What you mount your antenna on, mast/pole/tower/etc, depends on weather in most cases. It has to be strong enough to withstand the common wind etc, and not get blown over, you know? Typically, bigger is better till it get's ridiculous. Over doing is always cheaper/safer than re-doing.
Like others, I use some fence top-rail for masts. I limit it to two sections, about twenty feet, cuz' I'm too lazy to do it 'right' (guying etc.). It's not the best way in the world but it's okay for my particular situation.
It's a matter of what you may have on hand or can get rea$onably, and what you are 'allowed' to do by whoever/whatever.
- 'Doc