I can't afford a massive tower, but I can get a weather balloon. What kind of set up would work best to get the maximum efficiency out of altitude?
Hoisting a vertical antenna with a balloon seems to be a fairly-common trick for Field Day. A web search for
tethered balloon antenna returns many interesting results. Here are some points brought up by several of those web pages:
- Ordinary round weather balloons or 36" (91 cm) party balloons work okay, until the wind blows, which causes the antenna to be knocked flat.
- One poster suggested attaching the balloon-hoisted vertical antenna to a mast of some sort, so that when the wind blows the antenna becomes an inverted-L, rather than just laying along the ground.
- Several posters noted that kite-shaped balloons designed for tethering work best; the kite shape makes the balloon generate lift in the wind, so that the antenna stays more vertical. Apparently kite-shaped balloons meant for tethering are sold for advertising purposes, to host pennants over sales at car dealerships and such. Such balloons are also known as kytoons. Here's a commercial supplier.
- Governments may have something to say about your project. One poster suggested that in the US, tethered balloons have to be reported to the Federal Aviation Administration.
- Any balloon will surely leak helium over time. Several posters said that their balloons came down after a day or two.
- Safety concerns abound. Nearby power lines are obviously a concern. The balloon should probably have a tether line in addition to the antenna wire, in case the antenna wire breaks at the ground. A loose balloon dangling a copper wire would be dangerous.
Please reconsider your weather balloon hoisting scheme, at least if you plan to use helium as your lifting gas. Helium is a non-renewable resource derived from fossil fuel (natural gas wells are the only source). It migrates through all materials light enough to make good balloons, and can't be recaptured even if the balloon is recovered before significant leakage has taken place -- and helium lost into the atmosphere is effectively gone forever, no longer available for no-substitute applications like chilling superconducting coils in MRI machines.
If you can do the ground handling safely (requires care, but not at the level of rocket propellants), hydrogen will lift slightly more wire for the same balloon size, costs less than helium, and is renewable (commercially, it's made by electrolyzing water). As a bonus, it's not a greenhouse gas (as methane, the other alternative lifting gas, is), nor is its combustion product (water vapor) a significant greenhouse factor.
Hydrogen can be purchased from industrial gas suppliers -- you local welding shop can order it. It can also be made by a number of methods that predate wide use of electricity -- zinc dropped into acid produces hydrogen, and forcing steam past red hot iron will also produce the gas.
Safe handling mainly revolves around ignition control -- hydrogen will ignite/explode at concentrations from 4% to 95% (as I recall) in air, so during hydrogen handling and generation, one must take similar precautions to working around natural gas leaks or similar flammable/explosive gases. The good news is that hydrogen is so much lighter than air that leaked gas rises out of the work area very quickly. Worth noting that German zeppelins were lifted by hydrogen from before WWI until almost WWII with few accidents (zeppelins were destroyed by gunfire with incendiary bullets, and there was the Hindenburg, but those weren't mainly hydrogen problems).
The places where such a rig might be useful are in emergency areas with civil strife, wildfires, post storm ravaged landscapes. Of course this assumes the residents of the broadcast area would have receivers as well. Using such a setup might help in directing crowds to safety, to food and shelter, or other concerns re: public safety.... all for at most a couple of weeks.