Antenna gain figures.

So, now that you have seen how your setup may change your gain, let's get on to ways that the manufacturer may have got the figures higher than you will ever see in the real World. There are many ways to get high gain figures on paper, even if that antenna will perform poorly in the real World.
This can involve several methods...

Test conditions.
The exact performance depends upon the conditions at the time. As stated on the previous page, many things can affect the performance.
The height above ground is a major factor, as it has a big effect on the radiation pattern. At certain heights an antenna will radiate more power upwards, while at other heights it may increase the power sent across the ground.
Arranging the perfect test conditions is a great way to get the best figure, maybe selecting the conditions which favour your design while working against the comparison type, even if those conditions will not be matched in use.

Meaningless figures.
As gain is a comparison, a figure must state what the antenna has been compared to. If an antenna claims 3dB gain, that's twice the power - but twice as much as what? For all you know they may have compared it to having no antenna or using a dummy load.
If they state dBi or dBd, then they have told you what they compared it to. However, dB on it's own is meaningless. They could be comparing to anything, whatever gives them the highest figure to put on their sales literature or packaging.
Never pay too much attention to any gain figure that does not tell you what they compared to, it tells you nothing.

Dual standards.
The common standards are dBi and dBd, but they are not the same. One antenna specification may quote gain as dBi, another as dBd.
To compare a dBi to a dBd figure, simply subtract 1.64 from the dBi figure.

Gain direction.
Gain is measured in the direction of maximum signal for the test antenna. It is important to note that this is not necessarily the direction which works best in the real World. It is purely about which direction that model favours, whether or not that is useful in real use.
Consider the diagram on the right. antenna 'a' clearly has the higher gain. However, considering where your contact is (also on planet Earth), is that the best one for the job?
If an antenna puts out most of it's signal towards the sky, it is of very little use to you but it could still have an impressive gain figure.