So, anyone have any articles or data that shows that rated gain is bi-directional in an omni? I'm eager for fact.
*sigh* So the fact that I used to teach this stuff apparently isn't enough for you?
This is basic antenna and transmission line theory. I don't mean to offend, but anybody that truly
understands how antennas work knows this. But, tho it's "basic," it's also very, very complicated. It's not the kind of thing that can reasonably be taught on this site or a simple web page.
Best I can do in a short space is this: Examine the radiation pattern of an isotropic antenna element in free space. It'll be a balloon. Examine the radiation pattern of a half-wave dipole in free space. Notice how it's a big, fat
donut-shaped thing--with the hole along the axis of the antenna. (This dipole has 3dB gain over the isotropic source, btw, at right angles to the element.) Now examine the radiation pattern of an omni-directional "gain antenna" in free space. You'll notice it's a flattened along the antenna's axis and elongated at right angles to the antenna, as compared to the simple dipole. Now, in any of the two latter instances: Imagine an RF source that's outside the previous antenna's radiation patterns. E.g.: A source that's outside the isotropic antenna's pattern, but w/in the dipole's. A receiver connected to the lower-gain antenna would not be able to "hear" that RF source.
The power, or "value," of a signal is in its S/N ratio. A signal theoretically never truly disappears, it just becomes so weak (dissipated--in every sense of the word) its S/N ratio at the receiver has it buried in the noise. A gain antenna improves the energy received (more technically: The energy density), in a given direction, or, in the case of an omni-directional antenna, along a given plane (kind of--it's not really a "plane," per se). (In this respect, Bene505 was correct: It's all about S/N ratio.)
That's the best I can do. (And the most I'm willing to do.) I've forgotten more than I remember about this stuff. Like I said: I used to teach it. I could post my CV, but, if you don't believe me now, that probably wouldn't do any good, either.