There are quite a few dipoles mounted at less than a 1/2 wave length above ground, even on 10 and 11 meters. They are probably just as 'omnidirectional' as some verticals the way those verticals are mounted (stuff near them). And then, what's the great benefit in being omnidirectional?
Most dipoles are not all that directional even when above a 1/2 wave length, more of a flood light than a spotlight, and will never be a laser by any means. Does that mean you won't hear everything all around you? You probably won't. But then, the same can be said for an omnidirectional vertical antenna. Dipoles tend to 'favor' certain directions, but then, so do verticals.
This is Something that was revealing to me in regards to discussions about vertical antennas.
I realized that I needed to pay attention about if the vertical antenna being discussed was a ground based one with buried radials or a ground plane one elevated several feet above the earth.
There are significant differences in the performance and characteristics between the two type of verticals.
I know it is one of those Duuhh! moments, but it cleared up what appeared to be some contradictory observations in various text regarding vertical antennas for me.
Around 14 degrees seems very useful. That also happens when the Yagi is about 1 wavelength above ground. Verticals with elevated radials behave much the same way. While the nulls in a pattern can ruin the ability to communicate quickly, we have to remember that changing height effects lobes and nulls simultaneously.
If you noticed the 14 degree primary lobe was giving you good DX results in the desired target area, raising the antenna to two wavelengths would be disastrous for that same 14 degree target area. You just lined your deepest null up in the same place your strongest lobe was! Just some things to consider.
I remember on Copper's forum a few years back some guy was getting ready to put his beam up at around 50 feet agl. That idiot Lon 808 told him it would "perform better" at 36 feet because that was a wavelength for 11 meters.
I tried to post a reply suggesting that that was nonsense (in a polite way, of course) and 808 would not let my post through. The poor slob probably took the shitty advice and cheated himself out of 15 more feet of height for his beam.
As for having stacked over/under beams, yes it is possible that each antenna may be set up to have no meaningfull nulls and each will have a different optimum TOA and that switching between them one may indeed be able to select the proper wave angle to suit the path at the time. TOA does play a role in that but for a single antenna it is not nearly as important as the nulls or the distribution of power over that major lobe.With a single antenna one has to be prepared to work signals over a wide range of incoming wave angles which is why I say that TOA is not the important factor in selecting height of the antenna.
Horizontal antennas have what is called "ground gain" which is increased gain caused by the incident wave and a wave reflected from the ground combining in phase many, many wavelengths away from the antenna. This can add up to 5 or 6 dB gain to the "real" antenna gain. This is one of the reason you see such high gain figures on manufacturers antennas as they add the ground gain to their real gain and do not bother to tell you that. Unfortunately they also add it to vertical antennas which have no real ground gain and then there is the just plain bullshit and lie factor thrown in as well. This is why manufacturer's stated gains and program derived gain figures rarely if ever agree.