Originally posted by finnegan
Well this is getting interesting. Maybe the balloon did protect your pool and maybe it was redundant. This is the trouble with all forms of protection - if the event never happens we wonder if it might never have happened anyway - I have no tigers under my bed but maybe my precautions were less critical than the fact that I live in the UK? We need a contr ...[text shortened]... before. In the interests of science I implore you to remove the balloon and report the results.
Well in my case, I have had the above ground pool for more than 5 years and never had a problem with ice expanding to the point where it damaged anything, you have to have the water level lower than the skimmer to prevent ice from damaging the fragile plastic parts of the skimmer but that's all I did in previous years.
I never used a balloon before but this year I had to buy a new pool cover, it comes with about 22 meters of very strong but thin plastic covered steel twisted cable and a tightening device. The cover has regularly spaced grommet holes for the cable to thread through and after it tightens it resists high winds and such.
One effect I was a bit worried about, if the cable was tightened too much, would it add to the problem by forming a barrier to stop the expansion of the metal parts being forced outwards by the expanding ice. I will find out this year since before I used nylon rope which has a lot of give to it so if the sides were forced to expand, the rope would stretch a bit. This time, with steel cable running the circumference of the pool structure, it might change that dynamic. News at 11!
I'll weigh the balloon to see how much extra lift the H2 would provide. Obviously if it was made of lead, it would probably not float, eh. It has some weight of course but I think less than a Kg. Have to test that though. About the winds, I am talking about heights of less than 50 meters, not aircraft altitude!
It's tricky figuring out the radiation pattern of an antenna in a semivertical wire. If the antenna wire was going straight up and was exactly 1/2 wavelength long, like one of our ham bands is '40 meters', about 7 Megahertz, so exactly half at 20 meters high would make for an omnidirectional radiation pattern horizontally around the antenna, like a donut.
If it was way longer, say 200 meters high straight up, using that same 7 megahertz signal, the radiation would pretty much be going straight up in the air, which is good for rather short range communications, but not good for long range, which is what the donut shaped pattern is more likely to give you.
Short range in this case means a couple hundred km or so, long range, thousands of km. So if I used a fixed length, and that was 20 meters high, at 7 Mhz, you get a donut pattern but using the same antenna at 20 meters, 14 mhz (another ham band), the pattern would go from donut shaped to more like an inverted cone with more radiation going up following the length of the antenna and even more pronounced at 10 meters, 28 mhz.
So if I want a donut pattern with that wire antenna at ten meters, I have to lower the height of the vertical wire antenna to 5 meters or so to get the donut pattern you want for longer range.
So the longer wavelength bands, there is one at 160 meters, so the donut pattern would occur with a vertical wire 80 meters high. The problem is if the wind changed the balloon from vertical to say 45 degrees, two problems occur, one being if you have in fact a half wavelength antenna and it is at 45 degrees, it is called a sloper and the donut shaped radiation pattern will be still there but on the side of the antenna facing the ground, the donut pattern would be basically warming the grass on the ground and not being aimed into the air.
The other side facing the sky would now not shoot out horizontally but up in the air by some amount, say 45 degrees or so which can be good for very long range communications if you do it at the right angle. So the kind of problems you would face with a balloon supported antenna is variable angle of radiation depending on the wind velocity and whether there are high voltage power lines nearby which could really ruin your day if say 12,000 volts found its way into your radio equipment and you though touching your microphone or morse code key....
So it is more than academic the choice of length for such an antenna!
That's my 50 baht tour of antenna theory. BTW, for anyone interested, my ham call is AI3N, a great call for code: dit dah, dit dit, dit dit dit dah dah, dah dit.
Question about ice:
31 Oct '10 01:031 edit
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