the 1" rib pipe. Connect the zip cord to the valve. Polarity is not important. It should give a noticeable click when the trigger is pulled.


Here's a closup of the the valve end.	This shows the grip/trigger detail.

I mounted the fishing reel to the tank side of the launcher with hose clamps.

The projectile is made of two 1" end plugs glued end to end, and electrical tape wrapped around the flanges of the plugs for a critical fit down the barrel. Drill a small hole in one end for the screw eye and tie the line to it. Be sure to file off any excess mold sprue to make the flange smooth before taping.


These are the kind of PVC end plugs that make up the projectile.	Here is the completed assembly showing electrical tape used for critical fit.

Final Steps
I pressure tested the launcher before handling it. I connected my compressor to the schrader valve and let my compressor run up to 140PSI, as high as it would go while I waited outside the garage. Then when it was full I yanked on the hose pulling it off the workbench onto the concrete floor. If it was going to blow up I'm sure it would've under that stress. The pipe is rated for like 300PSI but you need to test your glue joints.

WARNING
When I first tested mine at 35PSI it shot the plug across the street and into my neighbor's garage door. So be careful with this thing. It can hurt you, an innocent bystander, pets or property if you don't respect it. I thought I'd be playing it safe testing it by shooting some shop rags, but at 50 PSI a shop rag will knock a full gallon jug of antifreeze over. Another safety warning- there's a little manual override control on the valve- cut it off. I found it all too easy to knock it accidently which could cause harm to you or someone else.

General Guidelines
Working it is as simple as giving it somewhere between 20 and 120PSI worth of charge, point and shoot. Even a tire inflator running on a gel cell will charge it up in a minute or so. Make up a couple of plugs as spares and you're ready to go. Start off with relatively low pressure, I've found that about 30PSI will clear just about any medium sized tree. If you use too much pressure you'll go way over the target and end up with a lot of line out, and possibly send the projectile where you can't get at it to tie a rope on (like your neighbor's roof). Try to go more 'up' than 'over'. Sometimes in order to hit the branch you want you may need more pressure to get through leaves and branches. With a few practice launches you'll be a pro in no time. When you're not launching antennas this thing makes a hell of a spud gun. I live near a school and easily clear one end of their football field to the other, at least 100 yards. A spud followed by a quart of water down the barrel makes a spectacular spray. Tossing a few glowsticks in there after a spud makes for some fun times with the kids at night, they love chasing after them in the dark. Makes the neighbors think there's UFO's going overhead. Whatever you fit down in there will come out in a hurry, and is a lot of fun. Potatoes and apples work well, tomatoes mostly spray. Half a quarter pounder with fries and some coke will come out in chunks. It won't take long before your imagination will take over and you'll be stuffing all kinds of objects in there, and will make the neighbors wonder where all the wierd stuff they find their yard is coming from.

Tom, WB5QYT and his Launcher.
Thanks Tom for providing the inspiration to build one of my own.

Additional Inspiration
Just so you don't think I'm completely crackers, there are others that have taken this idea much further than I have. Check out these links for other antenna launcher plans.

WB6ZQZ's Pneumatic Antenna Launching Site
Eric WD6EMU's Antenna Launcher Page

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