Just keep safety in mind. A balloon could be used as a high signal panel if tethered to the ground. Just remember that if it's raining, wet string will conduct electricity, i.e. lightning. If you are using a wire tether as a radio antenna, be aware that clear air lightning strikes have been known to happen. Rather than toting a tank of compressed helium, I was thinking of producing hydrogen by a chemical method. I haven't got the details worked out yet, but I'm thinking that a metal & a powdered acid could work (you add a little water to activate). Any chemists in the forum?