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A Ballistics Primer for the Radio Amateur
#1
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The Merriam-Webster dictionary defines ballistics as “the science of the motion of projectiles in flight”. I prefer a simpler definition: “what goes up must come down”. Ballistics doesn’t just refer to guns – although you would be forgiven for thinking that if you watch too much TV.

Hams employ ballistics when launching antennas. I witnessed a great ballistics demonstration at Winter Field Day when Tom, VA3TS and Frank, VA3GUF launched a lead fishing sinker into the stratosphere using a slingshot and fishing reel. The sinker soared over not one, but two tall trees (I estimated the trees were around 70 to 80 feet high). The team just managed to keep the sinker inside Grey County as it landed just behind Dan, VA3DNY who bravely waited under the projectile trajectory to attach the line that would be used to pull up the Winter Field Day antenna.

A slingshot is very effective for launching antennas into trees but, they have two drawbacks. The first is that they use a neoprene “band” which is stretched to create a huge amount of what physicists call “potential energy”. When the band is released that potential energy is converted into kinetic energy, launching our fishing sinker on a parabolic trajectory into the trees. Repeated stretching of the band causes it to eventually fail – usually at the point of maximum potential energy. When it fails, that potential energy is still converted into kinetic energy but with highly unpredictable results. I learned that from painful experience!

The second drawback of the slingshot is that its use is prohibited in some jurisdictions. Even where its use is permitted, firing lead weights into trees in a public place could be misinterpreted. I was once asked if I was trying to hit birds in the trees.

As well as being a physics graduate I’m a bit of a baseball fan, but without any playing talent myself. I have a lousy throw; my wife says I throw like a girl. If I were a baseball pitcher I would be great at picking off runners at the bases – but only while trying to get the ball over the plate! My analytical mind looked at how pitchers launch balls at high speed. Three techniques stood out. Their arms are stretched out so the ball describes a large arc as it is launched. Their wrists pivot sharply as the ball is released and the manner in which they grip the ball determines how it flies.

You can compare baseball pitching to the manner in which the medieval trebuchet was used to fire burning projectiles at castle walls during a siege. I have watched videos of modern trebuchet reconstructions being used. Medieval armies and baseball pitchers know a lot about physics; even if they don’t know they know a lot about physics.

So how does somebody who “throws like a girl” use physics, and particularly the branch of physics known as ballistics, to get his antennas high up into trees without using a slingshot? I do it every summer when I go what I like to call “radio-camping”.

What is more innocent than using a dog ball launcher in a public park? We see it all the time. A dog ball launcher is a flexible plastic arm that gives a mechanical advantage to somebody who throws like a girl. Dog balls are very light and won’t get very high into a tree. They will also tend to get stuck in a tree. For the outdoor ham operator it is very important that what goes up must come down again. I use a plastic balloon filled with 3 or 4 ounces of sand. Instead of fishing line I use a very thin, but enormously strong, cord. You can buy a 120ft reel of it at Deals in Owen Sound for a few dollars. With this arrangement I can get my line up at least 40ft which is usually good enough. And, importantly, the sand balloon will almost always pull my line right back down to the ground.

If I want to get even higher, I have another scientific trick. A fully extended trekking pole, with my “Deals” cord tied at the far end will get my sand balloon up even further – maybe 60 feet or more. I tried it with 20lb test fishing line but the kinetic energy was so high the line broke every single time! Trekking poles, like dog ball launchers, look innocent and can be seen in every park. And, unlike lead fishing sinkers, sand balloons create fewer casualties when they misfire!    
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#2
I couldn't find much on the web regarding much of any regulations in Canada. There are lots in the U.S. but for a country that have as many guns as citizens, I question why they make an issue of slingshots. Here is a website I came across by someone else that had the same question. 

https://www.theslingshotforum.com/thread...ada.42284/

Over the years I have had good luck with the slingshot to get a string up into a tree for field day or more often, right here at my qth.

A few things to remember:

It isn't necessary to pull the slingshot to it's maximum strength.
Heavy cord just means that if your launch is unsuccessful and gets tangled among the branches, then it's a problem to retrieve the line. I have found it is better to be able to break the line and start over. Fishing line is cheap and I use the 8 Lb test line because it does break easily. Cord also has a higher friction when moving across tree bark. I use a 1 ounce sinker, painted red or with red tape so it's easier to see. Nylon monofilament is the least UV resistant and is a bonus in this case because any line that is left in the tree, will deteriorate over time by itself.

A good amount of common sense should prevail and no matter what you do, there will always be somebody that doesn't like it.

My friend Ladislav Zarsky once said " You can't make happy everybody, ya"
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#3
I agree with you Tom. When I am in the bush, or on my own property I tend to prefer my slingshot and fishing line. When I am in places like provincial parks I am mindful of all the rules and regs, some of which carry quite steep fines.

(2020-02-06, 06:39:00)VA3TS Tom Wrote: I couldn't find much on the web regarding much of any regulations in Canada. There are lots in the U.S. but for a country that have as many guns as citizens, I question why they make an issue of slingshots. Here is a website I came across by someone else that had the same question. 

https://www.theslingshotforum.com/thread...ada.42284/

Over the years I have had good luck with the slingshot to get a string up into a tree for field day or more often, right here at my qth.

A few things to remember:

It isn't necessary to pull the slingshot to it's maximum strength.
Heavy cord just means that if your launch is unsuccessful and gets tangled among the branches, then it's a problem to retrieve the line. I have found it is better to be able to break the line and start over. Fishing line is cheap and I use the 8 Lb test line because it does break easily. Cord also has a higher friction when moving across tree bark. I use a 1 ounce sinker, painted red or with red tape so it's easier to see. Nylon monofilament is the least UV resistant and is a bonus in this case because any line that is left in the tree, will deteriorate over time by itself.

A good amount of common sense should prevail and no matter what you do, there will always be somebody that doesn't like it.

My friend Ladislav Zarsky once said " You can't make happy everybody, ya"
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