Monday, April 25, 2011

Digital Scope for the antenna launcher

Sometimes great ideas shine, just because of their simplicity and I think I had one the other day. (You can tell that I am modest person :-). I am claiming to have the most sophisticated aiming system of any pneumatic antenna deployment apparatus out there.

Here the Head Up Display of my "Augmented Reality Digital Scope". aiming at 56 degree up at a tree-top in my front yard. The scale for pitch (gun barrel angle) is displayed on the right.

The Scope mounted on my "steam-punk" antenna launcher. I know! The mount of the scope should have been made out of wood.

Front view. The Scope mount is attached with Velcro tape to the barrel for easy installation. Calibration of the scope is possible while mounted but it can be performed before installation on a perfectly level surfaces for greater precision.

This is the Scope's quick mount. I fabricated the mount out of aluminum L-stock. A piece of scrap 4" PVC pipe was used for the bottom part. I softened the plastic using heat-gun and formed it to fit tight around the 2.5" barrel. Small Velcro straps are used to secure the instrument in place. It takes about 30 seconds to install or remove the actual scope .

Cross-hair view down the barrel. Scope is secured in the mount and installed on the antenna launcher. Note the blue closed-cell foam padding between the barrel and the mount. It gives a very firm, non-slip grip for the mount

Ta-Daaah! Here is the secret :-))) The Scope is actually my Android OS smart-phone and it cost me absolutely nothing :) (I already had the phone). That's why it was important for me to be able to easily install and remove the phone from the mount. I fabricated the mount to fit my specific model phone - T-Mobile G2 (HTC Desire Z) but it can be modified for any phone.

Another view of the mount. Note the little cut-out for the phone's digital camera. The construction is very light and sturdy. The phone has a gel-skin to protect and further enhance the mounting.

The heart of the aiming system is a free app called GeoCam v.2.07, available on the Android Market or it can be DL from here. I came across this app accidentally and my first thought was - this could be useful for something one day (it used to be called Theodolite). It displays a wealth of information and it can triangulate objects in order to measure distance and height. The phone's accelerometer sensor is used to measure pitch (essential!!) and roll (the roll is kind of useless for the aiming system). It is using the internal magnetic sensor for the heading (compass) and GPS receiver for coordinates.
All this information changes dynamically and it is super-imposed over live video image from phone's digital camera. One can adjust colors of the HUD data and control the iris (exposure compensation) - very useful for use in bright, sunny day. (I am thinking of making some sort of rigid hood for the display to enhance further the contrast in bright ambient light and reduce glare). There is also a nice calibration procedure - very useful to account for differences between each installation.

The Digital Scope system works fantastic and brings the spud guns to the 21st century!! I think I'll apply for a patent on this one:)

Update: I am currently working on a BBTS or Basic Ballistic Trajectory Solver for the Android OS. The idea is to be able to input the distance to the tree, assuming the tree will be located under the peak of the trajectory, Tree Height + some padding , Tennis ball weight (for drag force calculation, I have data for the average drag coefficient of a tennis ball), Air density/Altitude (again for drag) and additional drag value (caused by line, the ball is pulling - there is no easy way to model this so it needs to be determined experimentally and entered as correction to the drag force)

The output will be a firing solution - Angle and required Muzzle Velocity. The tricky part will be to establish the correlation between the air tank pressure and the initial velocity of the ball leaving the barrel, but I'll work on a way to figure it out (police radar?).

Saturday, April 23, 2011

"Say Hello to my little friend!"

Update: If you are looking to buy an Antenna Launcher for Field Day, please check this post.

Tony Montana's famous words as the answer to the question: How do you get an antenna rope over a 100+ ft tree-top? Forget about slingshots or bow and arrows. Even the crossbow with optical sight is so last century :). Enter: the pneumatic antenna launcher or shall I say "pneumatic blunderbuss";-)Here is the result of a couple of evenings spent in the garage - cutting, gluing and painting PVC pipes.
This antenna launcher is based on (WB6ZQZ) Alan Biocca's CSV19 with some modifications / improvements on my part. His web site ( is an excellent source of information on these launchers and it has very detailed build instructions. I had to do something about the white PVC look which I REALLY hate! The paint job was inspired by K4ICY and his "Steampunk" antenna launcher.
The main changes from Alan's CSV19 design are:
- Slightly larger compressed air tank - my launcher is using 10 inch length of the 4" diameter pipe for the tank vs. Alan's 8 inches. The reducing coupler I am using as part of the tank gives a little extra volume too.
- Longer barrel - 18.5 inches vs. Alan's 16 inch barrel - I had lengthen the barrel a bit in order to account for the larger pressure vessel and have enough clearance for the Zip Reel.
-More reliable and safer pressure vessel - instead of drilling a hole for 1" pipe and epoxy gluing the pipe for the high pressure outlet in a 4" end cap, I am using a 4" to 2" reducing coupler and and 2" to 1" reducing bushing as part of my pressure vessel. Another advantage is that I don't have to drill precision large diameter hole - unfortunately I don't have a lathe.
-More reliable and safe coupling between the barrel and the high-pressure pipe - I am using 2.5" to 2" reducing coupler and 2" to 1.25" inch reducing bushing. It is much easier to assemble the launcher that way! Alan's design yields for drilling a 2.5" end cap and epoxying the 1.25" inlet (actually, a 90 degree elbow) in the hole
-I made the spacer between the pressure tank and the barrel out of two pieces PVC, sliced from 4" pipe scrap. I adjusted the curvature of each piece to follow the outside diameter of the corresponding pipe and glued the pieces back-to-back.
-In a moment of sheer brilliance, I came up with the Augmented Reality Digital Scope. The HUD (Head-Up Display) on the scope shows the firing angle of the barrel (pitch), heading, roll, and geographical coordinates. It is also capable of measuring distance and most importantly height of an object (tree). (Scope is not shown on the picture above)This is probably the most significant improvement to the launching system I am willing to take credit for as it allows to correct your shots in a precise manner by adjusting the exact angle of launching. About the only thing I am missing is on-screen display of the air pressure in the tank.More on this in a later post...

The main source of PVC hardware for this project was Trigger, pressure gauge and Schrader valve are available from McMaster (the trigger is part 6852K11). Rainbird 100DV-SS sprinkler valve is from eBay. The bow-fishing zip reel is from an online archery store. Brass fittings, brass street elbow and aluminum stock (for the Zip reel mount and support strut) - all from Home Depot. Tennis balls and Spectra line (150 yards spool / 50 lb test) from Sports Authority. For all threaded connections (sprinkler valve to trigger valve, pressure gauge, Schrader valve) one should use the yellow type teflon tape - it is made specifically for gas/high-pressure applications and seals much better than the standard white plumber's tape.I hated the rattling sound of the coins (used to bring the weight to 4 oz) inside the tennis balls so I injected the balls with polyurethane foam (used to fill gaps). For the tennis ball tie, I used a loop of string with a knot, drilled a penny right in the center and inserted the loop thru the hole. The knot should be large enough so it cant go through the hole. Then I inserted the penny vertically in the tennis ball thru the narrow slit I previously made. When I pull on the loop, the penny wedges flat across the slit - this solution works just fine and after I filled the ball with foam there was no need to stitch the slit - the foam glued the slit and the pennies inside.

This picture shows the installation of the Saunders Bowfishing Zip Reel. Two aluminum bars are attached to the zip reel and the 2.5" coupler is mounted in the center with countersunk screws. (the bottom side of the coupler was filed flat to form two "saddles" for the mounting bars). It is loaded with 150 yards of high-visibility Spectra-Line (50lb test). I even installed a little cutting blade (the yellow thing on the bottom) for added convenience. This line cutter was part of the Spectra Line packaging - i just had to cut it out from the plastic spool-holder.

Update: In the original design the trigger valve could loosen or over-tighten if one is not careful - the valve is not fixed - it relies entirely on the thread and because it must not go all the way in (the street elbow is just partially threaded, 2-3 turns max), accidentally rotating the valve in either direction could cause a variety of unwanted effects.

Alan, WB6ZQZ suggested to use a strut to support the trigger so this is what I came up with. Small piece of curved PVC (scrap 4" pipe, heat gun, 2.5" pipe used as a form for bending and sanding) is drilled for a countersunk screw then glued to the barrel with the screw in place to create an anchor point. An aluminum bar is used as a strut between the anchor point and a brass trigger outlet extension. The red cable-tie is the "safety" (currently in ON position) - it prevents accidental operation of the trigger.
Another solution for the strut anchor point is to drill, countersink and install the screw from inside of the 2.5" to 2" coupler BEFORE the 2.5" barrel pipe is glued. The hole for the screw should be drilled in the middle (or closer to the edge) of the 2.5" portion of the coupler and the countersinking should be deep enough to allow for smooth installation of the barrel after the screw is inserted. There is not much clearance for right-angle drill -the countersink can be done with RA Dremel attachment or manually by hand.