Folks, this has been a fun and hilarious winter project. Yesterday, when I built this air cannon antenna launcher it was -4 degrees outside and this was a perfect evening project to put together. For years, me and my two ham radio buddies (WR8S Bill Shultz and WV8TG Tom Graf), have enjoyed ARRL Field Day activities and usually used a slingshot and a 3/4 ounce fishing sinker weight and an old Zebco fishing reel (with 20 pound test line) to shoot and suspend our doublet dipole antennas high into the trees. Certainly, the slingshot worked pretty much flawlessly… but… boys will be boys, and the idea of an air cannon / spud launcher / potato gun type system seemed to be a new desire. (Most people, our wives included, wouldn’t trust our 3-man team with a slingshot, let alone a potato gun! But I digress.) Actually, our local ham radio club (Monongalia Wireless Association) had a version of a potato gun antenna launcher, so we decided we needed one of our own… and to make some design modifications in the interest of… “science”… plus our own sadistic pleasures. (Make sure you read below WV8TG’s initial “pressure test” experience.) The following pictures hopefully provide the basic concept and parts we used. Tom and I split the cost of the parts needed to make launchers and each built our own version, but they both are the same basic design with only length dimensions of the air chamber and barrel being the difference.
All parts where sourced from our local Lowes store in their plumbing department, except the Schrader valve which can be purchased at an automotive store. Total cost about $40 but you could make a couple of them as a joint project with a friend and reduce that cost per launcher a bit. I got a small rubber gasket for the outside nipple of the Schrade valve to act as another seal on the exterior of the air chamber. You will need to drill holes for Schrader valve, barrel slug stop and projectile slug caps to attach the screw eyes to attach the fishing line. I used 3 inch PVC for air chamber, 1.25 inch PVC for the barrel and short sections to mate the air chamber, trigger valve and threaded barrel. The projectile slugs were made from 3/4 inch PVC and caps and I filed off the nubs on the caps with a Dremel tool for a smooth fit into the barrel. I used PVC Prep on each joint before applying the glue. When gluing joints together, insert the sections together and twist a quarter turn for a solid adhesion. Let all glue set up for 24 hours before testing air pressure chamber. I will pressurize the chamber inside and let it sit overnight to see if it loses any pressure. It’s too cold right now to take outside in -4 degree temperatures to test, but I will use a bicycle pump with a pressure reading valve, and start at 40 psi… then 50 psi… and then 60 psi for test shots to see how it functions and check for any air pressure leaks. We use 60 psi for our club launcher.
WV8TG (Tom) charged his air chamber (barrel not attached) and let it set overnight to test for chamber air leakage. When he opened the trigger value… there was NO leakage… but there was a sudden LOUD release of 60 psi air gush out of a 30 inch long, 3 inch wide fully charged air chamber. He indicated the compressed air release was… impressive. However, his wife was not impressed… nor was she aware of the scientific test that was taking place. #surprise! #WHOOOOOOOSH #loudwifeexpressions I have no reason to doubt Tom will find his projectile slugs in the next county using his design. The club chamber was 12 inches long versus his 30 inch air chamber. #overkill? I designed my air chamber for 14 inches and will conduct all tests… outside. #potentialmeanwife
I have helped build and used two of these Doublet Antennas during the 2014 Field Day Contest and they worked great! Worked many stations on CW, Phone, and PSK31. Works great across many HF bands. A little over 120 feet long. WR8S (Bill Shultz), WD8WQK (Tom Graf) and I are going to make one for my ham shack as soon as I order the parts. Take a look at Ray Heffer’s explanation of the Doublet Antenna and a diagram by N4UJW below.