As others have already pointed out, it's likely not about your altitude, it's about whether you have obstructions, trees, or other things that impact your signal.
Since your question is essentially one of range, for what it's worth, I did range testing last week on an unobstructed beach mostly over water, in a couple different directions, at a height of 100 feet and distance of 5000+ ft. Never lost a bar of signal. Will do more over populated areas once I get a nice clear day.
