Mark,
I used an Edmund 8' balloon supported vertical in the ARRL 160M test a few
weeks ago. I inflated it to about 4', where I found it had plenty of lift
to hoist up my 129' piece of braided copper wire. I put it up on Friday
just before the contest and took it down on Tuesday night. By then, it had
deflated to approximately 2.5 ft diameter, but still had enough lift to keep
the wire in the air. I put it in the garage to see how long it would stay
inflated, and by the following Saturday it was laying on the ground.
I noticed a number of small "thin spots" on the top of the balloon when I
took it down. I'm not sure if they were there when it went up, because I
didn't inspect it immediately after it was filled. W7GNP has suggested that
these may be due to static discharge off the antenna. I did not use any DC
grounding, so I am inclined to agree with him. I'm not sure that a DC
ground path would even prevent static buildup at the top anyway, if a good
wind was blowing.
I would say that a spherical shape approximates the real shape for wind load
calculations. The 8' balloon inflated to 4' is only very slightly egg-shaped.
As far as the bursting altitude and maximum size, I don't know. Maybe we
could sacrifice one and blow it up with the shop-vac until it bursts :-) I
won't volunteer to hold it :-)) I would say the safe inflating size is the
size that the balloon is sold as (i.e. 8' for the 8' model, 3' for the 3'
model). According to the data sheet they are made of neoprene and are 5 mil
thick when uninflated and stretch to 2 mils when at full volume. At 2 mils,
it doesn't take much to tear them! That's why I used the bigger one and
only inflated it to 4'.
I calculated the exact lift at one point, but I use this rule of thumb: 1 oz
of lift per cu ft of helium. This is close enough to get you there.
I welcome any info on other sources for balloons, as I think these are kind
of flimsy. Any sources for mylar balloons? The mylar film is pretty light,
and it holds the gas in for much longer periods than rubber-types.
Other details of the 1/4 wave vertical:
The feed point was held at ~27 feet using a 3-section telescoping mast
bracketed to the garage. Two 129' radials were used, one to the NE and one
to the SW, sloping away and anchored at 6' to a block fence. The radials
were attached to the shield of the coax at the feedpoint, but not to the
mast. One of the radials was bent back along the fence (about 30') for lack
of space. The vertical radiator was connected directly to the center
conductor of the coax with no matching network. I fed it with 9913, and
coiled about 8 turns, 10" diameter at the feed point as a RF choke. I used
a pulley mounted at the top of the mast and a 1/4" nylon rope to haul the
feedline/balun/radials up and down.
It resonated at about 1830 kHz, and the SWR bandwidth was wide enough to cover
1800 - 1860 kHz easily.
After reading the comments on this list, I will add an RF choke to ground to
prevent static buildup (TNX K5AM for details).
It worked REALLY well on TX; I got through on the first call with few
exceptions with 100W. It out-heard both my horizontal loop and inverted L
at 25'. But it still was NOISY on RX! (What did you expect, right?) I was
definitely and alligator (big mouth, no ears).
Hope I didn't get to verbose, just my $.02 worth (well, maybe $.03 :-)
Merry Christmas to Mark and all on the list!
Scotty, WA2DFI
At 13:56 12/23/96 -0700, you wrote:
>I've been using the 3 ft Edmund balloons, with
>moderate success - when the wind lets up. These
>are called weather balloons, and are said to be
>designed to burst when they reach a certain altitude.
>I filled one to 4 ft, and it behaved much better in the
>wind. (The lift increases by a factor of 2.37, while
>the wind load area increases by a factor of only
>1.78.)
>
>1. What is the designed bursting altitude?
>
>2. What is the designed bursting size?
>
>3. What is the safe inflating size?
>
>4. I also have an Edmund 8 ft balloon which I haven't
>tried yet. I'll try inflating it to only 5 or 6 ft at first.
>(Compared to 3 ft, at 6 ft the lift increases by a factor
>of 8, while the wind load area increases by a factor
>of only 4.)
> Has anyone used these 8 ft balloons?
>
>5. The simple wind load area calculations ignore the
>balloon shape. Can someone calculate the wind load
>correctly?
>
>6. After 2 days aloft, a 3 ft balloon shrunk down to 2
>ft. This is a loss of 70 % of the helium in 2 days. Is
>that normal?
>
>7. Are there better balloons available?
>
>73,
>
>Mark, K5AM, DM62ni, k5am@lascruces.com
>
>Mark Mandelkern
>Las Cruces, New Mexico, USA
-----
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(602) 784-4649 Tempe, AZ 85285-7681
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