[HN Gopher] Pneumatic Antenna Launching Systems
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Pneumatic Antenna Launching Systems
Author : pubby
Score : 69 points
Date : 2023-06-04 19:58 UTC (1 days ago)
(HTM) web link (www.antennalaunchers.com)
(TXT) w3m dump (www.antennalaunchers.com)
| 1letterunixname wrote:
| There's no diagram or visual of what the heck it was for.
|
| Apparently, it launches a lead line over a high tree branch on
| the end of a tennis ball to hoist up wire for an antenna. PVC
| potato gun in Stinger form-factor.
|
| I'm curious why they didn't use fishing line considering it's
| much lighter.
| [deleted]
| mungoman2 wrote:
| They do use a fishing line.
| krisoft wrote:
| > I'm curious why they didn't use fishing line considering it's
| much lighter.
|
| They do:
|
| "In the launcher pictured above a Tennis Ball is propelled by
| compressed air, towing a fishing line over the tree."
|
| I don't know where you would even get a wire made out of lead.
| Sounds like that would need to be ordered/made specially.
|
| The only place they mention "lead" is in the context of a lead
| sinker being attached to the fishing wire. And they reject that
| approach as not safe enough compared to theirs which is a
| tennis ball attached to the fishing line.
| ishtanbul wrote:
| They mean the line "leads" in that its the first one up. Not
| that it is made of lead metal. They use fishing line as the
| lead line.
| aaron695 wrote:
| This is circa 2003. (2006 archive-
| http://web.archive.org/web/20060619223222/https://antennalau... )
|
| That's close to pre-internet society.
|
| As others point out, the tech and consensus is way different now.
|
| Cheap throw bags on eBay or Aliexpress for instance, no need for
| sinkers - https://www.aliexpress.com/w/wholesale-throw-bag-
| Arborist.ht...
|
| The world of confiscated Monkey's Fist's at ports is an
| interesting one -
| https://www.portskillsandsafety.co.uk/news/confiscated-monke...
| You can add it to your dangerous knots list.
| loph wrote:
| I use arborist throw bags for portable operations, but I cannot
| get them up 60-70 feet high, only 20-30 feet. For semi-
| permanent installations of wire antennas in tall trees I use a
| pneumatic tennis ball launcher. I can put a piece of 50 pound
| test fishing line over the top of 80-foot trees.
| resters wrote:
| For anyone who is interested in amateur radio but hasn't had much
| experience yet, please note that getting some wire high up into a
| tree is a great way to make HF operation more fun especially when
| running a low power transmitter.
| tyingq wrote:
| That's pretty interesting. I used normal long wire antennas,
| anchored to a mobile tower that was pretty fast to erect in the
| military. I'm somewhat surprised a launcher like this wasn't an
| existing option. Maybe it is for the highly mobile, "first in"
| type comm+other units (like ROMADs in the USAF)?
| yardie wrote:
| Be careful using PVC with compressed air.[0] It can shatter into
| hundreds of tiny daggers flying through the air.
|
| [0] https://www.engineeringtoolbox.com/pvc-cpvc-pipes-
| pressures-...
| dokem wrote:
| In reality this is not much of a concern at ~100psi. The bigger
| concern is to make sure that you get a good clean glue seal at
| all of the joints. That is the most likely error and point of
| failure. Also, don't drop it.
| MilStdJunkie wrote:
| Never use PVC pipe marked "DWV" - that's outflow, and it can
| break at pressures as small as 7 PSI.
|
| Schedule 40 is rated for 400-ish PSI but with epoxy, locktite,
| and some tape, it can go up to 1200. For repeat use, err on the
| side of underpressure. There's so many people pushing
| boundaries in "homegunner" circles, but every time I see
| someone doing it - with a cheek weld - I get the jibblies.
| "It's . . your face, dude"
| amluto wrote:
| This is sort of a general issue. Compressed air is much more
| dangerous than water under similar pressure, because air is far
| more compressible. If you pressurize water to, say, 100 psi,
| you're compressing the water a tiny bit (water is not very
| compressible) and stretching the container a bit. The stored
| energy is roughly 100 psi * (change in volume) / 2, and the
| change in volume is small.
|
| If you hit a water pipe. with a shovel and break it, and your
| pipe has a rather high 100 psi of water in it, you will regret
| it: you will get wet, you'll have a muddy hole, and you will be
| sad because you have to repair it.
|
| If you compress _air_ to 100 psi, you are compressing it to a
| rather small fraction of its original volume. So the change in
| volume is _greater than_ the entire volume of compressed air
| and is almost as large as the _uncompressed_ volume, so 100 psi
| * (change in volume) / 2. 1 cubic foot of air compressed to
| 100 psi is something like 10 kJ. Beware!
|
| If you winterize your irrigation system with compressed air to
| 100 psi (terrible idea), and you hit a pipe with a shove while
| it's still at 100 psi and break it, you will release a lot of
| energy.
| Certified wrote:
| Generally you are not compressing water though if dealing
| with compressed fluids (unless you are trying to simulate
| deep ocean depths for pressure testing.) You are compressing
| Hydraulic Fluid which can cause Hydraulic Injection Injury
| which can lead to loss of limb [1].
|
| [1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2532970/
| amluto wrote:
| You're generally not compressing water or hydraulic fluid
| because they're not very compressible. (Although I think
| many hydraulic fluids are actually more compressible than
| water, but still barely compressible. They're used for
| other reasons. The ideal hydraulic fluid for most
| applications is entirely incompressible.)
|
| But systems using hydraulic fluid to carry power have
| something that _keeps it pressurized as it moves_ : that
| is, something that can supply power. Often very large
| amounts of power. (So do water pipes that have some form of
| supply, of course, but the pressures are much lower.)
|
| So if you pressurize fluid in a pipe, you are storing some
| energy in the compressed fluid. You may _also_ have a lot
| of energy stored or power available in some other component
| of the system.
|
| In any event, the type of injury you're describing seems
| like more of a velocity issue. When you allow fluid to
| escape from a high pressure environment to a low pressure
| environment through an opening that doesn't have a whole
| lot of friction, the stored energy converts to kinetic
| energy. Some basic physics says that a small volume V of
| fluid has potential energy PV and mass rho * V.
| Conservation of energy says that, when that potential
| energy is gone (the fluid escapes), it has the same energy,
| but now it's 1/2 m v^2. You can solve for velocity, which
| scales like sqrt(P).
|
| (The real story is more complicated. Pressure in fast-
| moving things is a bit messy due to Bernoulli's Law, and
| different disciplines use annoyingly different terminology.
| Static pressure, total pressure, stagnation pressure, just
| plain old pressure, oh my!)
|
| Water (or air) jetting out of a hose through a tiny hole at
| even 100 psi is not likely to inject itself into a person.
| darkstar999 wrote:
| The article addresses this. These launchers operate at
| significantly lower pressures than PVC is rated for.
| dwpdwpdwpdwpdwp wrote:
| One should also be aware that the pressure rating is a
| function of temperature. Extra caution should be taken when
| operating in cold environments. Also the cooling effect that
| occurs when compressed air rapidly decompresses should be
| considered.
| jaywalk wrote:
| The article addresses this, too.
| voxadam wrote:
| My ham father built and uses one functionally similar to this
| one. It's slightly more involved than using a wrist-
| rocket/slingshot but it's also far more effective at reaching
| high branches.
| mungoman2 wrote:
| Curious how you get the equipment down after you're done?
| voxadam wrote:
| You launch a ball tethered by a spool of fishing line over a
| sufficiently high branch, locate the ball, remove the ball,
| attach one end of your dipole antenna to the line, pull the
| line back at the gun side until the antenna is in its desired
| location, cut the line, and finally secure the line to a tree
| trunk or other anchor point. Then, you do the same thing with
| the other end of the antenna. The the antenna's feed point
| and balun[0] is located at the midpoint of the dipole which
| is connected via coax to the tuner, amp, and radio.
|
| When you're done with the setup (e.g. field day[1], camping,
| etc.) you simply cut the anchor and lower your antenna down
| one end at a time.
|
| [0] https://en.wikipedia.org/wiki/Balun
|
| [1] https://en.wikipedia.org/wiki/Field_Day_(amateur_radio)
| gorgoiler wrote:
| I've always _loved_ the topography of friction savers / cambium
| savers:
|
| https://m.youtube.com/watch?v=sKEfLm066-4
|
| It's such a clever idea using the emergent properties of gravity,
| stiction, and geometry.
|
| A friction saver is a thick strap with two eyelets in it. With
| the strap placed over a limb, you can then run a thick rope
| through the eyelets instead of running the rope over the limb,
| which would otherwise abrade the rope and tree surface.
|
| But how do you get the friction saver up there in the first
| place? To install, once you have a throw line over a limb you
| then add the friction saver as a segment to the end of the throw
| line _but with the throw line also threaded into the eyelets of
| the friction saver_. Once the friction saver is over the limb it
| will stick with a sticktion much stronger than the throw line is
| stuck to the friction saver.
|
| Getting it back down is another clever trick which relies on one
| of the eyelets being larger than the other. To understand that
| (er, or indeed the whole system, as this is quite hard to
| explain) ... watch the video.
| myself248 wrote:
| That's interesting and has a lot of similarities to this ham-
| optimized version:
|
| https://www.youtube.com/watch?v=dN7wRupZ-AQ
|
| This is meant to be left in the tree long-term, but still
| removed easily when needed.
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(page generated 2023-06-05 23:05 UTC)