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[aprssig] baloons, etc.

Jim Lux jimlux at earthlink.net
Tue Apr 19 18:15:26 UTC 2005


Lots of clever ideas.. I've cut and pasted comments below..


"I just saw an article on slashdot about a stratolite... a communications
satellite that will hover over an area at 65,000 feet (yeah right....)
They say it's above the jet stream, but I know better... hope their
solar powered craft can "hover" in a 100mph head wind. "

This has been around for more than a decade, in one form or another. Check 
out the DARPA ISIS program, for instance.  Many, many practical problems 
(fuel, station keeping, etc.... winds are generally calm at 100kft, but can 
get high.. flying up to the station altitiude is challenging, because you 
have to go through the tropopause)

"Of course when the wind blows, it will surely pull the balloon off to
the side and cause it to descend."

You want what's called a kiteoon.. He or H2 holds it up in no wind, 
aerodynamic forces hold it up when there's wind.

"All that needs to be up that high is the antenna, right? Have you considered
just putting the antenna up suspended from the baloon with the antenna wire
running down to a digi on the ground?"

Coax/transmission line losses will eat you alive.  You could send DC power 
up the cable, but then there's two big problems:
1) Mass  (use high voltage, constant current, small wires, is the usual scheme)
2) Safety (conductive tethers are a no-no)

" You've got wind to worry about,
plus the difficulty of filling and deploying it in the field.  And carrying
around a heavy K size helium cylinder.
Might be more practical if you could get by with a smaller, tougher balloon."

You bet... As far as carrying around that cylinder, there are DOT rules, 
and throwing that bottle in the trunk of your car is a big no-no.  Strapped 
in the back of a truck, perhaps.  On-site H2 generators are popular for 
weather balloons and pibals.

"What about flying a WIFI range extender?  That would allow us to do
 > video conferencing from laptop to laptop....  What about using digined
 > running on linksys wrt54g with something like TNC-x on a serial port?
 > Then we'd have wifi access and an APRS digipeater in the sky."

Power, power, power....  Run some numbers.. those APs aren't particularly 
low power. Figure several watts at the least.

"We were going to try that once for a transmitter hunt - send up a reflector
and shoot a 4-element quad at it to see what confusing reflections we could
generate.  Can't see how that would help much on the receive side, though."

Clever idea- actually used in orbit as Echo I.. but RCS is pretty low 
unless the balloon is really, really big. RCS of a 4m diameter sphere at 
144 MHz is approximately 4.6 square meters at an angle of 1.19 radians.... 
Start calculating..

"What about a vertical omni connected via RG-174 coax to a short beam
pointed straight down?
I've seen commercial applications using two beams back-to-back in
order to connect two buildings together or multiple floors together
for 900 MHz spread-spectrum data radios.  The data radios themselves
had no physical connection to the antennas.  There were passive
repeaters.  Worked great."

This is an interesting approach.  Need to do some link budgets to figure 
out how well it will work, though.  I get about 100 dB loss for isotropes 
20m apart at 144 MHz.


"How about "un-manned aircraft"? You know, those little helicopters
that are radio control. The new thing now is to have a TV camera on
board and they fly around transmitting the image down to the pilot.
Very common. It would be easy to use a small plane as well, a plane
could fly in circles for a couple of hours with the camera pointed down
and the digipeater digipeating :) Actually sounds like fun."

This is a viable strategy, but requires someone to do the flying (which is 
non-trivial).  There's a wide variety of "giant scale" R/C model airplanes 
around that would be suitable (running off gasoline, too, not 
methanol/nitromethane) and can easily carry 5-10 pound payloads.  Winds, 
launch, and recovery are all issues you have to deal with.  I've been 
fooling around with a 1/3 scale powered parachute model (10 ft kite width), 
but it only goes about 10 mi/hr, so if the wind is faster, you'll lose it.

Figure about a kilobuck to get yourself flying in anything with a decent 
motor and payload capacity.


Blimps work quite well, and you can transport them (inflated) in a horse 
trailer.






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