Tennis Ball Antenna Launching

The Rest of the Story


version 0.53 10/31/2004

Table of Contents

So, you have a Pneumatic Tennis Ball Antenna Launcher, or you have decided to build one, or you are just interested in what it takes. What else is needed to Launch Lines? How exactly is it done? This note covers all the other stuff, the independent parts that go with a launcher.

PVC Safety

Extremes of temperature are bad for PVC. It becomes brittle in the cold, and loses strength quickly in the heat. It is best to use it only between 18 and 40 degrees C (65 to 100 degrees F).

The manufacturers of PVC plumbing parts do not recommend them for use with compressed gasses. The Pressure rated PVC is designed for working pressures in the range of 220 psi and up, so pressure itself is not the problem. The concern is the failure mode - the plastic can fracture when struck or otherwise broken, and the energy in the compressed gasses can accelerate the fragments to high velocity. Metal pipes to not behave this way, nor do compressed-air rated ABS plastic pipes. It is quite important to avoid breaking the PVC when pressurized and always wear eye protection when using launchers. Construction of a launcher from metal parts is another option to increase safety. The compressed-air rated ABS material is another option, but it is quite expensive.

Preparing Tennis Balls

Computer modelling has shown that 4 ounce tennis balls fly to the highest trajectory. Tests have verified that 4 ounces flies higher than 5 or 2. They fly at lower velocity than standard 2 ounce balls, so the reels feed line more reliably and the ball pulls out line better due to the higher momentum. Most importantly on the far side of the tree they pull line down better. Two ounces is often not enough to pull the line down if the tree is wet or sticky or many other reasons. This turns out to be the biggest time-waster of the entire process - fiddling with the line trying to get the ball to come down. I've used up to 8 ounces, which really comes down fast, but 4 ounces seems to be the best compromise. It generally comes down without delay but not so fast as to pose a danger on descent. To keep things simple you may want to use just one weight of ball, if so, 4 ounce should be the best. If you want to carry several weights I would recommend 2, 4 and 6 ounces. Since I started using 4 ounces I have not used the other weights very much, and I carry several. The more weight you use the more opportunity for damage as they return to earth. Be sure of your landing zone, and remember that balls can bounce. We had one bounce off a branch and come straight down on a trailer's open vent. The 4 ounce ball did no damage as it bounced off the vent.

To add weight to a tennis ball use a utility knife to make a straight slot cut into the ball about 5/8" long -- just long enough to slide pennies into the ball. Slide in pennies to bring the ball to the desired weight. Standard tennis balls weigh 2 ounces to start. It takes about 11 pennies per ounce, so 22 cents will bring it close to 4 ounces. I use a hot melt glue gun to reseal the slot.

To prepare a Tennis Ball for towing line a loop is attached to the ball. I use a Speedy Stitcher sewing awl tool to attach a heavy sewing thread to the ball going in on one side of the weight slot and coming out an inch or so away on the other side of the slot. The line that came with the tool is quite heavy - perhaps 80 pound line - so I use that. I tie a good knot in the line, leaving a loop sticking out of the ball 1-2 inches. Then I pull the line to slide the knot into the inside of the ball.

I have not had trouble losing balls in the field, but it is a good idea to prepare and carry several. Update - I did finally lose one 4 oz tennis ball in the field. Definitely carry a spare or two.

Attaching the Line to the Ball

A large loop is tied in the end of the line large enough to slip over the entire tennis ball. This large loop is pushed through the loop on the ball and around the entire ball, and then tightened. Thus it can be fastened to the ball and untied without knotting.

Loading the Tennis Ball into the Barrel

Some kind of Ramrod is required to push the ball into the barrel. It is best if the ball is pushed near the edges. Pushing on the center of the ball causes it to swell slightly and makes the loading process more difficult. I use a 1.5" to 1/2" bushing, and enough 1/2" PVC pipe so that the ramrod will extend slightly from the barrel even when no tennis ball is in there. The end of the ramrod can be colored red or orange to make it more noticeable. It is not good to accidentally launch the ramrod.

I position the glued slot and the loop on the forward face of the ball so they do not touch the barrel as the ball is launched.

Tennis Ball Trajectory

If we launch vertically, or close to vertically, what height can we expect? Choosing the Launch height and setting the pressure to achieve it is one of the nice features of Pneumatic Antenna Launchers.

The following table shows the predicted height for various velocities with both a standard 2 ounce tennis ball and a weighted 4 ounce ball. The table is computed based on the terminal velocity of a Tennis Ball being 70 mph, which is frequently referred to on the internet. The drag model used is a simple velocity squared model. The calculations are for standard atmospheric pressure, high altitude thin air changes thing considerably, allowing higher and longer flights for the same velocity due to reduced air friction. No allowance was made for line drag.

Tennis Ball Velocity vs Height, Time of Flight and Weight
Vel2 oz4 oz
FPS Ft Sec Ft Sec
50 34 2.9 36 3.0
60 48 3.5 51 3.6
70 62 3.9 68 4.1
80 77 4.4 86 4.6
90 93 4.8 106 5.1
100 109 5.2 127 5.6
110 125 5.6 148 6.1
120 140 5.9 170 6.5
130 156 6.3 192 6.9
140 171 6.6 215 7.3
150 187 6.8 237 7.7
160 201 7.1 260 8.1
170 215 7.4 282 8.4
180 229 7.6 304 8.7
190 243 7.8 326 9.0
200 256 8.0 348 9.3
210 269 8.2 369 9.6
220 281 8.4 390 9.9 Pro Tennis Serve (2 oz)
230 293 8.6 410 10.1
240 305 8.8 431 10.4
250 316 8.9 450 10.6

Reel and Line

Mounting the Reel

The simplest system I have found for mounting a reel on the launcher is to attach the reel to a 2.5" coupler. I use a piece of double sticky foam tape under the reel foot and a pair of hose clamps to hold the reel foot to the coupler. The reel should be as far forward on the coupler as it will go, with the foot up to the edge.

Eric came up with a nice mount for the Saunders Zip bowfishing reel. See his web pages http://www.qsl.net/wd6cmu/.

Line for the Reel

Nylon Twine

Sights

I have not tried sights on my launchers yet. Eric WD6CMU did put a telescopic sight on his launcher but he stopped using it after awhile. Generally just sighting down the barrel seems to be fairly sufficient. A tape, paint, or felt pen line straight down the barrel might be a help. A paintball gun red-dot type sight would be interesting. One problem with sights is breaking them. If they stick out much there is a good chance they will be damaged unless they are quite tough.

Source of Pressure = Powering the Tennis Ball Launcher

The choice of power source for your launcher depends a lot on how much launching you plan to do, and what you have available. It doesn't make much sense to consider a gas-powered compressor if you just want to launch two lines for a dipole. A hand pump or foot pump would be just fine. If you are going to put up ten antennas at a field day site then a DC powered compressor or a small CO2 setup begins to make sense. It is most convenient to fill the launcher right where the launch is going to take place, just prior to the launch. A portable pump or small CO2 setup does this well.

The pressures and volumes required to operate these launchers are similar to those required for bicycle and automotive tires, so the same sources of pressure will work for them. One easy and relatively low cost solution is the 12 volt DC powered compressor of the type sold for automotive tire filling. These can be powered from the car battery (or otoher 12 volt DC source) via the cigarette lighter. To make the system portable requires a portable 12 volt battery. Sealed lead-acid batteries are good for this, and my favorite is a 17 amp hour battery. These are commonly available from flea markets or surplus dealers for around $20. These pumps require one to two minutes to pressurize a launcher.

The portable emergency car-starting devices often have built in compressors, and they have the 12 volt gel-battery, so they can be ready-made self contained portable air sources, and double as an emergency car starter when required. This unit is also a source of 12 volts for the radio, so it is a good system component. This has become one of my favorites. It fits into a Rubbermaid tote of its own, and there is room for miscellaneous stuff left over.

Low pressure air tanks can be pre-filled and used to pressurize the launcher. I saw one recently at Wallmart for $18 that held 7 gallons of compressed air. Depending on the size of your launcher, this may not last very long. They are bulky and heavy and when they are out - well, you know. Carry a backup source.

I did some testing with a 6 gallon tank. Filled it to 100 or 110 psi (which gauge do you believe). Charged the 100 cubic inch CSV19 launcher to 50 psi from the tank 15 times. Of course there are 14 more charges in the tank, but we cannot get them out, at least not at 50 psi. So this is about equivalent to 6 ounces of liquid CO2 in terms of what is delivered to the launcher. Actually a gallon of CO2 at 100 psi is 1.8 ounces, but about half stays in the tank at 50 psi. This makes a 12 oz CO2 paintball tank approximately the equal of a 12 gallon air tank at 100 psi (when charging to 50 psi)!

Human powered pumping is quite practical for small launchers, especially of the number of launches is going to be small. I think the high pressure foot pump is the best (avoid the low pressure models intended for air mattresses). The leg muscles are the strongest and stepping on a pump doesn't tire one out much. These pumps are small and inexpensive, but they do not appear all that durable. I purchased one from Harbor Freight. It is a small unit, #37544 and cost about 5 bucks. I've tested it on a 100 cubic inch launcher and at 40 pumps the pressure was about 40 psi. As the pressure goes up the per-pump increase is reduced. The amount of effort required to pump this unit 40 times with the foot is rather small. For field use it might be good to mount the pump to a piece of plywood as the metal frame will sink into the dirt. It takes one to two minutes to pump it up, about the same time the 12 volt DC compressors require. I think this is a good backup system. This particular foot pump is a bit fragile, it might be wise to carry a spare if it is to be used as a primary source of pressure.

Various bicycle and car pumps that are operated with the arms can also be used. These also provide Great exercise...

Small Portable Compressor

Various manufacturers have small AC powered compressors, some of which are fairly inexpensive and quite portable. Harbor Freight has one #47407 that has a 2 amp 120 VAC motor and a 2 gallon reservoir that appears to be a possible choice. It can be operated from a small generator or an AC inverter (if you have sufficient battery capacity). I have one similar to this and it works well on a small generator. I use this when doing a lot of test launching such as making velocity measurements or height tests. There are even smaller compressors that do not have the pressure reservoir. These are basically AC versions of the DC tire compressor systems, though they may have greater pumping capacity. The nice feature of the reservoir is that filling does not have to wait for pumping. Make sure that the pressure capability is adequate - some of these small units are designed for airbrush or other applications that work at lower pressures than you might need for your launching...

Liquid CO2

My current favorite pressure source is the Paintball CO2 setup. Initially I did not think it worth the investment, but after using it awhile I am quite happy with the results. I have a 12 oz Paintball tank, and a Palmer Stabilizer pressure regulator to which I attached a tire filling adapter. A similar setup can be purchased from Palmer ready to go - see the "Palmer Tire Stabilizer" on their website. CO2 must be purchased locally from a paintball supply store. In my area it costs $3 to refill the bottle with 12 ounces of liquid CO2. It will provide about 30 to 50 launches depending on the chamber size and pressure used. If you need more capacity you can get tanks up to 24 ounces, and of course you can have several - it is easy to move the regulator to another tank. These CO2 setups fill the launcher in a few seconds and are portable enough to take with you right to where the launch is to be done. This is not only convenient, it increases the safety of the system. You can wait until you are ready to launch and then pressurize the launcher. This avoids walking around with a loaded pressurized launcher, and it reduces risk of accidental launch or accidental breakage while under pressure.

High Pressure Air - HPA / SCUBA / SCBA, Nitrogen - N2

Air Conditioning Compressor

Really high performance 12 volt compressors

Gas Powered Contractor's Compressor

Launching Mechanics

How Tall is that Tree?

It is useful to know how tall, or how far away the tree is that we're trying to get a line over. It is also useful to know what direction the antenna will be pointing at when a particular set of trees is used. Where there are lots of trees, there are lots of choices. Equipment we have found very useful for 'surveying' and planning our antennas, and our launches includes: Most any map compass (like the Boy Scouts use) will serve to help choosing trees in the right direction for your antenna. A long tape measure can be used to check the distance between trees, and to measure the wire for dipoles, etc. Harbor Freight tools has a nice fiberglass tape that is calibrated in meters and feet that is low in cost, long, and lightweight. The Rangefinder is really handy for determining the height of trees, and great for distances between them as well - much more convenient than dragging the tape out. The tape is still better for measuring dipoles, however.

For really long antennas a GPS may be useful to help survey the site. The compass and rangefinder are actually better tools, but many people have GPS receivers and not Rangefinders. You can use the GPS to determine how far away and what bearing you are from a starting point. Choose one end of the antenna first and set a waypoint there. Then explore the other trees and check your distance and bearing to the waypoint.

Rangefinders are really excellent for determining the height of trees, as well as the spacing for the antenna. Two common types of rangefinder have been employed. The older type is optical. These work by superimposing two images collected from mirrors spaced apart, measuring the angles and inferring the distance. Some of these are quite accurate, others not so. Generally the longer the baseline the more accuracy results. Our accuracy requirements are not great, so most of these are adequate. They have a lower range limit in the neighborhood of 5 to 15 yards. The accuracy falls off with distance. They usually give reasonable accuracy to half their claimed range, beyond that accuracy suffers. They require some practice to get consistent and accurate results. Some models do better - the longer the baseline the better. The Ranging 400 is built on the same baseline as the 1000 yard model, so it works quite well out to the full 400 yards. For antenna work a 150 yard model that is actually accurate at 75 yards is good. If you plan to put up longwires like vee beams you may want something with more range, like the Ranging 400. They are available from outdoor suppliers like www.cabelas.com, and probably can be found on Ebay.

The newer rangefinder type is the electronic laser rangefinder. They bounce an IR laser from the object and measure the time of flight. They read out digitally, generally to a yard accuracy or so. They are surprisingly inexpensive considering the technology they contain. These are excellent at measuring both tree height and spacing. They do have a lower range limit which can be as much as 20 yards or more. The upper limit varies with model but is often 400 to 1000 yards. These are also available from outdoor suppliers like www.cabelas.com, and may be found on Ebay as well.

A few minutes with a compass and rangefinder can really help lay out the antenna system. Use the tree height as a guide when selecting the pressure for the launch. Generally you want to launch just over the target tree or limb, going higher causes more tangling with other trees on the far side. One exception is the single-launch inverted L. There you intentionally overshoot several trees at once, pull the twine back to the reel, and then pull the wire up and over and tie it off - an inverted L shape. Hook up the tuner, throw out a counterpoise, and get on the air. One of the fastest wire antenna setups, not critical for height or direction, and very effective for short and medium range communications. The vertical section gives a respectable low angle signal as well for dx.

Safe System Transportation

Not everyone thinks about transporting the launcher when they start out. It may come as a revelation, but if it is hard to transport, it probably doesn't meet all your requirements. You don't want the sun on it - eventually the PVC will become brittle. You don't want it to fall over and get broken. It should be easy to take, and not get in the way when it is not being used. Archery gear is large and bulky, but slinghots are easy. Pneumatic Launchers for Antenna work can be easy also. Keep this in mind when designing and building your launcher. Start out with a size in mind and see if you can build to it. Using a tennis ball for a projectile limits the lower size limit to some degree. Barrels down to 8 or 12 inches can be effective.

I've been using the Rubbermaid 10 gallon totes to store and carry equipment in.

At this point I've built four different launcher designs that will fit into that volume, and all four can reach 150 feet of height easily. There is enough room in the box for a lot of accessories.
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